O-17 NMR AND CRYSTALLINE HYDRATES

O-17 NMR is complementary to H-2 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 e ffects of a large O-17 quadrupole coupling (e(2)qQ/h approximate to 7 MHz) on both the orientational dependence of the central transition re sonance frequency and its observation by cross-polarization are examin ed in powders and a single crystal of oxalic acid dihydrate. Compariso n with the exact energies shows that the variation of the central tran sition frequency with tensor orientation is accurately described as a second-order perturbation, even when e(2)qQ/h and the Larmor frequency are comparable. The combined chemical and quadrupole-induced shifts o f the central transition frequency are separated by their distinct tra nsformation properties, and we report the (H2O)-O-17 quadrupole coupli ng tensor in a-oxalic acid dihydrate, which is compared with the hydro gen bonding structure. Excitation of the central transition by cross-p olarization is examined in terms of the O-17 nutation frequency. Disto rtions in powder patterns and the loss of lines in single-crystal spec tra owing to the orientational dependence of the first-order quadrupol e coupling are verified, and simple methods for mitigating the problem in single-crystal and powder experiments are presented.