Infrared photodissociation spectroscopic studies of ScO(H2O)n=1-3Ar+ cluster cations: solvation induced reaction of ScO+ and water.

We investigate the gaseous ScO(H2O)1-3Ar+ cations prepared by laser vaporization coupled with supersonic molecular beam using infrared photodissociation spectroscopy in the O-H stretching region. The cation structures are characterized by comparing the experimentally observed frequencies with the simulated vibration spectra. We reveal that stoichiometric ScO(H2O)Ar+ is intrinsically the hydrated oxide cation expressed as H2O-ScOAr+ hydrate rather than Sc(OH)2Ar+ dihydroxide, although the former is higher in energy by 29.5 kcal mol-1 than the latter. Interestingly, when more water molecules are introduced to the complex, we find that the stoichiometric ScO(H2O)2-3Ar+ embraces the core subunit of Sc(OH)2+. Theoretical calculations suggest that the energy barrier of hydrogen transfer plays a critical role in the isomerization from hydrated complex to dihydroxide. When more than one water molecule is involved in the complex, the hydrogen transfer becomes nearly barrierless through a six-member cyclic transition state, leading to the reduction in the energy barrier from 21.8 kcal mol-1 to 4.2 kcal mol-1. Altogether, we conclude that the solvent molecules such as water can decrease the energy barrier and thus induce the formation of hydroxy species in the hydrolysis process.