Compositions and Structures of Vanadium Oxide Cluster Ions V_mO_n^± (m = 2-20) Investigated by Ion Mobility Mass Spectrometry

Stable compositions and geometrical structures of vanadium oxide cluster ions, V_mO_n^±, were investigated by ion mobility mass spectrometry (IM-MS). The most stable compositions of vanadium oxide cluster cations were (V₂O₄)(V₂O₅)_(m-2)/2⁺ and (VO₂)(V₂O₅)_(m-1)/2⁺, depending on the clusters with even and odd numbers of vanadium atoms. Compositions one-oxygen richer than the cations, such as (V₂O₅)_m/2^- and (VO₃)(V₂O₅)_(m-1)/2^-, were predominantly observed for cluster anions. Assignments of these stable cluster ion compositions, which were determined as a result of collision-induced dissociations in IM-MS, can partly be explained with consideration of spin density distribution. By comparing the experimental collision cross sections (CCSs) obtained from ion mobility measurement with CCSs of the theoretically calculated structures, we confirmed the patterned growth of geometrical structures partially discussed in previous theoretical and spectroscopic studies. We showed that even sized (V₂O₅)_m/2^± where m = 6-12 had right polygonal prism structures except for the anionic V₁₂O₃₀^-, and for the clusters of odd numbers of vanadium m, cations and anions can either have bridged or pyramid structures. Both of the odd sized structures proposed were derivatives from the even sized right polygonal prism structures. The exception, V₁₂O₃₀^-, which had a CCS almost equal to that of the neighboring smaller V₁₁O₂₈^-, should have a structure of higher density than the right hexagonal prism, in which it was proposed to be a captured pyramid structure, derived from V₁₁O₂₈^-.