INTERACTION OF 2 CHARGE CENTERS THROUGH BIPYRIDINES - IS THE CHEMICALMECHANISM POSSIBLE IN A FAVORABLE ELECTRON-TRANSFER

The interaction of two charge centres through bipyridine bridges is co nsidered, and the possibility that electron transfer is achieved by me ans of a ''chemical'' mechanism is investigated. The transition state of this reaction is compared with that corresponding to the bridge-iso lated radical cation: the behaviour of each pi-bond of the bridge is e xamined by applying multi-electron population analysis. It is found th at in the case of a favourable electron transfer, i.e. when the bridge has the topology of 4,4'-bipyridine, the transition state cannot bear similarities with the radical cation of para-bipyridine, rendering th e ''chemical'' mechanism impossible; this holds regardless of the oxid ation ability of the charge centres and is essentially due to the favo urable through pi-bonds resonance of the charge centres. In contrast, in a 3,3'-bipyridine topology (for which no favourable electron transf er is observed), the bridge can be quite similar to the isolated radic al cation. (C) 1998 Elsevier Science B.V.