Quantum diffusion and tunneling in the solid hydrogens (a short review)

The first observations 30 years ago of molecular clustering via quantum diffusion in solid ${\mathrm{H}}_{\mathrm{2}}$ in nuclear magnetic resonance (NMR) and pressure experiments are described. A previous review on quantum diffusion and tunneling up to 1986 is updated to reflect progress in the last ten years. In particular I review the effect of clustering on heat capacity in solid ${\mathrm{D}}_2$ and on the thermal conductivity in solid ${\mathrm{H}}_2.$ The configurational relaxation time observed in ${\mathrm{D}}_2$ at 1.75 K is found to scale with that in ${\mathrm{H}}_{\mathrm{2}}$ at the same temperature in terms of the predictions of ortho–para resonant conversion-induced hopping. The relaxation times measured simultaneously on the same sample in NMR and conductivity experiments reflect different configurational processes. The theory of molecular hopping and pairing in solid ${\mathrm{H}}_{\mathrm{2}}$ is outlined and compared with experiments. The tunneling frequency of HD in solid ${\mathrm{H}}_{\mathrm{2}}$ deduced from transverse and longitudinal NMR relaxation experiments between 0.03 and 14 K is discussed. The hopping of ortho-${\mathrm{H}}_{\mathrm{2}},$ invoked as a mechanism in the enhanced ortho–para conversion in presence of ${\mathrm{O}}_{\mathrm{2}}$ impurities, is mentioned. Finally, recent tunneling results for H, D and ${\mathrm{H}}_{\mathrm{2}}^{\mathrm{-}}$ anions in solid ${\mathrm{H}}_2$ during the studies of low temperature reaction dynamics are also briefly described. In the Appendix, the hopping frequency determination in HD from NMR transverse relaxation time measurements is reviewed.