17O NMR and crystalline hydrates

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.