Preferential solvation of CH3F by ortho-H2 in cryogenic solid hydrogen

Infrared spectroscopic studies of solid hydrogen crystals enriched with the parahydrogen (pH2) nuclear spin state and doped with part per million concentrations of methyl fluoride (CH3F) show that CH3F preferentially clusters with residual orthohydrogen (oH2) present in the crystal. At low oH2 concentrations (<2%) small CH3F(oH2)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 CH3F. At higher oH2 concentrations (>2%) where larger clusters (n>12) are observed, the outer oH2 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 oH2 molecules in large CH3F(oH2)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 oH2 in the solid. The ortho–para nuclear spin conversion of hydrogen molecules in the first solvation shell of CH3F is observed with a rate constant ten times larger than the self-conversion rate constant in solid hydrogen.