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Abstract

The interaction of hydrogen with doubly vanadium doped aluminum clusters, Al_nV₂⁺ (n = 1–12), is studied experimentally by time-of-flight mass spectrometry and infrared multiple photon dissociation spectroscopy. The hydrogen binding geometry is inferred from comparison with infrared spectra predicted by density functional theory and shows that for the more reactive clusters the hydrogen adsorbs dissociatively. Three sizes, n = 4, 5 and 7, are remarkably unreactive compared to the other clusters. For larger sizes the reactivity decreases, a behavior that is similar to that of singly vanadium doped aluminum clusters, and that might be attributed to geometric and/or electronic shielding of the dopants. By examining the electronic structure of Al₆V₂⁺ and Al₇V₂⁺, interactions between the frontier orbitals of the clusters and those of H₂ that explain the size-dependent reactivity are identified.