Gas phase vibrational spectroscopy of mass-selected vanadium oxide anions

The vibrational spectra of vanadium oxide anions ranging from V2O6- to V8O20- are studied in the region from 555 to 1670 cm(-1) by infrared multiple photon photodissociation (IRMPD) spectroscopy. The cluster structures are assigned and structural trends identified by comparison of the experimental IRMPD spectra with simulated linear IR absorption spectra derived from density functional calculations, aided by energy calculations at higher levels of theory. Overall, the IR absorption of the VmOn- clusters can be grouped in three spectral regions. The transitions of (i) superoxo, (ii) vanadyl and (iii) V-O-V and V-O-center dot single bond modes are found at similar to 1100 cm(-1), 1020 to 870 cm(-1), and 950 to 580 cm(-1), respectively. A structural transition from open structures, including at least one vanadium atom forming two vanadyl bonds, to caged structures, with only one vanadyl bond per vanadium atom, is observed in-between tri- and tetravanadium oxide anions. Both the closed shell (V2O5)(2,3)VO3- and open shell (V2O5)(2-4)(-) anions prefer cage-like structures. The (V2O5)(3,4)(-) anions have symmetry-broken minimum energy structures (C-s) connected by low-energy transition structures of C-2v symmetry. These double well potentials for V-O-V modes lead to IR transitions substantially red-shifted from their harmonic values. For the oxygen rich clusters, the IRMPD spectra prove the presence of a superoxo group in V2O7-, but the absence of the expected peroxo group in V4O11-. For V4O11-, use of a genetic algorithm was necessary for finding a non-intuitive energy minimum structure with sufficient agreement between experiment and theory.