Orientation of the quadrupole and dipole tensors of hydroxyl groups by O-17 quadrupole separated local field NMR

The static O-17 NMR spectra of Mg(OH)(2) and amorphous ME(OH)(x)(OCH3)(2-x) were measured. Simulation of these spectra gave e(2)qQ/h=6.8MHz, eta=0, and delta(iso)=20 ppm, and e(2)qQ/h =7.25 MHz, eta=0, and delta(iso)=-25 ppm for the hydroxyl oxygen in, respectively, Mg(OH)(2) and Mg(OH)(x)(OCH3)(2-x). An OH distance in Mg(OH)(2) of 1.001 Angstrom was obtained using Lee-Goldberg decoupling to obtain the OH dipolar coupling constant. Dipolar oscillations in the H-1-O-17 cross-polarization curve provided an OH distance of 0.995 Angstrom in Mg(OH)(x)(OCH3)(2-x); these oscillations were not observed in Mg(OH)(2). Based on differences in the OH distance and in the O-17 quadrupole coupling constant it was concluded that the OH bond in Mg(OH)(x)(OCH3)(2-x) was more covalent. O-17 2D quadrupole separated local field experiments were performed on both samples in order to obtain the relative orientation of the O-17 quadrupole and OH dipole tensors. In both cases the interaction tensors were found to be collinear. Lee-Goldberg decoupling during the dipolar evolution time t(1) improved the resolution in the dipolar dimension. The resolution in the dipolar dimension of the 2D spectra obtained without proton decoupling during t(1) was better for the Mg(OH)(x)(OCH3)(2-x) sample than for the Mg(OH)(2) sample. Since the effect of the homonuclear proton dipolar coupling was less prominent in the amorphous sample, as evidenced by difference in CP-dynamics and spectral resolution, it was concluded that the hydroxyl groups were more isolated in the amorphous sample. The simulation of 2D separated quadrupole local field spectra showed that the experiment is very sensitive to differences in the relative quadrupole and dipole tensor orientation.