In the previous post, I discussed what we could learn from ethane by forcing it into a pericyclic dyotropic rearrangement. We saw how it voraciously scavenged two electrons from the C-C bond to achieve this. What if we give it more electrons? Thus 1,2-dibromoethane undergoing the same reaction.
Subjected to B3LYP/6-311G(d,p) (with or without solvent field) yields a transition state with only one negative force constant. No tendency to distort from D2h symmetry then. Notice also how the migrating hydrogens did all the moving for ethane, but with a much heavier bromine replacing them, it is now the relatively light carbons and the hydrogens attached to them that instead carry the reaction.What is more? Well, each Br has more valence electrons than H (7 vs 1), but of course the C-Br bond is also more ionic. Time for ELF to try to tell us what the electrons are up to.
This shows that the C-C region has 2.4 electrons; its actually gained some! The bromines each have 7.9 electrons in the valence shell in the form of two lone pair monosynaptic basins (and 27.5 in the core), and the remaining hydrogens on the carbon have ~2.15 each (0.15 having been ~extracted from the core of the bromine). The system has distorted from a pericyclic transfer of electrons to an ionic mechanism; an ion-triple to be precise. This is also not anti-aromatic. So here we see yet another way in which a forbidden (anti-aromatic) pericyclic reaction can distort. There are other ways still!