Hydrogen nuclear spin relaxation in hydrogen-ice clathrate.

H2 in D2O ice clathrate has been studied by hydrogen NMR. In a previous report, the H2 line shape was shown to be due to incompletely averaged intramolecular dipolar interactions. Here the relaxation times T1, T1rho, and T2 are reported. T1 passes through a minimum at 10 K, indicating that the rotational transition rate Gamma between the three sublevels of J = 1 passes through the resonance frequency at this temperature. On the cold side, T1 varies as T(-2.6); on the hot side, the rate T1(-1) varies as T(-2) plus a constant (due to paramagnetic impurities). These indicate a two-phonon process drives the rotational transitions Gamma. The spin-echo T2 is nearly independent of temperature and in reasonable agreement with the Van Vleck intermolecular H2-H2 second moment. T1rho deviates from the expected T1rho = T1 behavior above 85 K, revealing an additional slow-motion source of relaxation. The deviation is driven by the hopping of H2 between large cages. Ortho-para conversion is measured to be much slower in the clathrate than in the bulk solid, reflecting the greater distances between the H2 molecules.