Preferential solvation of CH 3F by ortho-H 2 in cryogenic solid hydrogen

Abstract

Infrared spectroscopic studies of solid hydrogen crystals enriched with the parahydrogen ( pH 2) nuclear spin state and doped with part per million concentrations of methyl fluoride (CH 3F) show that CH 3F preferentially clusters with residual orthohydrogen ( oH 2) present in the crystal. At low oH 2 concentrations (<2%) small CH 3F( oH 2) n ( n=1–12) clusters are observed and the cluster spectra display no temperature dependence over the 1.8–4.4 K temperature range studied. This cluster size range corresponds to filling the first solvation shell around the CH 3F. At higher oH 2 concentrations (>2%) where larger clusters ( n>12) are observed, the outer oH 2 molecules in the clusters can be reversibly condensed or boiled off the cluster over the same temperature range. The measured shift in the cluster distribution indicates the outer oH 2 molecules in large CH 3F( oH 2) n clusters are bound by energies comparable to kT≈3 cm −1 at 4.4 K. The kinetics of the growth or break up of the clusters is limited by the quantum diffusion rate of oH 2 in the solid. The ortho– para nuclear spin conversion of hydrogen molecules in the first solvation shell of CH 3F is observed with a rate constant ten times larger than the self-conversion rate constant in solid hydrogen.