Gas-phase infrared photodissociation spectroscopy of tetravanadiumoxo and oxo-methoxo cluster anions.

The infrared spectra of the binary vanadium oxide cluster anions V(4)O(9)(-) and V(4)O(10)(-) and of the related methoxo clusters V(4)O(9)(OCH(3))(-) and V(4)O(8)(OCH(3))(2)(-) are recorded in the gas phase by photodissociation of the mass-selected ions using an infrared laser. For the oxide clusters V(4)O(9)(-) and V(4)O(10)(-), the bands of the terminal vanadyl oxygen atoms, nu(V-O(t)), and of the bridging oxygen atoms, nu(V-O(b)-V), are identified clearly. The clusters in which one or two of the oxo groups are replaced by methoxo ligands show additional absorptions which are assigned to the C-O stretch, nu(C-O). Density functional calculations are used as a complement for the experimental studies and the interpretation of the infrared spectra. The results depend in an unusual way on the functional employed (BLYP versus B3LYP), which is due to the presence of both V-O(CH(3)) single and V=O double bonds as terminal bonds and to the strong multireference character of the latter.