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

The solvation of a proton by up to six rare-gas atoms He, Ne, Ar, Kr, and X e is investigated by B3LYP density functional theory with large basis sets, forming the first systematic study of all rare gases, He through Xe, on th e same high level of theory. The solvation energy for regular two-fold, tri gonal planar, tetrahedral and octahedral coordination shows, as known previ ously, that the protonated rare gas dimer is the most stable configuration in every case. Solvation of a point charge by hard polarizable spheres yiel ds the same preference for two-fold coordination. Two rare gas atoms shield the proton efficiently, and additional rare gas atoms may be coordinated i n 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 co rresponding rare gas solids. The influence of additional solvent atoms on t he harmonic stretching frequencies is minor and cannot explain the large sh ift observed in low temperature matrices. Proton diffusion is examined by c alculating the transition state for isomerization of Rg(3)H(+) species, whi ch 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, bo nd dissociation energies and partial charges of mixed protonated rare gas d imers reveal a consistently smooth trend of these properties with size and polarizability of the rare gas atoms. Based on these findings, the assignme nt of spectral lines attributed to the mixed ArH+Kr, ArH+Xe, and KrH+Xe spe cies is questioned. The stabilization of positive charge centers in solid X e in the presence of hydrogen atoms is also discussed.