At the end of the article, it says that resonance diagrams are soon to come. That was 6 months ago. Is anybody going to be adding resonance diagrams soon here? H Padleckas 16:17, 25 Jan 2005 (UTC)
They don't really switch between the two possible states constantly, do they? Then the electrons would be net going in a circle and would create a magnetic field. Rather, they exist in a less-than particle state or a "cloud" or something, right? - Omegatron 16:47, Jan 25, 2005 (UTC)
Has anyone actually heard of the term "resonant bonds"? I've look in texts before, and while I've definately heard of resonance structures, "resonant bonds" just doesn't seem standard.
There's a merge tag over at benzene. That section on resonance would look mighty good here. Isopropyl 21:08, 11 April 2006 (UTC)
I agree that some of that material should be added here, but it should also be retained in the benzene article. Resonance is a crucial part of the benzene story and readers should not have to come over here to read about it. -- Bduke 21:27, 11 April 2006 (UTC)
Maintain separate articles, sharing whichever sections would improve the other. They are sufficiently separate entities. - Ayeroxor 21:41, 1 May 2006 (UTC)
The article states that sigma bonds are meaningless in MO theory for benzene. This is strictly accurate but goes against useage. Pi orbitals in benzene are also meaningless strictly. The terms sigms and pi strictly apply only to linear molecules giving the spectroscopic terms for angular momentum about the molecular axis. We can talk therefore about sigma and pi molecular orbitals even in long linear molecules where the MOs are delocalised. For planar congugated systems such as benzene the useage has come to mean how the MOs change on reflection in the molecular plane. If it changes sign, it is pi. If it does not change sign it is sigma. The term "sigma-pi separation" is very common in both MO and VB discussions of benzene, so the current wording could serious confuse people. I'm not clear how to rewrite the section. The section is:-
"Often when describing benzene the VB picture and the MO picture are intermixed, talking both about sigma 'bonds' (a meaningless concept in MO) and 'delocalized' pi electrons (a meaningless concept in VB). This is not a good practice, because mathematically the models are incompatible."
Any body got any ideas? -- Bduke 22:24, 18 December 2006 (UTC)
MO has sigma orbitals, but no sigma bonds. A bond implies that an electron pair belongs to two and only two nucleus, whereas in MO the electron state vectors are calculated with the )approximate) potential of the entire molecule and not any two particular atoms. That is why in MO it does not make sense to say that every carbon has 1 sigma bond with each of its neighbours. At best we could say that these electrons are in highly localised orbitals. As is indicated in the article, sigma-pi in VB is somewhat different from sigma-pi in MO. Loom91 07:07, 19 December 2006 (UTC)
The problem is that sigma orbitals and sigma bonds do not appear that different and indeed they are not as the delocalised MOs can be transformed into localised orbitals. This means that sigma bonds are not a meaningless concept in MO although I agree that sigma in MO is not exactly the same as sigma in VB. I note also that most quantitative VB treatments of benzene such as the famous spin coupled calculations of Gerratt, Cooper and Raimondi use VB only for the pi electrons and treat the sigma as MOs. It is true that MOs are calculated with the potential of the entire molecule, but so are the VB wave functions. It is the orbitals that are delocalised or localised. The potential is always the whole molecule included the repulsion of all other electrons. We are not too far apart but I think the current wording will lead to misunderstandings and there is already too much misunderstanding in the MO/VB debate. -- Bduke 07:46, 19 December 2006 (UTC)
The point made in the article is that VB and MO are two different mathematical models with different approaches to solving a problem. The sigma-orbital approach in MO and the sigma-bond approach in VB give the same predicted expectation of observables (otherwise one theory would be more 'wrong' than the other), but that doesn't mean that the theories are the same and you can go around taking a slice from VB and a scoop from MO. Also, MO is a more 'natural' theory than VB. The postulates or methodology of MO are closer to the actual quantum mechanical picture, which exists exactly only for the simplest molecules like H2+. Loom91 09:51, 20 December 2006 (UTC)
There are two major problems with what you say. (1) MO and VB do NOT give the same predicted expectation values of observables and indeed one is more wrong than the other. I never said they are the same. They are different. (2) MO is not more natural than VB. Just because the exact solution of H2+ (a one electron problem) is a MO does not mean that MO treats electron repulsion in cases with more than one electron correctly. Both are approximations. MO is much easier to do and is very popular. Computationally a lot of methods build on it or add to it. However in fact simple VB energies almost always lie below simple MO energies (and hence are better) and MO does not dissociate homonulcear diatomic molecules correctly. VB is less wrong. I would also add that it may seems odd to take "a slice from VB and a scoop from MO" but that is exactly what a lot of people do, including as I said above, some of the best VB calculations on benzene. Have you studied quantum chemistry to any depth? Of course this article is not the place for advanced quantum chemistry, but we must try to avoid too many "lies for children" as we simplify the quantum chemistry. -- Bduke 10:16, 20 December 2006 (UTC)
I think the confusion is resulting from talking about VB and MO as theories in the ordinary sense. I haven't studied VB in detail, but as far as I know they are two different approaches to approximating the solution, not methods of approximation as such. As you say, computational methods build on them. There are different levels of approximation in VB and different levels of approximation in MO (you could start with Huckel and proceed to DFT). If a certain level of approximation in VB gives a result, MO will also give similar or better results at some level of approximation. You say simple VB energies lie below simple MO energies, but we can't decide superiority between the two simply by looking at the 'simple' limit.
The framework provided by MO (or VB) can not be identified with any particular method or level of approximation in either. The MO road is closer to what a physicist will do if asked to solve a system, but that does not mean the VB road will not give the same results at sufficient approximation. The converse is also true. You claim "MO does not dissociate homonuclear diatomic molecules correctly"-I've never read this, but VB fails to predict that hybridisation in CH4 does not produce equivalent orbitals. VB is not more correct than MO.
As for mixing MO and VB, you are talking about mathematical calculations where we have quantitative ways of verifying whether we are mixing in a valid manner. The article however talks about mixing the two in qualitative descriptions ("lies for children"), where there is no way to know (without having actually done the calculations) whether the pictorial descriptions reflect the actual mathematics. It is such situations that trying to mix the two models is dangerous. Also, I don't think that this article is not a place for advanced quantum chemistry. Wikipedia is a specialised as well as general purpose encyclopedia, so feel free to add a section on the detailed mathematics behind the two-headed arrows. It will be best if you cite references. Loom91 07:48, 21 December 2006 (UTC)
You state that you have not studied VB in detail. Sorry, but I'm afraid that is pretty clear. Let us take the dissociation of MO theory for homonuclear diatomics first. The simple MO wavefunction is an equal mixture at all values of the bond length of the simple VB covalent and ionic terms, so it dissociates into an equal mixture of 2 H atoms and (H+ + H-). The energy is the mean at infinite bond length of these two and thus very much higher in energy than 2 H atoms which H2 experimentally dissociates into. VB dissociates correctly. The predicted MO dissociation energy is too large. The predicted VB dissocaiation energy is closer to experiment but two small because the two atoms at large distance are exact and the VB energy lies above the exact energy at the predicted bond length. The MO energy lies even higher. These facts matter because both methods are variational - they lie above the exact result. The simple Heitler-London VB lies below the simple MO energy curve and is thus better. The similar case of F2 is so bad that the MO energy lies higher than the correct dissociation of 2 F atoms. This is in many texts. By MO I mean for H2 a single molecular orbital built as a linear combination of atomic orbitals (with simple MO using just the two 1s orbitals). Of course one can add configuration interaction and do as well as VB. Adding all possible excited configurations to MO and all possible VB structures to the simple VB gives identical results but such calculations, called full configuration interaction, are only possible for fairly small molecules with fairly small basis sets. "VB fails to predict that hybridisation in CH4 does not produce equivalent orbitals" - please explain more carefully or give a source. Hybridisation is not a physical think. It is an artifact of VB theory and the hybrids can and are closen to be equivalent. DFT is not MO although I grant you it looks like it. VB can easily do better than the best MO wavefunction - called the Hartree-Fock limit. It does exactly because it get bond breaking better.
The bottom line is that the simple pictures grew out of calculations. Pauling would have done nothing without the Heitler-London calculation on H2 and some extensions of it. We forget this at our peril. Qualitative ideas did not grow out of the air. "Also, I don't think that this article is not a place for advanced quantum chemistry". I think you meant "Also, I do think that this article is not a place for advanced quantum chemistry". I agree. It is about the simple pictures but we must not mislead. I'll think about it more after the holiday period which is going to be very busy for me. -- Bduke 08:42, 21 December 2006 (UTC)
What you are saying is not that VB is a more accurate theory than MO, but that VB is computationally less intensive. As for hybridisation, the 4 CH bonds are not exactly equivalent as calculated in VB. As predicted by MO, one of the bonding pairs have a different energy from the other three. The photoelectron spectra shows two characteristic bands [1]. Also, why are you excluding post-Hartree-Fock methods from the umbrella of MO? And I meant exactly what I said: "Also, I don't think that this article is not a place for advanced quantum chemistry." That is, this article IS a place for advanced quantum chemistry. An accurate section on what exactly is mathematically meant by resonance will add greatly to the article. It may even become a GA. So feel free to add such a section. Loom91 07:12, 22 December 2006 (UTC)
No, I am saying that VB is more accurate at equivalent levels than MO, but that MO is computationally less intensive. The orbitals in MO are orthogonal and that simplifies things. In VB they are not orthogonal and it has taken a long time to get code that competes with MO. Why am I excluding post-Hartree-Fock (HF) methods? Because while based on a MO reference function they are not MO. Configuration interaction at the full level is entirely equivalent to full VB, so no comparision is fruitfull. Bond breaking is still badly handled by post-HF that uses a single determinant reference. To handle bond breaking correctly the MO guys use multi-configuration SCF where you can no longer say that 2n electrons are in n MOs for a closed shell singlet. Also it can be shown that these methods are very similar and in some cases identical to some spin-coupled VB methods, so again comparision is not fruitfull. The only meaningfull comparision is between the methods Pauling and Mulliken and their respective supporters fought about in the 1930s, for example simple MO for H2 - (a + b)(1)(a + b)(2) and Heitler London for H2 - a(1)b(2) + b(1)a(2) where a and b are the 2 is orbitals on the 2 H atoms. In passing note that expanding out the MO function, you get a(1)b(2) + b(1)a(2) + a(1)a(2) + b(1)b(2). The first 2 terms are the Heitler London terms and the last 2 are ionic terms - H- H+ and H+ H- so as I said earlier MO is a mixture of the VB covalent term that dissociates in 2 H atoms and the VB ionic terms that dissociate into two ions at a higher energy.
I have no idea where you have got the idea that the 4 CH bonds are not equivalent in VB. They are. The photoelectron spectrum with 2 peaks is best explained by the fact that there are only 2 energy-distinct MOs - 1 triply degenerate group and a single degenerate one for the 4 pairs of valence electrons. Ionisation is certainly best explained by MO theory because the electron does not leave one bond but the whole molecule. A VB description of CH4+ would have to include resonance between the 4 structures each with 3 two electron bonds and 1 one electron bond. In this way the ion would come out symmetric and there are indeed 2 solutions just as in MO theory. Yes, MO theory is simpler to describe ionisation and spectroscopy. VB can be simpler to describe bonding and generally gives better numbers. Getting numbers to agree with your PE spectra from MO theory is not easy, but the simple picture is. The orbital energies, for example, will only predict the position of your peaks well, using Koopman's approximation, if the massive correlation energy corrections and relaxation energy corrections are of opposite sign and similar magnitude which they often are for organic molecules but rarely are for metal complexes. -- Bduke 08:12, 22 December 2006 (UTC)
What do you mean by equivalent levels? How would you say a particular VB method is equivalent to a particular MO method? Loom91 18:07, 23 December 2006 (UTC)
I assume you are asking about my statement that, for example, full CI is equivalent to full VB. OK, let us take the classic example of H2. The simple MO is (a + b)(1)(a + b)(2) which expands to:-
The excited state with both electrons in the antibonding orbital is:-
Now mix these and collect terms (K is the mixing weight):-
The above is the full CI result for this small basis set of 2 1s orbitals. The full VB is ionic - covalent resonance, which is (C is the mixing coefficient):-
Neither of these are normalised. In both cases the mixing coefficent is determined by finding the value that minimises the energy. Since both allow any proportion of the covalent - {a(1)b(2) + b(1)a(2)} - and ionic terms - {a(1)a(2) + b(1)b(2)}, the final results will be the same. This result is general. If we take a simple MO and mix in all possible excitatations that mix with the ground state, and then take all possible VB structures from the same set of atomic orbitals, the results are equivalent. The general result is perhaps surprising - approximations that look very different and start from different ideas, can actually be completely identical.
To our other readers, I apologise. This is getting over complicated and technical. Loom91, if you want to continue this, please move it to e-mail. I have e-mail set from my user page. I am happy to continue helping you to learn about VB theory, but I think the discussion is getting beyond relevence to this article. -- Bduke 21:42, 23 December 2006 (UTC)
You misunderstand me. I know VB == MO in the high accuracy limit. I was asking in lower accuracy levels how you say that VB is more accurate than 'equivalent' MO. As for the article, what changes do you propose? Loom91 07:02, 27 December 2006 (UTC)
Let us take H2. The simplest MO approach just using the two hydrogen atom like 1s orbitals is as above. The simplest VB using the same orbitals is the Heitler-London. These are at an equivalent level, yet give different results. The latter lies lower in energy than the former at all interatomic distances and particularly at large distance and so is better. We can then optimise the orbital exponent of the 2s orbital in both cases. These are at equivalent levels. Again VB is better. That is what I mean by equivalent - same basis set and simplest possible MO or VB approach or comparable improvements to simplest approach.
I have made the changes to the article that I think should be made. My reasons are many. First, it is quite common to mix MO and VB ideas. Coulson in both "Valence" and in McWeeny's "Coulson's Valence" says this about the sigma bonds in benzene, "These bonds can be described either in MO or VB language; their essential character is the same in either case". I know of no book that criticises this statement. He goes on to give the VB and MO approaches. This mixing of language is commonly done in simple qualitative explanations and a mixing of methods is commonly done in quantitative calculations as I mention above. I do not think sigma is "meaningless in MO" or delocalised is "meaningless in VB". I do agree it is best to use "delocalisation energy" in MO descriptions, but note that somewhere on WP is a reference to a Journal of Chemical Education article that recommends delocalisation rather than resonance for VB descriptions. I also suggest it is stretching it to say about the two methods that "mathematically the models are incompatible". Different, yes, but not incompatible. To say, for example that MO for H2 is entirely identical to VB resonance between the covalent and ionic structures, but with equal weights, demonstrates this lack of incompatability. The article is best made simpler at this point. The wording was confusing and not clarifying matters, so is best removed. -- Bduke 02:10, 28 December 2006 (UTC)
Reasonance diagrams for heteroaromtic compounds would be nice -- Quantockgoblin 23:47, 20 March 2007 (UTC)
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Just out of interest, why is resonance referred to as "a tool used (predominantly in organic chemistry) to represent certain types of molecular structures."? , it's a bit of a LARGE generalisation; aside from being referred to as canonical forms, Miessler refers to it as when there is "more than one possible way in which valence electrons can be placed in a lewis[-based] structure.", Chambers refers to it as "when [a] true structure of .. [a molecule or compound] cannot be accurately represented by a single structure, ... several resonance structures are suggested." ♥♥ ΜÏΠЄSΓRΘΠ€ ♥♥ slurp me! 10:40, 6 April 2007 (UTC)
To clarify this point "Resonance hybrids are always more stable than any of the canonical structures", the wave function Ψ is given by:
where Ψ is the resonance hybrid function and C1, C2, .. are the canonical structure functions. a1, a2 , a3, .. are coefficients chosen to minimise the energy. It follows from the variation theorem that the energy of Ψ is less than or equal to the energy of all of C1, C2, .. taken separately. It would be equal if one of the a1, a2, .. was 1 and all the others zero, and lower otherwise. Loom91 is correct and he gives a good simple reference. In antiaromaticity, the geometry changes to a more stable form. A good discussion is chapter 4 of "Facts and Theories of Aromaticity" by David Lewis and David Peters, Macmillan, 1975. -- Bduke 13:37, 12 April 2007 (UTC)
Quotation:
When separating charge (giving rise to ions), usually structures where negative charges are on less electronegative elements have little contribution, but this may not be true if additional bonds are gained.
I believe this statement should be rewritten. Not being a native english speaker, my opinion may be misguided; anyway, to me it looks convoluted and is nearly uncomprehensible.
A statement like this, being a list item, should speak for itself. It remains unclear however, to what phelomenon contribution should contribute.
Bertus van Heusden 10:53, 4 September 2007 (UTC)
I see someone changed the units under "resonance energy" to kcal/mol without changing the numeric values. These should definitely stay as SI units, but someone should now check the correct values. Unfortunately I don't have time right now. -- Slashme 06:53, 10 October 2007 (UTC)
You have a point, it's probably a good idea to keep the kcals, but a quick browse through the literature shows that kJ is gaining ground. I can't yet find a good ref. for the values quoted, because my chem. books are at the lab. If I get around to it, I'll sort it out, but I might not... -- Slashme 13:21, 10 October 2007 (UTC)
The original estimates were in kcal, probably in the era of Pauling, but kJ are now preferred. So yes, we need both units, so I have now inserted kJ values. I just multiplied the kcal values by 4.184 and rounded off to two figures. Dirac66 13:33, 10 October 2007 (UTC)
To me, as a physicist, there are a couple of problems with this article. One is that the article never really explains the reason for the term "resonance," and there is no obvious (to me) physical phenomenon going on that is in any way (that's obvious to me) analogous to resonance. The section near the end about Pauling's introduction of the term doesn't really explain anything very clearly: why the quantum-mechanical treatment of H2+ was relevant to Pauling, or why he used the word "resonance." The other problem IMO is that the article never gives any very transparent physical explanation of what's going on. Although I understand the general argument made above on the talk page that a superposition of trial wavefunctions can be optimized variationally to lower its energy, that argument is so generic that it really has nothing in particular to do with chemical bonds, or even chemistry. If I had to take a stab at it, I would guess that the general physical mechanism is that, compared to a structure made of single and double bonds, the actual structure delocalizes the electrons, which means that they have a larger wavelength, thus a lower momentum and kinetic energy. In the case of an ion like CH3CO2-, I can also imagine that the delocalization would lead to a lower Coulomb energy.-- 207.233.87.196 ( talk) 23:56, 11 December 2007 (UTC)
Rectifico ( talk) 19:27, 18 June 2010 (UTC)
Hmm...okay, I think I understand the reason for the term now. See http://www.nap.edu/readingroom/books/biomems/lpauling.html . The paragraph beginning "Resonance: In attempting to explain ..." seems to be saying clearly what the WP article is saying unclearly. I'd take a whack at it myself, but I'm not a chemist, so I don't want to get this wrong.-- 207.233.87.196 ( talk) 00:08, 12 December 2007 (UTC)
I know I'm coming in late into this discussion, but ...
There are several small problems:
The biggest problem I find with this article is that it is not well targeted to the intended audience.
After all, who will look up 'Resonance' on Wikipedia? The average Joe wouldn't ever. Practitioners don't need to. Only students would -- students who have encountered resonance in their studies and in their textbook, and who need help in understanding it. Their first encounter would be with main group examples (e.g. sulfate ion), but this article barely acknowledges this reality. My evaluation is that this article would confuse more than enlighten.
Let me be clear: I can live with this article, because I see no substantial problem with the content, but I would never refer any of my students to it. It's simply not written for them and assumes knowledge.
Specific problems:
Pgpotvin ( talk) 00:23, 11 January 2009 (UTC)
I largely agree and will look at this, if others do not, when I have more time. It will not be easy.
"In the mathematical discipline of graph theory, a Kekulé structure is a matching or edge-independent set in a graph." I added this to get the links in. In particular, Kekulé structure rather oddly redirects to matching. I could not see a better way of drawing attention to this. Can you think of a better way of getting the links right to all the related articles? -- Bduke (Discussion) 23:12, 10 January 2009 (UTC)
The term "Kekulé structure" is used elsewhere. The redirection can be changed, or the link can be to Kekulé structure.
Pgpotvin ( talk) 00:30, 11 January 2009 (UTC)
On the other hand, maybe I'm off. I followed Bduke's links to Wikiversity. I'd never looked at that before. There is there an article on Resonance with many of the same elements as here (and which I presume was also written by Bduke). Maybe the didactic approach for which I argued earlier rightly belongs there, whereas the article here can speak to other, more general audiences. Perhaps a link to the Wikiversity article can be added to direct students? Then this article can be greatly simplified. Pgpotvin ( talk) 01:56, 11 January 2009 (UTC)
In any case I don't really think it is useful to have two sets of articles named Wikipedia and Wikiversity. There is enough work to do without having two articles on each subject. Sans compter les articles en d'autres langues. Dirac66 ( talk) 04:47, 11 January 2009 (UTC)
Sorry, I only assumed without evidence that Bduke had written both articles. The other article is indeed a Wikibooks entry, the link to which I got from Wikiversity. Here it is: Resonance (Wikibooks). I agree that two sets of articles is much to create and maintain, but I can see value in both. Pgpotvin ( talk) 22:06, 11 January 2009 (UTC)
Because there have been no further comments, I've gone ahead and re-written this article. See my sandbox draft here. Depending on your comments, this will replace the existing article in a few days. The changes include removal of material that is not germane, correction of some falsehoods, and extensive examples of various resonance situations. The tone is different, as well, with not a single mention of Valence Bond theory. Pgpotvin ( talk) 21:06, 18 January 2009 (UTC)
I also think that both VB and MO theories should be mentioned in order to properly show the conceptual development of the subject. First note that some early chemists did occasionally represent a molecule by multiple structures without any reference to quantum theory, including Thiele in 1899 (!) and Arndt more systematically starting from 1924. (For Arndt see references in the Kerber article.)
Pauling of course was the first to relate this practice to quantum mechanics by writing the molecular wave function as a combination of VB functions. The inclusion of VB is justified in this article because of its extensive use in the semi-quantitative theories of the chemical bond by Pauling and Wheland (about 1930-1955), since the concepts of these theories are still used in qualitative bonding theory. [As for the date of “Nature of the Chemical Bond”, Kerber notes that Pauling first used this title for a series of papers starting in 1931, and then for the book, first edition 1939]
As for MO theory, we can note that although it is less clearly related to resonance, it is better suited to systematic quantitative calculations of “delocalization energy” which may be considered another form of “resonance energy”. From the early calculations of Huckel and Coulson to modern ab initio quantum chemistry methods, MO theory has evolved to provide the most reliable way to evaluate “resonance energy”, which justifies its mention in the article as well. In sum: explain that VB is for bonding concepts, MO for quantitative calculations. Dirac66 ( talk) 03:12, 20 January 2009 (UTC).
I am not disagreeing in general. “Resonance energy” and “delocalization energy” are about the same thing. The first is from VB and the second from MO. It is no longer a turf war as it was in the 1940s. However to say that resonance is using a combination of Lewis structures without mentioning VB is odd, since here the Lewis structures are VB structures. I would also mention that "ab initio quantum chemistry methods" now include both MO and VB calculations. The latter are no longer just the empirical calculations of Pauling. So "explain that VB is for bonding concepts, MO for quantitative calculations" is just wrong (both VB and MO are for both), but these issues do not need to be mentioned here but in the articles on MO and VB. -- Bduke (Discussion) 03:28, 20 January 2009 (UTC)
Pauling makes the statement about conditions for resonance that "the two structures must involve the same numbers of unpaired electrons" (Nature of Chemical bond, 1940).Is this (still?) true - and if it is, shouldn't the statement be made in the section "writing resonance structures". -- Axiosaurus ( talk) 10:04, 16 February 2009 (UTC)
Good point, and still true since it is related to the quantum mechanical statement that total spin is constant. I have now added this point (slightly reworded) to the article. Dirac66 ( talk) 14:25, 16 February 2009 (UTC)
Yesterday Wickey-nl inserted Template:Essay-like which says that "This article is written like a personal reflection or essay and may require cleanup." The template links to WP:Not#Essay, which I have just read, but it is not at all clear that it refers to this article. In fact the content appears to me quite similar to what is found in chemistry textbooks at various levels (some paragraphs are more advanced than others). Please explain what is personal about this article.
It is true that very few sources are given. Perhaps a more appropriate notice would be Template:Refimprove which asks for more sources. Dirac66 ( talk) 15:39, 4 April 2010 (UTC)
In the section Writing resonance structures, rule 4 does not really belong because it is not helpful at an elementary level. Rules 1,2,3 and 5 can each be illustrated in a first-year lecture with simple examples of acceptable and unacceptable structures, and perhaps such examples should be included in the article.
However rule 4 says "Resonance hybrids can not be made to have lower energy than the actual molecules." This is a special case of the variational principle of quantum mechanics and therefore a true statement. But it is not a useful rule because one cannot quickly determine the energy by inspection, unlike for example the number of unpaired electrons. I propose that this rule be deleted from the article. Dirac66 ( talk) 14:14, 4 May 2010 (UTC)
Added yesterday: "If the bond lengths are measured, for example with NMR spectroscopy, no single and multiple bonds can be distinct. All bonds appear to have the same bond length, somewhere between single bond and multiple bond length."
Two comments: 1. Bond lengths are not usually measured with NMR, but with x-rays in solid state, or by microwave, ir or uv-vis spectroscopy in gas. 2. Can we specify exactly which bonds have the same bond length? Even in benzene, C-C and C-H are not the same length. Do we mean all bonds involved in the resonance? No, because today the thiocyanate ion was added, and the S-C and C-N bonds are not the same length. Do we mean all bonds between similar atoms? No, in ethylbenzene the side-chain C-C is a single bond and longer than the ring bonds. And in naphthalene there are ring bonds which are longer than other ring bonds.
I think what we really mean here as a general statement is that bonds with different bond orders in different contributing structures usually have intermediate bond lengths. Then we can give some examples of equal bond lengths (benzene, carbonate) without claiming that all bond lengths are equal. Dirac66 ( talk) 20:20, 8 May 2010 (UTC)
Can we delete the chapter True nature of resonance? If not, what part is worth to keep?-- Wickey-nl ( talk) 11:19, 10 May 2010 (UTC)
"In fact, resonance energy, and consequently stability, increase with the number of canonical structures possible, especially when these (non-existent) structures are equal in energy."
This can only be true if all canonical structures have comparable energy and have low energy. A canonical structure with higher energy would, by definition, not change the resonance energy and make the compound even less stable.
Furthermore, I think you cannot say that contributing structures are non-existent structures. Although they do not represent the actual compound because they "ignore" the other contributing structures, no one can say this structure never exists at certain moments.-- Wickey-nl ( talk) 14:06, 13 May 2010 (UTC)
This article now contains the word "compound" 14 times, and I suggest they all be replaced by "molecule". First, resonance can be a property of elements too; ozone, graphite and fullerenes come to mind. More fundamentally, resonance structures are a property of the microscopic molecular unit and not of a molecular compound (or element). It is not correct to talk about (Lewis) structures of a "compound".
Some may argue that "molecule" excludes ions, but we can add "or ion" a few times, or else specify "molecule (neutral or charged)". In any case, "compound" also does not include ions. Dirac66 ( talk) 23:21, 1 June 2010 (UTC)
I still think that "molecule" is the best word in most cases (in this article), because electrons are usually delocalized over one molecule and not over a macroscopic sample of a compound or element. (Graphite is an exception and has no molecules). I will however add "or polyatomic ion" and "or ion" a few times to make it clear that ions are included. The idea that "molecules exclude ions" comes from the simple chemistry of small molecules. Organic chemists and biochemists routinely refer to ions as molecules - try telling a biochemist that proteins and DNA are not molecules just because they are charged!
We can however make the point explicitly that the molecules (and ions) can be of both compounds and elements. Since most of the molecules are compounds, we might include an explicit list of elements: I mentioned ozone, graphite and fullerenes which are all neutral molecules, and an example of an ion is the azide ion N3-. As for "structure" we can use it in a few places where the distinction is clear between the hybrid structure and the contributing structures, but I think it would be confusing to use it everywhere.
I will try to make these changes now. Dirac66 ( talk) 01:16, 8 June 2010 (UTC)
Um... DNA isn't a molecule. Perhaps a pair of molecules held together by hydrogen bonds, but not a single molecule. Proteins can be single molecules though. Biologists are terribly sloppy with nomenclature, and can often fail to tell a compound and an ion apart. The IUPAC definition of a molecule is an electrically neutral entity containing more than one atom. Thus zwitterions can be molecules, while atoms like argon or helium are not. This squares exactly with my understanding as a chemist. -- Rifleman 82 ( talk) 15:34, 9 June 2010 (UTC)
Is this an axiom?-- Wickey-nl ( talk) 16:19, 22 July 2010 (UTC)
This helps, but I like to make some remarks:
Unfortunately, I have to do with online references.-- Wickey-nl ( talk) 16:32, 29 July 2010 (UTC)
Comment, point by point:
Re 2. "Relative minimum" is an accepted synonym for "local minimum"; see Maxima and minima. However the statement is incorrect here since as I have already stated, the contributing structures do not in fact correspond to local minima. Dirac66 ( talk) 03:10, 30 July 2010 (UTC)
I have reverted the last edit. First, the hyperconjugation heading is for a section that is not about hyperconjugation. It is about 3-centre 2-electron bonding with sigma orbitals. As it says, hyperconjugation is about pi electrons. Second, I suggest the gallery is inappropriate. These diagrams could just as well be describing delocalisation with MOs. They are certainly not resonance structures. -- Bduke (Discussion) 02:18, 5 August 2010 (UTC)
I am not sure if this (may be boring) discussion has lead to some agreement. Otherwise it has been a waste of time. My logic is: the essence of resonance is the existence of a pi-system with delocalized electrons. The pictures with dotted bonds represent delocalized electrons, thus they represent resonance structures ("identical to signs" in the image; was not just a simple correction from me). This differs fundamentally from the view of DMacks that delocalized electrons are only the result of resonance. We can also reverse the question: Are there compounds with delocalized electrons, but without resonance?-- Wickey-nl ( talk) 10:15, 19 August 2010 (UTC)
I just see "resonance hybrid" is actually a synonym of "contributing structure" → http://goldbook.iupac.org/RT07094.html. That means the intro should be adapted.-- Wickey-nl ( talk) 16:17, 17 August 2010 (UTC)
This subject is already discussed above →
What is the definition of "resonance hybrid"
I will cite now what Linus Pauling said about the resonance hybrid:
"In this case the best wave funtion ψ would be formed in part from ψI and in part from ψII and the normal state of the system would be described correspondendingly as involving both structure I and structure II. It has become conventional to speak of such a system as resonating between structures I and II, or as being a resonance hybrid of structures I and II."
[2] Linus Pauling, The Nature of the chemical bond - An Introduction to Modern Structural Chemistry . Third Edition 1960, p.12
Pauling is speaking of the normal state, the state with the lowest possible value of energy (page 11). Structures I and II are contributing structures. In the next paragraph (page 12) he says (in my words):
The resonance hybrid is not exactly intermediate in character between structures I and II, because the resonance stabilized hybrid is lower in energy than either of the contributing structures. As we can suppose the real structure will have the state of lowest possible energy (in the normal state), we can say the real structure is the resonance hybrid.--
Wickey-nl (
talk) 08:52, 19 August 2010 (UTC)
The sentence in the second paragraph, "Each contributing structure can be represented by a Lewis structure, with normal single, double or triple covalent bonds between every pair of adjacent atoms within the structure." is wrong. I see it is supported by the reference to the Gold Book, but it is contradicted by a classic example of resonance for the bridge region of diborane, where the resonance structures have alternating "single" and "no bond" bonds giving each bond in the resonance hybrid to be a half bond. Can anyone suggest a better wording? Note also that diborane also contradicts the statements that imply that resonance is only about pi bonds. -- Bduke (Discussion) 22:24, 5 September 2010 (UTC)
The following link ( Resonance Theory) clarifies the topic and enhances the material covered. Material covered in this website is available to anyone and was written by a tenured PhD Organic Chemistry Professor at Utah Valley University. The website is a non-profit website and is intended to advance students understanding of Organic Chemistry. Thanks for your time, Nickcc20 ( talk) 14:45, 19 January 2011 (UTC)
Pauling's principle of electroneutrality is still taught as being the method by which favourable and unfavourable resonanace structures are "selected". A simple statement is that the charge on an atom should be between +/- 1 (formal charge)with the corollary that the negative charges should reside on the most electronegative atom and positive on more electropositive. Is this worth a mention? Axiosaurus ( talk) 13:04, 17 March 2013 (UTC)
I have reread this article and see that it is very molecular in its scope- why? Ionic structures with "covalent character", metals, intermetallics and other unusual solid state substances were all tackled by Pauling, was he wrong? Axiosaurus ( talk) 16:52, 17 March 2013 (UTC)
The statement regarding ionic contributions reads as if it applies to both to homonuclear and heteronuclear bonds. I am not familiar with the referenced book- however other books by Shaik discuss in detail the Heitler-London treatment of H2, is this where this quote comes from? . Historically ionic contributions in A-B bonds were the basis of the electronegativity concept. Axiosaurus ( talk) 06:24, 12 April 2015 (UTC)
Charge shift bonding is not the the same as ionic-covalent resonance. The claim is that some molecules are stable only because of ionic-covalent resonance. In F2 for example the calculations using just covalent terms do not show bonding. Ionic terms on their own are not that good. The claim is that it is resonance between them that is responsible for bonding. I want to stress that charge shift bonding is controversial. For example a generalised valence bond (GVB) function function gives a reasonable description of bonding for F2. Some workers argue that this GVB is just a description of covalent bonding. There is no ionic-covalent resonance. Others argue that it disguises the ionic terms and thus the ionic-covalent resonance. There are wide differences of opinion between researchers on valence bond theory. Take care. -- Bduke (Discussion) 11:45, 26 April 2015 (UTC)
I am wondering about the exact meaning and validity of the energy diagram added today for benzene in the section Resonance in quantum mechanics. The file description (obtained by clicking on the image) says it is based on the MO diagram for H2, which is of course well known. However that diagram and all the other diatomic MO diagrams are for individual orbitals (one-electron wave functions). This diagram for benzene appears to show the combination of two many-electron (at least six pi-electron) wave functions, one for each Kekulé structure. Is there a source for combining 2 Kekulé structures in an energy-level diagram, as opposed to the elementary diagram which joins the 2 structures with a simple ↔ ? The lower energy level is acceptable, as it is true that the wave function may be written as a (normalized) sum of two functions representing Kekulé structures. But the upper level is quite mysterious - is it antibonding at all 6 C-C bonds? Or bonding and antibonding at alternate positions, so that there are 3 double pi bonds and 3 antibonds? or null bonds? In the absence of a source, a proper answer to this question would require detailed mathematical analysis of the proposed wave function, which of course would be original research. In any case, I think the upper state would be at very high energy and inaccessible by one- or even two-electron transitions, so it is of no experimental interest.
In summary, I have never seen such a diagram for benzene and I think it requires a better explanation with a source. If this is not available, then I recommend the diagram be deleted. Dirac66 ( talk) 15:45, 3 May 2015 (UTC)
The last section on Charge delocalization contains mysterious acronyms and other terms. In encyclopedia articles, terms likely to be unfamiliar to many readers should either be explained at first usage or else linked to another article which does provide an explanation. So would someone please provide the answers to the following questions?
I just corrected a bunch of statements in the lead which imply that resonance structures have different distributions of electrons. That is extremely poorly worded if not flatout wrong. Only our *depiction* of where the electrons reside (in Lewis structures) changes, not the location (density) of the electrons themselves. Alsosaid1987 ( talk) 00:55, 11 May 2018 (UTC)
I have very carefully rewritten the second and third paragraphs of the lead to fix these issues. Also, there are some delicate issues with respect to logic and terminology. On the one hand, we rationalize the structure of a resonance hybrid based on the expected geometries of the individual Lewis structures and taking the "average". On the other hand, we later need to assert that contributing forms of a resonance hybrid do not differ in the geometry or overall electron density but are simply different representations of the real molecule.
Basically, we need to distinguish between standalone Lewis structures and Lewis structures that are part of resonance hybrids (i.e., contributing forms). I welcome changes that clarify this point. Alsosaid1987 ( talk) 06:26, 11 May 2018 (UTC)
I think using NO2 as the example is problematic for several reasons. Like nitric oxide or triplet O2, nitrogen dioxide has a 2c3e bond due to its unpaired electron. However, Lewis structures are unable to correctly show the extra half bond, and averaging the structures gives an incorrectly low estimate of the bond order. Also, there are at least two other important Lewis structures that one can draw. Though charge separation is less favorable, they cannot be neglected when determining the bond order or structure. The bond angle is 134 degrees, which reflects the formation of the extra half-pi-bond, and the hybridization is somewhere between sp2 and sp, while the bond lengths are also shorter than expected. There is no simple way of estimating the bond order in this case (which is somewhere between 1.5 and 2). For these reasons, NO2 is really a pathological example that shows the limits of the Lewis representation. On the other hand, NO2-, nitrite is much more straightforward, and I tentatively chose this example to illustrate the concept. Unfortunately, it's hard to find a neutral example of resonance that is simple enough to give. Alsosaid1987 ( talk) 03:17, 12 May 2018 (UTC)
@ Dirac66:@ DMacks:@ Alsosaid1987: It has come to my attention that this article seems to be struggling a lot to explain that resonance structures do not exist but the resonance hybrid does (the very long introduction and repeated assertions in the article is testament to this). It has also occurred to me that nowhere in this article, except the valence bond (ie quantum mechanics) part explains what resonance truly is. Could I suggest an overhaul of this article in that we introduce in the outset that resonance is basically the equivalent of linear combination of atomic orbitals in Valence Bond theory, where the actual molecular wavefunction is a weighted sum of individual resonance structures just like molecular orbitals are a weighted sum of individual atomic orbitals (the analogy goes quite far in fact. We can even take antisymmetric, ie. antibonding, combinations of resonance structures to attain excited states etc).
I'm putting the above as an idea, not sure how to better write this article to be both concise and succinct. From reading this talk page it appears the writing of this article being problematic goes back some time. Opinions?-- Officer781 ( talk) 14:19, 19 January 2019 (UTC)
Just posting an idea here. This article currently talks about major and minor contributors but does not go into the types of resonance structures that can be drawn. As far as I know there are three types (doesn't just refer to aromatic molecules but to any molecule where contributing structures can be drawn):
The Kekule-type structures are the ones currently covered in the article and in elementary discussions of resonance. I am busy currently and if anybody else would like to have a go at giving a brief mention of these in the major and minor contributors section can go ahead.-- Officer781 ( talk) 15:25, 23 January 2019 (UTC)
What is difference between resonance and hyperconjugation? Umer ilyas shaaheen ( talk) 15:55, 26 January 2020 (UTC)
At the end of the article, it says that resonance diagrams are soon to come. That was 6 months ago. Is anybody going to be adding resonance diagrams soon here? H Padleckas 16:17, 25 Jan 2005 (UTC)
They don't really switch between the two possible states constantly, do they? Then the electrons would be net going in a circle and would create a magnetic field. Rather, they exist in a less-than particle state or a "cloud" or something, right? - Omegatron 16:47, Jan 25, 2005 (UTC)
Has anyone actually heard of the term "resonant bonds"? I've look in texts before, and while I've definately heard of resonance structures, "resonant bonds" just doesn't seem standard.
There's a merge tag over at benzene. That section on resonance would look mighty good here. Isopropyl 21:08, 11 April 2006 (UTC)
I agree that some of that material should be added here, but it should also be retained in the benzene article. Resonance is a crucial part of the benzene story and readers should not have to come over here to read about it. -- Bduke 21:27, 11 April 2006 (UTC)
Maintain separate articles, sharing whichever sections would improve the other. They are sufficiently separate entities. - Ayeroxor 21:41, 1 May 2006 (UTC)
The article states that sigma bonds are meaningless in MO theory for benzene. This is strictly accurate but goes against useage. Pi orbitals in benzene are also meaningless strictly. The terms sigms and pi strictly apply only to linear molecules giving the spectroscopic terms for angular momentum about the molecular axis. We can talk therefore about sigma and pi molecular orbitals even in long linear molecules where the MOs are delocalised. For planar congugated systems such as benzene the useage has come to mean how the MOs change on reflection in the molecular plane. If it changes sign, it is pi. If it does not change sign it is sigma. The term "sigma-pi separation" is very common in both MO and VB discussions of benzene, so the current wording could serious confuse people. I'm not clear how to rewrite the section. The section is:-
"Often when describing benzene the VB picture and the MO picture are intermixed, talking both about sigma 'bonds' (a meaningless concept in MO) and 'delocalized' pi electrons (a meaningless concept in VB). This is not a good practice, because mathematically the models are incompatible."
Any body got any ideas? -- Bduke 22:24, 18 December 2006 (UTC)
MO has sigma orbitals, but no sigma bonds. A bond implies that an electron pair belongs to two and only two nucleus, whereas in MO the electron state vectors are calculated with the )approximate) potential of the entire molecule and not any two particular atoms. That is why in MO it does not make sense to say that every carbon has 1 sigma bond with each of its neighbours. At best we could say that these electrons are in highly localised orbitals. As is indicated in the article, sigma-pi in VB is somewhat different from sigma-pi in MO. Loom91 07:07, 19 December 2006 (UTC)
The problem is that sigma orbitals and sigma bonds do not appear that different and indeed they are not as the delocalised MOs can be transformed into localised orbitals. This means that sigma bonds are not a meaningless concept in MO although I agree that sigma in MO is not exactly the same as sigma in VB. I note also that most quantitative VB treatments of benzene such as the famous spin coupled calculations of Gerratt, Cooper and Raimondi use VB only for the pi electrons and treat the sigma as MOs. It is true that MOs are calculated with the potential of the entire molecule, but so are the VB wave functions. It is the orbitals that are delocalised or localised. The potential is always the whole molecule included the repulsion of all other electrons. We are not too far apart but I think the current wording will lead to misunderstandings and there is already too much misunderstanding in the MO/VB debate. -- Bduke 07:46, 19 December 2006 (UTC)
The point made in the article is that VB and MO are two different mathematical models with different approaches to solving a problem. The sigma-orbital approach in MO and the sigma-bond approach in VB give the same predicted expectation of observables (otherwise one theory would be more 'wrong' than the other), but that doesn't mean that the theories are the same and you can go around taking a slice from VB and a scoop from MO. Also, MO is a more 'natural' theory than VB. The postulates or methodology of MO are closer to the actual quantum mechanical picture, which exists exactly only for the simplest molecules like H2+. Loom91 09:51, 20 December 2006 (UTC)
There are two major problems with what you say. (1) MO and VB do NOT give the same predicted expectation values of observables and indeed one is more wrong than the other. I never said they are the same. They are different. (2) MO is not more natural than VB. Just because the exact solution of H2+ (a one electron problem) is a MO does not mean that MO treats electron repulsion in cases with more than one electron correctly. Both are approximations. MO is much easier to do and is very popular. Computationally a lot of methods build on it or add to it. However in fact simple VB energies almost always lie below simple MO energies (and hence are better) and MO does not dissociate homonulcear diatomic molecules correctly. VB is less wrong. I would also add that it may seems odd to take "a slice from VB and a scoop from MO" but that is exactly what a lot of people do, including as I said above, some of the best VB calculations on benzene. Have you studied quantum chemistry to any depth? Of course this article is not the place for advanced quantum chemistry, but we must try to avoid too many "lies for children" as we simplify the quantum chemistry. -- Bduke 10:16, 20 December 2006 (UTC)
I think the confusion is resulting from talking about VB and MO as theories in the ordinary sense. I haven't studied VB in detail, but as far as I know they are two different approaches to approximating the solution, not methods of approximation as such. As you say, computational methods build on them. There are different levels of approximation in VB and different levels of approximation in MO (you could start with Huckel and proceed to DFT). If a certain level of approximation in VB gives a result, MO will also give similar or better results at some level of approximation. You say simple VB energies lie below simple MO energies, but we can't decide superiority between the two simply by looking at the 'simple' limit.
The framework provided by MO (or VB) can not be identified with any particular method or level of approximation in either. The MO road is closer to what a physicist will do if asked to solve a system, but that does not mean the VB road will not give the same results at sufficient approximation. The converse is also true. You claim "MO does not dissociate homonuclear diatomic molecules correctly"-I've never read this, but VB fails to predict that hybridisation in CH4 does not produce equivalent orbitals. VB is not more correct than MO.
As for mixing MO and VB, you are talking about mathematical calculations where we have quantitative ways of verifying whether we are mixing in a valid manner. The article however talks about mixing the two in qualitative descriptions ("lies for children"), where there is no way to know (without having actually done the calculations) whether the pictorial descriptions reflect the actual mathematics. It is such situations that trying to mix the two models is dangerous. Also, I don't think that this article is not a place for advanced quantum chemistry. Wikipedia is a specialised as well as general purpose encyclopedia, so feel free to add a section on the detailed mathematics behind the two-headed arrows. It will be best if you cite references. Loom91 07:48, 21 December 2006 (UTC)
You state that you have not studied VB in detail. Sorry, but I'm afraid that is pretty clear. Let us take the dissociation of MO theory for homonuclear diatomics first. The simple MO wavefunction is an equal mixture at all values of the bond length of the simple VB covalent and ionic terms, so it dissociates into an equal mixture of 2 H atoms and (H+ + H-). The energy is the mean at infinite bond length of these two and thus very much higher in energy than 2 H atoms which H2 experimentally dissociates into. VB dissociates correctly. The predicted MO dissociation energy is too large. The predicted VB dissocaiation energy is closer to experiment but two small because the two atoms at large distance are exact and the VB energy lies above the exact energy at the predicted bond length. The MO energy lies even higher. These facts matter because both methods are variational - they lie above the exact result. The simple Heitler-London VB lies below the simple MO energy curve and is thus better. The similar case of F2 is so bad that the MO energy lies higher than the correct dissociation of 2 F atoms. This is in many texts. By MO I mean for H2 a single molecular orbital built as a linear combination of atomic orbitals (with simple MO using just the two 1s orbitals). Of course one can add configuration interaction and do as well as VB. Adding all possible excited configurations to MO and all possible VB structures to the simple VB gives identical results but such calculations, called full configuration interaction, are only possible for fairly small molecules with fairly small basis sets. "VB fails to predict that hybridisation in CH4 does not produce equivalent orbitals" - please explain more carefully or give a source. Hybridisation is not a physical think. It is an artifact of VB theory and the hybrids can and are closen to be equivalent. DFT is not MO although I grant you it looks like it. VB can easily do better than the best MO wavefunction - called the Hartree-Fock limit. It does exactly because it get bond breaking better.
The bottom line is that the simple pictures grew out of calculations. Pauling would have done nothing without the Heitler-London calculation on H2 and some extensions of it. We forget this at our peril. Qualitative ideas did not grow out of the air. "Also, I don't think that this article is not a place for advanced quantum chemistry". I think you meant "Also, I do think that this article is not a place for advanced quantum chemistry". I agree. It is about the simple pictures but we must not mislead. I'll think about it more after the holiday period which is going to be very busy for me. -- Bduke 08:42, 21 December 2006 (UTC)
What you are saying is not that VB is a more accurate theory than MO, but that VB is computationally less intensive. As for hybridisation, the 4 CH bonds are not exactly equivalent as calculated in VB. As predicted by MO, one of the bonding pairs have a different energy from the other three. The photoelectron spectra shows two characteristic bands [1]. Also, why are you excluding post-Hartree-Fock methods from the umbrella of MO? And I meant exactly what I said: "Also, I don't think that this article is not a place for advanced quantum chemistry." That is, this article IS a place for advanced quantum chemistry. An accurate section on what exactly is mathematically meant by resonance will add greatly to the article. It may even become a GA. So feel free to add such a section. Loom91 07:12, 22 December 2006 (UTC)
No, I am saying that VB is more accurate at equivalent levels than MO, but that MO is computationally less intensive. The orbitals in MO are orthogonal and that simplifies things. In VB they are not orthogonal and it has taken a long time to get code that competes with MO. Why am I excluding post-Hartree-Fock (HF) methods? Because while based on a MO reference function they are not MO. Configuration interaction at the full level is entirely equivalent to full VB, so no comparision is fruitfull. Bond breaking is still badly handled by post-HF that uses a single determinant reference. To handle bond breaking correctly the MO guys use multi-configuration SCF where you can no longer say that 2n electrons are in n MOs for a closed shell singlet. Also it can be shown that these methods are very similar and in some cases identical to some spin-coupled VB methods, so again comparision is not fruitfull. The only meaningfull comparision is between the methods Pauling and Mulliken and their respective supporters fought about in the 1930s, for example simple MO for H2 - (a + b)(1)(a + b)(2) and Heitler London for H2 - a(1)b(2) + b(1)a(2) where a and b are the 2 is orbitals on the 2 H atoms. In passing note that expanding out the MO function, you get a(1)b(2) + b(1)a(2) + a(1)a(2) + b(1)b(2). The first 2 terms are the Heitler London terms and the last 2 are ionic terms - H- H+ and H+ H- so as I said earlier MO is a mixture of the VB covalent term that dissociates in 2 H atoms and the VB ionic terms that dissociate into two ions at a higher energy.
I have no idea where you have got the idea that the 4 CH bonds are not equivalent in VB. They are. The photoelectron spectrum with 2 peaks is best explained by the fact that there are only 2 energy-distinct MOs - 1 triply degenerate group and a single degenerate one for the 4 pairs of valence electrons. Ionisation is certainly best explained by MO theory because the electron does not leave one bond but the whole molecule. A VB description of CH4+ would have to include resonance between the 4 structures each with 3 two electron bonds and 1 one electron bond. In this way the ion would come out symmetric and there are indeed 2 solutions just as in MO theory. Yes, MO theory is simpler to describe ionisation and spectroscopy. VB can be simpler to describe bonding and generally gives better numbers. Getting numbers to agree with your PE spectra from MO theory is not easy, but the simple picture is. The orbital energies, for example, will only predict the position of your peaks well, using Koopman's approximation, if the massive correlation energy corrections and relaxation energy corrections are of opposite sign and similar magnitude which they often are for organic molecules but rarely are for metal complexes. -- Bduke 08:12, 22 December 2006 (UTC)
What do you mean by equivalent levels? How would you say a particular VB method is equivalent to a particular MO method? Loom91 18:07, 23 December 2006 (UTC)
I assume you are asking about my statement that, for example, full CI is equivalent to full VB. OK, let us take the classic example of H2. The simple MO is (a + b)(1)(a + b)(2) which expands to:-
The excited state with both electrons in the antibonding orbital is:-
Now mix these and collect terms (K is the mixing weight):-
The above is the full CI result for this small basis set of 2 1s orbitals. The full VB is ionic - covalent resonance, which is (C is the mixing coefficient):-
Neither of these are normalised. In both cases the mixing coefficent is determined by finding the value that minimises the energy. Since both allow any proportion of the covalent - {a(1)b(2) + b(1)a(2)} - and ionic terms - {a(1)a(2) + b(1)b(2)}, the final results will be the same. This result is general. If we take a simple MO and mix in all possible excitatations that mix with the ground state, and then take all possible VB structures from the same set of atomic orbitals, the results are equivalent. The general result is perhaps surprising - approximations that look very different and start from different ideas, can actually be completely identical.
To our other readers, I apologise. This is getting over complicated and technical. Loom91, if you want to continue this, please move it to e-mail. I have e-mail set from my user page. I am happy to continue helping you to learn about VB theory, but I think the discussion is getting beyond relevence to this article. -- Bduke 21:42, 23 December 2006 (UTC)
You misunderstand me. I know VB == MO in the high accuracy limit. I was asking in lower accuracy levels how you say that VB is more accurate than 'equivalent' MO. As for the article, what changes do you propose? Loom91 07:02, 27 December 2006 (UTC)
Let us take H2. The simplest MO approach just using the two hydrogen atom like 1s orbitals is as above. The simplest VB using the same orbitals is the Heitler-London. These are at an equivalent level, yet give different results. The latter lies lower in energy than the former at all interatomic distances and particularly at large distance and so is better. We can then optimise the orbital exponent of the 2s orbital in both cases. These are at equivalent levels. Again VB is better. That is what I mean by equivalent - same basis set and simplest possible MO or VB approach or comparable improvements to simplest approach.
I have made the changes to the article that I think should be made. My reasons are many. First, it is quite common to mix MO and VB ideas. Coulson in both "Valence" and in McWeeny's "Coulson's Valence" says this about the sigma bonds in benzene, "These bonds can be described either in MO or VB language; their essential character is the same in either case". I know of no book that criticises this statement. He goes on to give the VB and MO approaches. This mixing of language is commonly done in simple qualitative explanations and a mixing of methods is commonly done in quantitative calculations as I mention above. I do not think sigma is "meaningless in MO" or delocalised is "meaningless in VB". I do agree it is best to use "delocalisation energy" in MO descriptions, but note that somewhere on WP is a reference to a Journal of Chemical Education article that recommends delocalisation rather than resonance for VB descriptions. I also suggest it is stretching it to say about the two methods that "mathematically the models are incompatible". Different, yes, but not incompatible. To say, for example that MO for H2 is entirely identical to VB resonance between the covalent and ionic structures, but with equal weights, demonstrates this lack of incompatability. The article is best made simpler at this point. The wording was confusing and not clarifying matters, so is best removed. -- Bduke 02:10, 28 December 2006 (UTC)
Reasonance diagrams for heteroaromtic compounds would be nice -- Quantockgoblin 23:47, 20 March 2007 (UTC)
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Just out of interest, why is resonance referred to as "a tool used (predominantly in organic chemistry) to represent certain types of molecular structures."? , it's a bit of a LARGE generalisation; aside from being referred to as canonical forms, Miessler refers to it as when there is "more than one possible way in which valence electrons can be placed in a lewis[-based] structure.", Chambers refers to it as "when [a] true structure of .. [a molecule or compound] cannot be accurately represented by a single structure, ... several resonance structures are suggested." ♥♥ ΜÏΠЄSΓRΘΠ€ ♥♥ slurp me! 10:40, 6 April 2007 (UTC)
To clarify this point "Resonance hybrids are always more stable than any of the canonical structures", the wave function Ψ is given by:
where Ψ is the resonance hybrid function and C1, C2, .. are the canonical structure functions. a1, a2 , a3, .. are coefficients chosen to minimise the energy. It follows from the variation theorem that the energy of Ψ is less than or equal to the energy of all of C1, C2, .. taken separately. It would be equal if one of the a1, a2, .. was 1 and all the others zero, and lower otherwise. Loom91 is correct and he gives a good simple reference. In antiaromaticity, the geometry changes to a more stable form. A good discussion is chapter 4 of "Facts and Theories of Aromaticity" by David Lewis and David Peters, Macmillan, 1975. -- Bduke 13:37, 12 April 2007 (UTC)
Quotation:
When separating charge (giving rise to ions), usually structures where negative charges are on less electronegative elements have little contribution, but this may not be true if additional bonds are gained.
I believe this statement should be rewritten. Not being a native english speaker, my opinion may be misguided; anyway, to me it looks convoluted and is nearly uncomprehensible.
A statement like this, being a list item, should speak for itself. It remains unclear however, to what phelomenon contribution should contribute.
Bertus van Heusden 10:53, 4 September 2007 (UTC)
I see someone changed the units under "resonance energy" to kcal/mol without changing the numeric values. These should definitely stay as SI units, but someone should now check the correct values. Unfortunately I don't have time right now. -- Slashme 06:53, 10 October 2007 (UTC)
You have a point, it's probably a good idea to keep the kcals, but a quick browse through the literature shows that kJ is gaining ground. I can't yet find a good ref. for the values quoted, because my chem. books are at the lab. If I get around to it, I'll sort it out, but I might not... -- Slashme 13:21, 10 October 2007 (UTC)
The original estimates were in kcal, probably in the era of Pauling, but kJ are now preferred. So yes, we need both units, so I have now inserted kJ values. I just multiplied the kcal values by 4.184 and rounded off to two figures. Dirac66 13:33, 10 October 2007 (UTC)
To me, as a physicist, there are a couple of problems with this article. One is that the article never really explains the reason for the term "resonance," and there is no obvious (to me) physical phenomenon going on that is in any way (that's obvious to me) analogous to resonance. The section near the end about Pauling's introduction of the term doesn't really explain anything very clearly: why the quantum-mechanical treatment of H2+ was relevant to Pauling, or why he used the word "resonance." The other problem IMO is that the article never gives any very transparent physical explanation of what's going on. Although I understand the general argument made above on the talk page that a superposition of trial wavefunctions can be optimized variationally to lower its energy, that argument is so generic that it really has nothing in particular to do with chemical bonds, or even chemistry. If I had to take a stab at it, I would guess that the general physical mechanism is that, compared to a structure made of single and double bonds, the actual structure delocalizes the electrons, which means that they have a larger wavelength, thus a lower momentum and kinetic energy. In the case of an ion like CH3CO2-, I can also imagine that the delocalization would lead to a lower Coulomb energy.-- 207.233.87.196 ( talk) 23:56, 11 December 2007 (UTC)
Rectifico ( talk) 19:27, 18 June 2010 (UTC)
Hmm...okay, I think I understand the reason for the term now. See http://www.nap.edu/readingroom/books/biomems/lpauling.html . The paragraph beginning "Resonance: In attempting to explain ..." seems to be saying clearly what the WP article is saying unclearly. I'd take a whack at it myself, but I'm not a chemist, so I don't want to get this wrong.-- 207.233.87.196 ( talk) 00:08, 12 December 2007 (UTC)
I know I'm coming in late into this discussion, but ...
There are several small problems:
The biggest problem I find with this article is that it is not well targeted to the intended audience.
After all, who will look up 'Resonance' on Wikipedia? The average Joe wouldn't ever. Practitioners don't need to. Only students would -- students who have encountered resonance in their studies and in their textbook, and who need help in understanding it. Their first encounter would be with main group examples (e.g. sulfate ion), but this article barely acknowledges this reality. My evaluation is that this article would confuse more than enlighten.
Let me be clear: I can live with this article, because I see no substantial problem with the content, but I would never refer any of my students to it. It's simply not written for them and assumes knowledge.
Specific problems:
Pgpotvin ( talk) 00:23, 11 January 2009 (UTC)
I largely agree and will look at this, if others do not, when I have more time. It will not be easy.
"In the mathematical discipline of graph theory, a Kekulé structure is a matching or edge-independent set in a graph." I added this to get the links in. In particular, Kekulé structure rather oddly redirects to matching. I could not see a better way of drawing attention to this. Can you think of a better way of getting the links right to all the related articles? -- Bduke (Discussion) 23:12, 10 January 2009 (UTC)
The term "Kekulé structure" is used elsewhere. The redirection can be changed, or the link can be to Kekulé structure.
Pgpotvin ( talk) 00:30, 11 January 2009 (UTC)
On the other hand, maybe I'm off. I followed Bduke's links to Wikiversity. I'd never looked at that before. There is there an article on Resonance with many of the same elements as here (and which I presume was also written by Bduke). Maybe the didactic approach for which I argued earlier rightly belongs there, whereas the article here can speak to other, more general audiences. Perhaps a link to the Wikiversity article can be added to direct students? Then this article can be greatly simplified. Pgpotvin ( talk) 01:56, 11 January 2009 (UTC)
In any case I don't really think it is useful to have two sets of articles named Wikipedia and Wikiversity. There is enough work to do without having two articles on each subject. Sans compter les articles en d'autres langues. Dirac66 ( talk) 04:47, 11 January 2009 (UTC)
Sorry, I only assumed without evidence that Bduke had written both articles. The other article is indeed a Wikibooks entry, the link to which I got from Wikiversity. Here it is: Resonance (Wikibooks). I agree that two sets of articles is much to create and maintain, but I can see value in both. Pgpotvin ( talk) 22:06, 11 January 2009 (UTC)
Because there have been no further comments, I've gone ahead and re-written this article. See my sandbox draft here. Depending on your comments, this will replace the existing article in a few days. The changes include removal of material that is not germane, correction of some falsehoods, and extensive examples of various resonance situations. The tone is different, as well, with not a single mention of Valence Bond theory. Pgpotvin ( talk) 21:06, 18 January 2009 (UTC)
I also think that both VB and MO theories should be mentioned in order to properly show the conceptual development of the subject. First note that some early chemists did occasionally represent a molecule by multiple structures without any reference to quantum theory, including Thiele in 1899 (!) and Arndt more systematically starting from 1924. (For Arndt see references in the Kerber article.)
Pauling of course was the first to relate this practice to quantum mechanics by writing the molecular wave function as a combination of VB functions. The inclusion of VB is justified in this article because of its extensive use in the semi-quantitative theories of the chemical bond by Pauling and Wheland (about 1930-1955), since the concepts of these theories are still used in qualitative bonding theory. [As for the date of “Nature of the Chemical Bond”, Kerber notes that Pauling first used this title for a series of papers starting in 1931, and then for the book, first edition 1939]
As for MO theory, we can note that although it is less clearly related to resonance, it is better suited to systematic quantitative calculations of “delocalization energy” which may be considered another form of “resonance energy”. From the early calculations of Huckel and Coulson to modern ab initio quantum chemistry methods, MO theory has evolved to provide the most reliable way to evaluate “resonance energy”, which justifies its mention in the article as well. In sum: explain that VB is for bonding concepts, MO for quantitative calculations. Dirac66 ( talk) 03:12, 20 January 2009 (UTC).
I am not disagreeing in general. “Resonance energy” and “delocalization energy” are about the same thing. The first is from VB and the second from MO. It is no longer a turf war as it was in the 1940s. However to say that resonance is using a combination of Lewis structures without mentioning VB is odd, since here the Lewis structures are VB structures. I would also mention that "ab initio quantum chemistry methods" now include both MO and VB calculations. The latter are no longer just the empirical calculations of Pauling. So "explain that VB is for bonding concepts, MO for quantitative calculations" is just wrong (both VB and MO are for both), but these issues do not need to be mentioned here but in the articles on MO and VB. -- Bduke (Discussion) 03:28, 20 January 2009 (UTC)
Pauling makes the statement about conditions for resonance that "the two structures must involve the same numbers of unpaired electrons" (Nature of Chemical bond, 1940).Is this (still?) true - and if it is, shouldn't the statement be made in the section "writing resonance structures". -- Axiosaurus ( talk) 10:04, 16 February 2009 (UTC)
Good point, and still true since it is related to the quantum mechanical statement that total spin is constant. I have now added this point (slightly reworded) to the article. Dirac66 ( talk) 14:25, 16 February 2009 (UTC)
Yesterday Wickey-nl inserted Template:Essay-like which says that "This article is written like a personal reflection or essay and may require cleanup." The template links to WP:Not#Essay, which I have just read, but it is not at all clear that it refers to this article. In fact the content appears to me quite similar to what is found in chemistry textbooks at various levels (some paragraphs are more advanced than others). Please explain what is personal about this article.
It is true that very few sources are given. Perhaps a more appropriate notice would be Template:Refimprove which asks for more sources. Dirac66 ( talk) 15:39, 4 April 2010 (UTC)
In the section Writing resonance structures, rule 4 does not really belong because it is not helpful at an elementary level. Rules 1,2,3 and 5 can each be illustrated in a first-year lecture with simple examples of acceptable and unacceptable structures, and perhaps such examples should be included in the article.
However rule 4 says "Resonance hybrids can not be made to have lower energy than the actual molecules." This is a special case of the variational principle of quantum mechanics and therefore a true statement. But it is not a useful rule because one cannot quickly determine the energy by inspection, unlike for example the number of unpaired electrons. I propose that this rule be deleted from the article. Dirac66 ( talk) 14:14, 4 May 2010 (UTC)
Added yesterday: "If the bond lengths are measured, for example with NMR spectroscopy, no single and multiple bonds can be distinct. All bonds appear to have the same bond length, somewhere between single bond and multiple bond length."
Two comments: 1. Bond lengths are not usually measured with NMR, but with x-rays in solid state, or by microwave, ir or uv-vis spectroscopy in gas. 2. Can we specify exactly which bonds have the same bond length? Even in benzene, C-C and C-H are not the same length. Do we mean all bonds involved in the resonance? No, because today the thiocyanate ion was added, and the S-C and C-N bonds are not the same length. Do we mean all bonds between similar atoms? No, in ethylbenzene the side-chain C-C is a single bond and longer than the ring bonds. And in naphthalene there are ring bonds which are longer than other ring bonds.
I think what we really mean here as a general statement is that bonds with different bond orders in different contributing structures usually have intermediate bond lengths. Then we can give some examples of equal bond lengths (benzene, carbonate) without claiming that all bond lengths are equal. Dirac66 ( talk) 20:20, 8 May 2010 (UTC)
Can we delete the chapter True nature of resonance? If not, what part is worth to keep?-- Wickey-nl ( talk) 11:19, 10 May 2010 (UTC)
"In fact, resonance energy, and consequently stability, increase with the number of canonical structures possible, especially when these (non-existent) structures are equal in energy."
This can only be true if all canonical structures have comparable energy and have low energy. A canonical structure with higher energy would, by definition, not change the resonance energy and make the compound even less stable.
Furthermore, I think you cannot say that contributing structures are non-existent structures. Although they do not represent the actual compound because they "ignore" the other contributing structures, no one can say this structure never exists at certain moments.-- Wickey-nl ( talk) 14:06, 13 May 2010 (UTC)
This article now contains the word "compound" 14 times, and I suggest they all be replaced by "molecule". First, resonance can be a property of elements too; ozone, graphite and fullerenes come to mind. More fundamentally, resonance structures are a property of the microscopic molecular unit and not of a molecular compound (or element). It is not correct to talk about (Lewis) structures of a "compound".
Some may argue that "molecule" excludes ions, but we can add "or ion" a few times, or else specify "molecule (neutral or charged)". In any case, "compound" also does not include ions. Dirac66 ( talk) 23:21, 1 June 2010 (UTC)
I still think that "molecule" is the best word in most cases (in this article), because electrons are usually delocalized over one molecule and not over a macroscopic sample of a compound or element. (Graphite is an exception and has no molecules). I will however add "or polyatomic ion" and "or ion" a few times to make it clear that ions are included. The idea that "molecules exclude ions" comes from the simple chemistry of small molecules. Organic chemists and biochemists routinely refer to ions as molecules - try telling a biochemist that proteins and DNA are not molecules just because they are charged!
We can however make the point explicitly that the molecules (and ions) can be of both compounds and elements. Since most of the molecules are compounds, we might include an explicit list of elements: I mentioned ozone, graphite and fullerenes which are all neutral molecules, and an example of an ion is the azide ion N3-. As for "structure" we can use it in a few places where the distinction is clear between the hybrid structure and the contributing structures, but I think it would be confusing to use it everywhere.
I will try to make these changes now. Dirac66 ( talk) 01:16, 8 June 2010 (UTC)
Um... DNA isn't a molecule. Perhaps a pair of molecules held together by hydrogen bonds, but not a single molecule. Proteins can be single molecules though. Biologists are terribly sloppy with nomenclature, and can often fail to tell a compound and an ion apart. The IUPAC definition of a molecule is an electrically neutral entity containing more than one atom. Thus zwitterions can be molecules, while atoms like argon or helium are not. This squares exactly with my understanding as a chemist. -- Rifleman 82 ( talk) 15:34, 9 June 2010 (UTC)
Is this an axiom?-- Wickey-nl ( talk) 16:19, 22 July 2010 (UTC)
This helps, but I like to make some remarks:
Unfortunately, I have to do with online references.-- Wickey-nl ( talk) 16:32, 29 July 2010 (UTC)
Comment, point by point:
Re 2. "Relative minimum" is an accepted synonym for "local minimum"; see Maxima and minima. However the statement is incorrect here since as I have already stated, the contributing structures do not in fact correspond to local minima. Dirac66 ( talk) 03:10, 30 July 2010 (UTC)
I have reverted the last edit. First, the hyperconjugation heading is for a section that is not about hyperconjugation. It is about 3-centre 2-electron bonding with sigma orbitals. As it says, hyperconjugation is about pi electrons. Second, I suggest the gallery is inappropriate. These diagrams could just as well be describing delocalisation with MOs. They are certainly not resonance structures. -- Bduke (Discussion) 02:18, 5 August 2010 (UTC)
I am not sure if this (may be boring) discussion has lead to some agreement. Otherwise it has been a waste of time. My logic is: the essence of resonance is the existence of a pi-system with delocalized electrons. The pictures with dotted bonds represent delocalized electrons, thus they represent resonance structures ("identical to signs" in the image; was not just a simple correction from me). This differs fundamentally from the view of DMacks that delocalized electrons are only the result of resonance. We can also reverse the question: Are there compounds with delocalized electrons, but without resonance?-- Wickey-nl ( talk) 10:15, 19 August 2010 (UTC)
I just see "resonance hybrid" is actually a synonym of "contributing structure" → http://goldbook.iupac.org/RT07094.html. That means the intro should be adapted.-- Wickey-nl ( talk) 16:17, 17 August 2010 (UTC)
This subject is already discussed above →
What is the definition of "resonance hybrid"
I will cite now what Linus Pauling said about the resonance hybrid:
"In this case the best wave funtion ψ would be formed in part from ψI and in part from ψII and the normal state of the system would be described correspondendingly as involving both structure I and structure II. It has become conventional to speak of such a system as resonating between structures I and II, or as being a resonance hybrid of structures I and II."
[2] Linus Pauling, The Nature of the chemical bond - An Introduction to Modern Structural Chemistry . Third Edition 1960, p.12
Pauling is speaking of the normal state, the state with the lowest possible value of energy (page 11). Structures I and II are contributing structures. In the next paragraph (page 12) he says (in my words):
The resonance hybrid is not exactly intermediate in character between structures I and II, because the resonance stabilized hybrid is lower in energy than either of the contributing structures. As we can suppose the real structure will have the state of lowest possible energy (in the normal state), we can say the real structure is the resonance hybrid.--
Wickey-nl (
talk) 08:52, 19 August 2010 (UTC)
The sentence in the second paragraph, "Each contributing structure can be represented by a Lewis structure, with normal single, double or triple covalent bonds between every pair of adjacent atoms within the structure." is wrong. I see it is supported by the reference to the Gold Book, but it is contradicted by a classic example of resonance for the bridge region of diborane, where the resonance structures have alternating "single" and "no bond" bonds giving each bond in the resonance hybrid to be a half bond. Can anyone suggest a better wording? Note also that diborane also contradicts the statements that imply that resonance is only about pi bonds. -- Bduke (Discussion) 22:24, 5 September 2010 (UTC)
The following link ( Resonance Theory) clarifies the topic and enhances the material covered. Material covered in this website is available to anyone and was written by a tenured PhD Organic Chemistry Professor at Utah Valley University. The website is a non-profit website and is intended to advance students understanding of Organic Chemistry. Thanks for your time, Nickcc20 ( talk) 14:45, 19 January 2011 (UTC)
Pauling's principle of electroneutrality is still taught as being the method by which favourable and unfavourable resonanace structures are "selected". A simple statement is that the charge on an atom should be between +/- 1 (formal charge)with the corollary that the negative charges should reside on the most electronegative atom and positive on more electropositive. Is this worth a mention? Axiosaurus ( talk) 13:04, 17 March 2013 (UTC)
I have reread this article and see that it is very molecular in its scope- why? Ionic structures with "covalent character", metals, intermetallics and other unusual solid state substances were all tackled by Pauling, was he wrong? Axiosaurus ( talk) 16:52, 17 March 2013 (UTC)
The statement regarding ionic contributions reads as if it applies to both to homonuclear and heteronuclear bonds. I am not familiar with the referenced book- however other books by Shaik discuss in detail the Heitler-London treatment of H2, is this where this quote comes from? . Historically ionic contributions in A-B bonds were the basis of the electronegativity concept. Axiosaurus ( talk) 06:24, 12 April 2015 (UTC)
Charge shift bonding is not the the same as ionic-covalent resonance. The claim is that some molecules are stable only because of ionic-covalent resonance. In F2 for example the calculations using just covalent terms do not show bonding. Ionic terms on their own are not that good. The claim is that it is resonance between them that is responsible for bonding. I want to stress that charge shift bonding is controversial. For example a generalised valence bond (GVB) function function gives a reasonable description of bonding for F2. Some workers argue that this GVB is just a description of covalent bonding. There is no ionic-covalent resonance. Others argue that it disguises the ionic terms and thus the ionic-covalent resonance. There are wide differences of opinion between researchers on valence bond theory. Take care. -- Bduke (Discussion) 11:45, 26 April 2015 (UTC)
I am wondering about the exact meaning and validity of the energy diagram added today for benzene in the section Resonance in quantum mechanics. The file description (obtained by clicking on the image) says it is based on the MO diagram for H2, which is of course well known. However that diagram and all the other diatomic MO diagrams are for individual orbitals (one-electron wave functions). This diagram for benzene appears to show the combination of two many-electron (at least six pi-electron) wave functions, one for each Kekulé structure. Is there a source for combining 2 Kekulé structures in an energy-level diagram, as opposed to the elementary diagram which joins the 2 structures with a simple ↔ ? The lower energy level is acceptable, as it is true that the wave function may be written as a (normalized) sum of two functions representing Kekulé structures. But the upper level is quite mysterious - is it antibonding at all 6 C-C bonds? Or bonding and antibonding at alternate positions, so that there are 3 double pi bonds and 3 antibonds? or null bonds? In the absence of a source, a proper answer to this question would require detailed mathematical analysis of the proposed wave function, which of course would be original research. In any case, I think the upper state would be at very high energy and inaccessible by one- or even two-electron transitions, so it is of no experimental interest.
In summary, I have never seen such a diagram for benzene and I think it requires a better explanation with a source. If this is not available, then I recommend the diagram be deleted. Dirac66 ( talk) 15:45, 3 May 2015 (UTC)
The last section on Charge delocalization contains mysterious acronyms and other terms. In encyclopedia articles, terms likely to be unfamiliar to many readers should either be explained at first usage or else linked to another article which does provide an explanation. So would someone please provide the answers to the following questions?
I just corrected a bunch of statements in the lead which imply that resonance structures have different distributions of electrons. That is extremely poorly worded if not flatout wrong. Only our *depiction* of where the electrons reside (in Lewis structures) changes, not the location (density) of the electrons themselves. Alsosaid1987 ( talk) 00:55, 11 May 2018 (UTC)
I have very carefully rewritten the second and third paragraphs of the lead to fix these issues. Also, there are some delicate issues with respect to logic and terminology. On the one hand, we rationalize the structure of a resonance hybrid based on the expected geometries of the individual Lewis structures and taking the "average". On the other hand, we later need to assert that contributing forms of a resonance hybrid do not differ in the geometry or overall electron density but are simply different representations of the real molecule.
Basically, we need to distinguish between standalone Lewis structures and Lewis structures that are part of resonance hybrids (i.e., contributing forms). I welcome changes that clarify this point. Alsosaid1987 ( talk) 06:26, 11 May 2018 (UTC)
I think using NO2 as the example is problematic for several reasons. Like nitric oxide or triplet O2, nitrogen dioxide has a 2c3e bond due to its unpaired electron. However, Lewis structures are unable to correctly show the extra half bond, and averaging the structures gives an incorrectly low estimate of the bond order. Also, there are at least two other important Lewis structures that one can draw. Though charge separation is less favorable, they cannot be neglected when determining the bond order or structure. The bond angle is 134 degrees, which reflects the formation of the extra half-pi-bond, and the hybridization is somewhere between sp2 and sp, while the bond lengths are also shorter than expected. There is no simple way of estimating the bond order in this case (which is somewhere between 1.5 and 2). For these reasons, NO2 is really a pathological example that shows the limits of the Lewis representation. On the other hand, NO2-, nitrite is much more straightforward, and I tentatively chose this example to illustrate the concept. Unfortunately, it's hard to find a neutral example of resonance that is simple enough to give. Alsosaid1987 ( talk) 03:17, 12 May 2018 (UTC)
@ Dirac66:@ DMacks:@ Alsosaid1987: It has come to my attention that this article seems to be struggling a lot to explain that resonance structures do not exist but the resonance hybrid does (the very long introduction and repeated assertions in the article is testament to this). It has also occurred to me that nowhere in this article, except the valence bond (ie quantum mechanics) part explains what resonance truly is. Could I suggest an overhaul of this article in that we introduce in the outset that resonance is basically the equivalent of linear combination of atomic orbitals in Valence Bond theory, where the actual molecular wavefunction is a weighted sum of individual resonance structures just like molecular orbitals are a weighted sum of individual atomic orbitals (the analogy goes quite far in fact. We can even take antisymmetric, ie. antibonding, combinations of resonance structures to attain excited states etc).
I'm putting the above as an idea, not sure how to better write this article to be both concise and succinct. From reading this talk page it appears the writing of this article being problematic goes back some time. Opinions?-- Officer781 ( talk) 14:19, 19 January 2019 (UTC)
Just posting an idea here. This article currently talks about major and minor contributors but does not go into the types of resonance structures that can be drawn. As far as I know there are three types (doesn't just refer to aromatic molecules but to any molecule where contributing structures can be drawn):
The Kekule-type structures are the ones currently covered in the article and in elementary discussions of resonance. I am busy currently and if anybody else would like to have a go at giving a brief mention of these in the major and minor contributors section can go ahead.-- Officer781 ( talk) 15:25, 23 January 2019 (UTC)
What is difference between resonance and hyperconjugation? Umer ilyas shaaheen ( talk) 15:55, 26 January 2020 (UTC)