Chemical effects from strained molecules

The major evidence supporting the fact that cyclopropane permits electron delocalization was that like ethylene, it stabilizes adjacent positive charge, and it stabilizes the excited states of many molecules when the cyclopropane ring is adjacent to the unsaturation. My argument was that the same conclusion arises from standard electromagnetic theory.

Why is the ring strained (i.e. at a higher energy)? Either the molecule is based on distorted ordinary C – C bonds (the “strain” model) or it involves a totally new form of bonding (required for delocalization). If we assume the strain model, the orbitals around carbon are supposed to be at 109.4 degrees to each other, but the angle between nuclei is 60 degrees. The orbitals try to follow the reduced angle, but as they move inwards, there is increased electron-electron repulsion, and that is the source of the strain energy. That repulsion “lifts” the electron up from the line between the nuclei to form a “banana” shaped bond. Of the three atoms, each with two orbitals, four of those orbitals come closer to a substituent when the bonds are bent, and the two on the atom to which the substituent is attached maintain a more or less similar distance, because the movement is more rotational.

If so, the strained system should be stabilized by adjacent positive charge. Those four orbitals are destabilized by the electron repulsion from other electrons in the ring; the positive charge gives the opposite effect by reducing the repulsion energy. Alternatively, if four orbitals move towards a substituent carrying positive charge, then as they come closer to a point, the negative electric field is stronger at that point, in which case positive charge is stabilized. The problem is to put numbers on such a theory.

My idea was simple. The energy of such an interaction is stored in the electric field, and therefore it is the same for any given change of electric field, irrespective of how the change of field is generated. Suppose you were sitting on the substituent with a means of measuring the electric field, and the electrons were on the other side of a wall. You see an increase in electric field, but what generates it? It could be that the electrons have moved closer, and work is done by their doing so (because the change of field strength requires a change of energy) OR you could have left them in the same place but added charge, and now the work corresponding to the strain energy would be done by adding the charge. There is, of course, no added charge, BUT if you pretend there is, it makes the calculation that relates the strain energy to the effects on adjacent substituents a lot simpler. The concept is a bit like using centrifugal force in an orbital calculation. Strictly speaking, there is no such force – if you use it, it is called a pseudoforce – but it makes the calculations a lot easier. The same here, because if one represented the change of electric field as due to a pseudocharge, there is an analytic solution to the integration. One constant still has to be fixed, but fix it for one molecule and it applies to all the others. So an alternative reason why adjacent positive charge is stabilized was obtained, and my calculation was very close to the experimental value that was obtained. So far, so good.

The UV spectra could also be easily explained. From Maxwell’s electromagnetic theory, to absorb a photon and form an excited state, there has to be a change of dipole moment, so as long as the positive end of the dipole can be closer to the cyclopropane ring than the negative end, the excited state is stabilized. More importantly, when this effect was applied to various systems, the changes due to different strained rings were proportional to my calculated changes in electric field at substituents. Very good news.

If positive charge were stabilized due to delocalization, so should negative charge be stabilized, but if it were due to my proposed change of electric field, then negative charge should be destabilized. This is where wheels fell off, because a big name published asserting negative charge was stabilized (Maerker, A.; Roberts, J. D. J. Am. Chem.Soc. 1966, 88, 1742-1759.) They reported numerous experiments in which they tried to make the required anion, and they all failed. Not exactly a great sign of stabilization. If they used a method that cannot fail, the resultant anion rearranged. That is also not a great sign of stabilization, but equally it does not necessarily show destabilization because stabilization could be there, but it changes to something even more stable.

Their idea of a clinching experiment was to make an anion adjacent to a cyclopropane ring and two benzene rings. The anion could be made provided potassium was a counterion. How that got through peer review I have no idea because that anion would be far less stable than the same anion without the cyclopropane ring. Even one benzene ring adjacent to an anion is well known to stabilize it. The reason why potassium was required was because the large cation could not get near the nominal carbon atom carrying the charge and that ballowed the negative charge to be destabilized away from the cyclopropane.. If lithium were used, it would get closer, and focus the negative charge closer to the cyclopropane ring. This was a case of a big name able to publish just about anything, and everyone believed him because they wanted to.

Which is all very well, but it is one thing to argue an experiment could have been interpreted some other way, but that is hardly conclusive. However, there was an “out”. The very lowest frequency ultraviolet spectral absorptions of carbonyl compounds were found to involve charge moving from the oxygen towards the carbon atom, and the electric moment of the transition was measured for formaldehyde. My theory now could make a prediction: strained systems should move the transition to higher frequency, whereas if delocalization were applicable, it should move to lower frequency. My calculations got the change of frequency for cyclopropane as a substituent correct to within 1 nm, whereas the delocalization argument could not even get the direction of the shift correct. It also explained another oddity: if there were a highly strained system such as bicyclobutyl as a substituent, you did not see this transition. My reason was simple: the signal moved to such a higher frequency that it was buried in another signal. So, I was elated.

When my publications came out, however, there was silence. Nobody seemed to understand, or care, about what I had done. The issue was settled; no need to look further. So much for Popper’s philosophy. And this is one of the reasons I am less than enthused at the way alternative theories to the mainstream are considered. However, there is a reason why this is so. Besides the occasional good theory, there is a lot of quite spurious stuff circulating. It is easy to understand why nobody wants to divert their attention from the work required for them to get more funding. Self-interest triumphs.

Does it matter? It does if you want to devise new catalysts, or understand how enzymes work.


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