17O NMR and crystalline hydrates

✍ Scribed by Q.W. Zhang; H.M. Zhang; M.G. Usha; R.J. Wittebort

• Book ID: 104357810
• Publisher: Elsevier Science
• Year: 1996
• Tongue: English
• Weight: 768 KB
• Volume: 7
• Category: Article
• ISSN: 0926-2040
• DOI: 10.1016/s0926-2040(96)01258-1

✦ Synopsis

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 (e2qQ/h approximately 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 e2qQ/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 H2(17)O quadrupole coupling tensor in alpha-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.