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

Abstract

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 87Rb NMR experiments wih Rb2SO4 and Rb2CrO4 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 2ω2 Q/(ω0ωRF) > 0.1, where ωQ = 2πC Q/(4I(2I—1)), ωRF is the RF amplitude, and ω0 the Larmor frequency. Using numerical simulations and iterative fitting the magnitudes and relative orientation of 87Rb quadrupolar coupling and chemical shielding tensors for Rb2SO4 and Rb2CrO4 have been determined.