Proton solvated by noble-gas atoms: simplest case of a solvated ion

Abstract

The solvation of a proton by up to six rare-gas atoms He, Ne, Ar, Kr, and Xe is investigated by B3LYP density functional theory with large basis sets, forming the first systematic study of all rare gases, He through Xe, on the same high level of theory. The solvation energy for regular two-fold, trigonal planar, tetrahedral and octahedral coordination shows, as known previously, that the protonated rare gas dimer is the most stable configuration in every case. Solvation of a point charge by hard polarizable spheres yields the same preference for two-fold coordination. Two rare gas atoms shield the proton efficiently, and additional rare gas atoms may be coordinated in an equatorial plane or along the axis of the central protonated rare gas dimer, with binding energies and bond lengths comparable to those of the corresponding rare gas solids. The influence of additional solvent atoms on the harmonic stretching frequencies is minor and cannot explain the large shift observed in low temperature matrices. Proton diffusion is examined by calculating the transition state for isomerization of Rg₃H⁺ species, which yields barrier heights of 8.8, 11.5, 29.7, 32.3, and 35.5 kJ mol^-1 for He, Ne, Ar, Kr, and Xe, respectively. Geometries, harmonic frequencies, bond dissociation energies and partial charges of mixed protonated rare gas dimers reveal a consistently smooth trend of these properties with size and polarizability of the rare gas atoms. Based on these findings, the assignment of spectral lines attributed to the mixed ArH⁺Kr, ArH⁺Xe, and KrH⁺Xe species is questioned. The stabilization of positive charge centers in solid Xe in the presence of hydrogen atoms is also discussed.