Is science sometimes in danger of getting tunnel vision? Recently published ebook author, Ian Miller, looks at other possible theories arising from data that we think we understand. Can looking problems in a different light give scientists a different perspective?

C2

In some of my previous posts, I have bemoaned the absence of public discussions between chemists on matters of theoretical importance to chemistry, and so, when one actually appears, I must first congratulate the participants and the journal. This specific issue relates to two recent discussions (Angew. Chem. Int. Ed. 52: 5922-5925; 5926-5928) relating to whether there is a quadruple bond in C2. Whether the molecule is important is a matter of opinion, but the point that I have tried to make previously in these posts is simply publishing papers is not sufficient to lead to greater understanding. What I believe is needed is subsequent analysis, so that we better know what we know as opposed to what we think. It therefore follows that to be useful, the discussion should be in a form comprehensible to the educated chemist who is not directly involved in the field, and it is with in mind that I wish to consider, were the criticisms worth making, and were they answered satisfactorily in that the general chemist would learn anything? There are obviously other issues, but I shall leave them for further posts.
 
The first article was a criticism by Frenking and Hermann of a previous publication in which the existence of the quadruple bond was proposed. Their main points were:
(a)  The force constant of C2 < force constant acetylene. The stretching frequency of C2 was 1855 cm-1 while that of acetylene is 1974 cm-1. Their argument was that these data are evidence that the bond in C2 is weaker than that of acetylene.
(b)  The claim for C2 to have a stronger bond lies in measurement of the dissociation energies of acetylene. Thus when the first hydrogen is removed, the energy required is 133.5 kCal/mol, and the second 116.7 kCal/mol, a difference of 16.8 kCal/mol. This 16.8 kCal/mol is supposedly the additional energy arising from the formation of the quadruple bond, however the criticism is that the framework is not constant, in that in the second dissociation, the carbon-carbon bond length increases by 0.035 A. They argue there is no reason to assume that a smaller C – H bond dissociation energy arises through strengthening of the C – C bond; there may be other reasons.
(c)  The remaining arguments were largely dependent on computational procedures and they may or may not be correct. The outside observer merely has to either accept or not the points. However, there was one point made that irritated me. The criticism was that the original paper adopted incorrect reference states. In general physics, the end conclusion eliminates the frame of reference, and hence the results are independent of it. The reference points eliminated from the calculation are chosen for ease of calculation, and should not affect the conclusion.
(d)  In the footnotes, they write "A bonding model is not right or wrong, but it is more or less useful." Their argument is the quadruple bond model is not useful because it does not agree with the properties of the molecule. Whether or not this criticism is correct or not, it is important because it focuses attention on the critical issues that lead to further understanding.
 
The response by Danovich, Shaik, Rzepa and Hoffmann is of interest. They argue first that the rule that stronger bonds have stronger force constants may not be universal. Given that there is no firm relationship (at least that I know of) relating bond strength and stretching force constant, that may be true, but equally it may not. As an outside observer, I think the F&H point has validity, although it is not conclusive. They also argue that computations show that the energy change in the C – C distance changing from 1.21 to 1.24 A is negligible. If so, the point (b) fails. However, we must ask, were the computations 100% guaranteed true? I am not convinced. On the other hand, the lowering of the energy is unambiguous and uncontested, so any argument thereafter really must be based on what this means. The responders argue that this means additional bonding, and to defeat that argument, there has to be some alternative for this energy lowering.
 
Does it matter? I think conceptually, yes, because it makes us think more about what is a bond. (More on this in subsequent posts.) Consider the energy argument above, and transfer that to dinitrogen. The triple bond of N2 is no simple extrapolation from single and double bonded nitrogen species. One likely reason is, like the acetylide anion, the triple bond configuration stabilizes the lone pair, and extrapolating Coulson's "bent bond" model, the orbitals in the triple bond are bent away from the lone pair, thus exposing the lone pair electrons to greater positive field.
 
The skeptical chemist should now ask, what is the exact electron configuration in C2? Are all electrons paired? Unfortunately, this was not specifically stated in the article, however by observation the species is actually a singlet. To be a singlet as opposed to being a triplet diradical, within standard MO theory, the two electrons must be in a common wave function. If they are, it is either bonding or antibonding, and since there is a net energy lowering, it must be bonding. So, within MO theory, the fourth bond exists because there is an energy lowering of 16 kCal/mol. Suppose we wish to go outside MO theory. If so, and have the two electrons in separable wave functions, then to get a singlet there has to be a phase relationship between the two waves, and an interaction that leads to the energy lowering, and if so, the question then is, why is that not within the description of a bond? In fact Shaik et al. (Nature Chem DOI:10.1038/NCHEM.1263) show by VB treatment, that the reality is in line with that proposition. Thus I believe this omission of the singlet nature of the state was unfortunate, because it is the omitted observational evidence that settles the issue, at least for me.
 
Finally, a quote from Roald Hoffmann: Could it be that “this most rigorous theory,” the one that affords “deep insight,” in fact has failed (so far) to provide pragmatic chemists with a way of thinking about real chemistry—whether it is that of “synthetic” or of short-lived molecules—that is as useful as are Lewis structures, arrow-pushing, and molecular orbitals?
 
My guess is, so far, yes, but if we had more of these discussion-type articles more directed towards the general chemist, perhaps the answer would change.
Posted by Ian Miller on Jun 17, 2013 4:45 AM Europe/London

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