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?

Cyclopropane conjugation: the definitive observation?

How do you tell which of two theories is likely to be correct? The answer is that each gives a set of predictions, and you have to find an experiment where the two theories predict discernibly different effects. More formally, you cannot state that one theory applies and the other does not from data in the intersection of the two sets. Thus one could not decide whether cyclopropane conjugates with adjacent unsaturation from the fact that positive charge adjacent to a cyclopropane ring is stabilized, because both the electron delocalization theory and my polarization field theory gave essentially the same prediction that positive charge would be stabilized. Worse, calculations showed that to within the uncertainties inherent in each calculation, the two gave essentially the same degree of stabilization: a little over 100 kJ/mol. for the bare carbenium ion in a vacuum. On the other hand, the effects were qualitatively opposite for negative charge. As noted in an earlier post, a case could be made that the required destabilization occurred, and there was certainly no evidence of significant stabilization, but it was difficult to say this was definitive. Then I got lucky: key evidence was published.
One further piece of evidence sometimes quoted in favour of cyclopropane conjugating was that cyclopropane adjacent to a chromophore generally gave a bathochromic shift, and an enhanced extinction coefficient. Now, to absorb electromagnetic radiation, to reach the excited state, the system must undergo a change in electric moment, and the probability of a photon being absorbed is proportional to the change in electric moment. Thus something like benzene must have an instantaneous dipole moment in the excited state. The net effect is probably most easily seen using the canonical structure representation, even if it is not strictly accurate. The net result is that for most transitions, a positive charge can be adjacent to the cyclopropane ring in the excited state, hence the polarization field interpretation predicts a bathochromic shift and an enhanced electric moment, exactly the same as does the conjugation theory.
It was some time after this that for me a key observation was made: the change of electric moment was measured for the n → π* UV transition of formaldehyde. The important point was the change of electric moment was from oxygen to carbon, hence the same transition on a carbonyl adjacent to a cyclopropane ring would lead to a change of dipole moment with the negative end directed towards the cyclopropane ring. Now, that change of electric moment would interact with my proposed polarization field, which would lead to any strained compound giving a hypsochromic shift to that transition when compared with alkyl. This was important, because it was well-known that conjugative effects give a pronounced bathochromic shift to all such transitions. For example, the transition in acrolein has a bathochromic shift of approximately 25 nm from a saturated aldehyde. I used my pseudocharge to calculate the magnitude of the hypsochromic shift for some strained systems, and got the shift for cyclopropane to within a half a nanometer. (There was probably a certain amount of luck there because observation of these transitions gives broad signals, and picking the maximum is a little subjective.) Of course I could also calculate proportional shifts for π → π* transitions, which have bathochromic shifts. An interesting point here is that it was thought that a carbonyl adjacent to the bridgehead of bicyclobutyl had no n → π* transition. According to my calculation, it would have one but it would be buried underneath the π → π* transition, a consequence of the larger shifts due to the higher strain moving them in opposite directions and thus eliminating most of their separation.
So, a triumph? Well, actually, no. Two reviews on the issue of electron delocalization in cyclopropane came out around this period. The first (Bul. Soc. Chim. France  1967, 357-370.) simply stated that the hypsochromic shifts occurred, but they were unimportant! The second (Angew. Chem. Internat. Ed. (Engl.) 1979, 18, 809-886) got around this problem by simply ignoring it. It also ignored my work, and worse, it ignored all the references I had found to work that suggested there was no electron delocalization. That is not the science that I signed up for.
The problem with reviews is that once one is declared definitive, there is no place to debate a review. I later wrote a review that found over sixty different types of observation that falsified the delocalization theory but I could not get it published. Accordingly, I and the textbooks disagree on this matter.
Posted by Ian Miller on Mar 11, 2013 2:35 AM Europe/London

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