4-NUCLEAR MIXED-VALENCE VANADIUM CLUSTERS - DIVERGENCE OF PHENOMENOLOGICAL AND MOLECULAR-ORBITAL MODELS

The electronic structure of four-nuclear vanadium cluster representing the well-known vanadyl pyrophosphate catalytic system is studied on t he basis of model Hamiltonian and semi-empirical molecular orbital app roaches. The possible charge redistribution resulting from the vibroni c interaction is analyzed for different mixed-valence combinations of V-V and V-IV. The adiabatic surfaces are calculated in the space of no rmal modes built from the V-O vibrations of vanadyl groups. It is show n that in some cases the two approaches may lead to different localiza tion patterns of excess electron(s). The molecular orbital model is a more adequate way to describe the structural features of real compound s, whereas the phenomenological model allows to analyze the qualitativ e influence of the different electronic interactions.