Gas phase vibrational spectroscopy of mass-selected vanadium oxide anions

Abstract

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 V_mO_n⁻clusters can be grouped in three spectral regions. The transitions of (i) superoxo, (ii) vanadyl and (iii) V–O–V and V–O˙ single bond modes are found at ∼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)_2,3VO₃⁻ 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.