Polyoxometallates as models for oxide catalysts: Part II. Theoretical semi-empirical approach to the influence of the inner and outer Mo coordination spheres on the electronic levels of polyoxomolybdates

Abstract

In complement to Part I, theoretical MO calculations (EHMO level) show that the main factor which influences the electronic energy levels of polyoxomolybdates is not always the local Mo symmetry (tetrahedral or octahedral). The effect of protonation and/or grafting on a surface oxide is weak but slightly larger for tetrahedral than for octahedral species and could lead to wrong attributions. The condensation of several sites in polyoxomolybdates has a greater effect since it involves a MO delocalization over the whole compound, and therefore a loss of the individuality of each octahedral or tetrahedral unit. It leads to a decrease in the HOMO-LUMO gap yielding to an absorption red shift. The effect of distortion of local symmetry by bond elongation tends to decrease the HOMO-LUMO gap in an isolated species. In a condensed system, this distortion lowers the MoMo interactions and moderates the red shift due to the effect of condensation. This latter effect, however, remains predominant.