COMBINED EFFECT OF 2ND-ORDER QUADRUPOLE COUPLING AND CHEMICAL SHIELDING ANISOTROPY ON THE CENTRAL TRANSITION IN MAS NMR OF QUADRUPOLAR NUCLEI - RB-87 MAS NMR OF RBCLO4

The combined effect of the tensorial interactions from both second-ord er quadrupole coupling and chemical shielding on the central transitio n in magic-angle spinning (MAS) NMR of half-integer quadrupolar nuclei has been convincingly observed and analyzed for the first time. Rb-87 MAS NMR spectra of RbClO4 at 7.1, 9.4, and 11.7 T and at different sp inning speeds serve as the illustrative example. The spectra are analy zed by iterative fitting of their complex line shapes using a modifica tion of our simulation software recently described and employed for de termination of the same two interactions from first-order MAS NMR spec tra of all (central and satellite) transitions. Accurate and consisten t values for the eight parameters describing the magnitudes and relati ve orientation of the two tenser interactions are obtained from the di fferent spectra. The parameters determined from Rb-87 MAS NMR are of h igher accuracy than the values earlier reported from static-powder Rb- 87 NMR of RbClO4. The relative orientation (Euler angles psi, chi, and xi) determined for the two tensorial interactions conforms to the cry stal structure for RbClO4 in an excellent manner.