Common features of various mechanisms of electron transfer across a 4,4'-bipyridine bridge: A theoretical evaluation of resonance structures of the transition state

The transition state of electron transfer occurring in mixed-valence systems, which involve the 4,4'-bipyridine as bridge, is examined in the framework of three mechanisms: a 'double exchange' (or 'two state') and two 'super exchange' mechanisms involving positive or negative polaron components. The common features of these mechanisms as well as effects which are due to the topology of the bridge are investigated. The electron transfer is described within 'Mesomeric or Resonance' theory, and the corresponding weights of various resonance structures are calculated by means of the poly-electron population analysis. Even though the one-electron characteristics (as charge and spin densities) are totally different for the three considered mechanisms, common features are revealed from the multi-particle structures. It is shown that in the field of two equally resonating charge centers, the electron pairs (ionic + covalent) of 4,4'-bipyridine show a trend towards quinoid localization. This effect occurs regardless of the mechanism with which the thermal electron transfer is realized; it is more pronounced for the two 'super exchange' mechanisms than in the 'double exchange' one. The main observed effects are essentially controlled by the topology.