Electronic structure and conformational analysis of paraquat in three oxidation states

Electronic structure calculations are reported for biphenyl and for three oxidation states of paraquat (N,N′-dimethyl-4,4′-bipyridinium). The fully oxidized species, PQ+2, is found to be twisted about the inter-ring bond. One-electron reduction yields a radical cation, PQ+1, which is predicted to be planar by virtue of the bonding characteristics of the singly occupied molecular orbital. Addition of a second electron enhances the propensity for planarity. The relevance of these calculations to the action of paraquat as an electron relay is discussed.