Fast diffusion of molecular hydrogen in hydrogen hydrates

In order to analyse the dynamics of molecules at high pressures, we are applying high-resolution nuclear magnetic resonance (NMR) spectroscopy for samples at gigapascals of pressures in a diamond anvil cell. Here we report some results of its application to various phases of hydrogen hydrates. These hydrates are stable only at high pressures and have never been analyzed in situ by NMR. The observed ¹H-NMR spectra of filled-ice hydrogen hydrates at pressures 1 to 4 GPa gave anomalously narrow resonances of the H₂ guests encapsulated into hydrogen-bonded H₂O frameworks. Observed effects of pressure on NMR relaxation times of these H₂ guests indicate that molecular rotation and translational diffusion contribute together to their spin relaxation. We determined the two motional correlation times of the H₂ guest molecules as a function of pressure. From the diffusion correlation time, liquid-like fast diffusion of the H₂ guests within the hydrate, of the order of 10^-8 cm²/s, has been deduced. For hydrogen clathrate hydrate stable at much lower pressure, such diffusion is even faster, which was separately confirmed by pulsed-gradient field NMR method using a sapphire gas-pressure cell.