High-field QCPMG-MAS NMR of half-integer quadrupolar nuclei with large quadrupole couplings

The quadrupole Carr-Purcell-Meiboom-Gill NMR experiment using magic-angle s pinning (QCPMG-MAS) is analysed as a means of determining quadrupolar coupl ing and anisotropic chemical shielding tensors for half-integer (I > 1/2) q uadrupolar nuclei with large quadrupole coupling constants (C-Q) This is ac complished by numerical simulations and Rb-87 NMR experiments wih Rb2SO4 an d Rb2CrO4 using different magnetic fields. It is demonstrated that (i) QCPM G-MAS experiments typically provide a sensitivity gain by more than an orde r of magnitude relative to quadrupolar-echo MAS experiments, (ii) non-secul ar second-order terms do not affect the spin evolution appreciably, and (ii i) the effect of finite RF pulses needs to be considered when 2 omega(Q)(2) /(omega(0)omega(RF)) > 0 1, where omega(Q) = 2 pi C-Q/(4I(2I-1)), omega(RF ) is the RF amplitude, and omega(0) the Larmor frequency. Using numerical s imulations and iterative fitting the magnitudes and relative orientation of Rb-87 quadrupolar coupling and chemical shielding tensors for Rb2SO4 and R b2CrO4 have been determined.