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 spinning (QCPMG-MAS) is analysed as a means of determining quadrupolar coupling and anisotropic chemical shielding tensors for half-integer (I > 1/2) quadrupolar nuclei with large quadrupole coupling constants (C Q ). This is accomplished by numerical simulations and 87 Rb NMR experiments wih Rb 2 SO 4 and Rb 2 CrO 4 using different magnetic fields. It is demonstrated that (i) QCPMG-MAS experiments typically provide a sensitivity gain by more than an order of magnitude relative to quadrupolar-echo MAS experiments, (ii) non-secular second-order terms do not affect the spin evolution appreciably, and (iii) the effect of finite RF pulses needs to be considered when 2ω 2 Q /(ω 0 ω RF ) > 0.1, where ω Q = 2πC Q /(4I(2I-1)), ω RF is the RF amplitude, and ω 0 the Larmor frequency. Using numerical simulations and iterative fitting the magnitudes and relative orientation of 87 Rb quadrupolar coupling and chemical shielding tensors for Rb 2 SO 4 and Rb 2 CrO 4 have been determined.