Electric-field perturbations of ring currents in π systems

Ipsocentric distributed-gauge calculations are used to map differential ring currents induced in [4n+2] and [4n] π systems by simultaneous application of an axial magnetic and electric field (field gradient). In benzene, electric fields and gradients produce stratified first-order π ring currents, opposing/reinforcing the intrinsic diatropic current, according to the height profile of charge depletion/enhancement induced by the electric perturbation. In (planarised) cyclooctatetraene, action of a uniform electric field is similarly rationalised, but response to the (totally symmetric) field gradient is dominated by a specific orbital effect, closing/widening of the small HOMO-LUMO gap leading to reinforcement/reduction of the intrinsic paratropic current.