Abstract
17O NMR is complementary to 2H NMR for studies of hydrate structure and dynamics in solids, insofar as the quadrupole coupling parameters can be accurately determined. At a Larmor frequency of 34.2 MHz, the effects of a large 17O quadrupole coupling ( e 2qQ h ≈ 7 MHz ) on both the orientational dependence of the central transition resonance frequency and its observation by cross-polarization are examined in powders and a single crystal of oxalic acid dihydrate. Comparison with the exact energies shows that the variation of the central transition frequency with tensor orientation is accurately described as a second-order perturbation, even when e 2qQ h and the Larmor frequency are comparable. The combined chemical and quadrupole-induced shifts of the central transition frequency are separated by their distinct transformation properties, and we report the H 2 17O quadrupole coupling tensor in α-oxalic acid dihydrate, which is compared with the hydrogen bonding structure. Excitation of the central transition by cross-polarization is examined in terms of the 17O nutation frequency. Distortions in powder patterns and the loss of lines in single-crystal spectra owing to the orientational dependence of the first-order quadrupole coupling are verified, and simple methods for mitigating the problem in single-crystal and powder experiments are presented.
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