The unusual electronic mechanism of the [1,5] hydrogen shift in (Z)-1,3-pentadiene predicted by modern valence bond theory

The combination of modern valence bond theory in its spin-coupled (SC) form [SC(6)/6-31G*] and an intrinsic reaction coordinate calculation (MP2/6-31G*IRC) is used in order to obtain a model for the electronic mechanism of the gas-phase [1(s),5(s)] hydrogen shift in (Z)-1,3-pentadiene. It is shown that this reaction follows an unusual heterolytic mechanism consistent with the C-s symmetry of the transition state and involving the simultaneous movements of three well-defined orbital pairs. One of these is responsible for the sigma bond which initially attaches the migrating hydrogen to carbon 5 and, later on, to carbon 1. The second one realises the p bond between carbons 3 and 4 which during the course of the reaction moves over carbons 2 and 3, while the third pair, initially involved in the p bond between carbons 1 and 2 has no other choice but to embark on a long-range journey across the ring, ending up over carbons 4 and 5. While at first sight, it might appear that an electronic mechanism of this type would preclude the existence of an aromatic transition state, we have been able to show that the electronic structure of the transition state for this sigmatropic hydrogen shift has much in common with a hitherto apparently unknown alternative modern valence bond description of benzene involving 'antipairs' and so it can be considered to be aromatic.