We have obtained high-resolution solid-state NMR spectra of a variety of nonintegral-spin quadrupolar nuclei (23Na, 51V, and 55Mn) under conditions of "magic-angle" sample-spinning (MASS) and "variable-angle" sample-spinning (VASS). We show for systems in which the second-order quadrupole interaction dominates the breadth of the (1/2,-1/2) spin transition, that optimum line narrowing is achieved by rapid sample rotation at angles other than the familiar θ = 54.7° "magic-angle." The effect originates in the more complex angle dependence of the second-order quadrupole interaction, rather than the more familiar P2 (cos θ) dependence of dipolar and chemical shift interactions, and theoretical VASS line shapes for a variety of spinning angles, and electric field gradient tensor asymmetry parameters (η), are presented. We show that VASS NMR at low fields generates complex spectral line shapes, which at some angles exhibit well-resolved centerbands and spinning sidebands. We also discuss briefly, with examples, the complexities introduced by the presence of dipolar and/or chemical shift anisotropy interactions in such experiments and investigate the question of the optimum field-strengths for NMR of quadrupolar nuclei. Our results indicate that the VASS NMR technique appears to have considerable utility in obtaining high-resolution NMR spectra of a wide variety of quadrupolar nuclei in inorganic solids, especially those with relatively small (≲ 5 MHz) quadrupole coupling constants.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry