| Abstract
| - A recently reported hydrogen−ice clathrate carries up to four H2 in each large cage and one H2 in each smallcage. We report pulsed proton NMR line shape measurements on H2−D2O clathrate formed at 1500 bar and250 K. The behavior of the two-pulse spin−echo amplitude with respect to the nutation angle of the refocusingpulse shows that intramolecular dipolar broadening, modulated by H2 molecular reorientations, dominatesthe line width of the ortho-H2. Dipolar interaction between H2 guests and host D atoms explains the echovariation with the relative phases of the pulses. From 12 to 120 K, the line width varies as 1/T, demonstratingthat the three sublevels of J = 1 are split by a constant energy, ε. The splitting arises from distortion in theotherwise high-symmetry cages from frozen-out D2O orientational disorder. Above 120 K, furtherline-narrowing signals the onset of H2 diffusion from cage to cage. At the lowest temperature, 1.9 K, thespectrum has Pake powder doublet-like features; the doublet is not fully developed, indicating a broaddistribution of order parameters and energies ε.
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