Rotation and diffusion of H2 in hydrogen -: Ice clathrate by 1H NMR

A recently reported hydrogen-ice clathrate carries up to four H-2 in each large cage and one H2 in each small cage. We report pulsed proton NMR line shape measurements on H-2-D2O clathrate formed at 1500 bar and 250 K. The behavior of the two-pulse spin-echo amplitude with respect to the nutation angle of the refocusing pulse shows that intramolecular dipolar broadening, modulated by H-2 molecular reorientations, dominates the line width of the ortho-H-2. Dipolar interaction between H-2 guests and host D atoms explains the echo variation with the relative phases of the pulses. From 12 to 120 K, the line width varies as 1/T, demonstrating that the three sublevels of J = 1 are split by a constant energy, epsilon. The splitting arises from distortion in the otherwise high-symmetry cages from frozen-out D(2)0 orientational disorder. Above 120 K, further line-narrowing signals the onset of H-2 diffusion from cage to cage. At the lowest temperature, 1.9 K, the spectrum has Pake powder doublet-like features; the doublet is not fully developed, indicating a broad distribution of order parameters and energies epsilon.