Ab initio calculations on 1,2-hydrogen shifts in the benzene radical cation and on carbon scrambling via an isomerization to the fulvene structure.

Multireference configuration interaction (MRCI//6-31G**) ab initio calculations show that the barrier for hydrogen scrambling in the benzene radical cation is about 50 kcal mol(-1). Once the internal energy is sufficient for a 1,2-hydrogen shift, the moving hydrogen can go to any position in the ring. The barrier for carbon scrambling via an isomerization to the fulvene structure is about 17 kcal mol(-1) higher than that for hydrogen scrambling. Both of these values are far below the dissociation limit.