Two-Dimensional Multiple-Quantum MAS NMR of Quadrupolar Nuclei. A Comparison of Methods

Abstract
Multiple-quantum magic-angle-spinning (MQMAS) NMR experiments have recently been used to remove second-order broadening from the central transition of half-integer quadrupolar nuclei. In this paper, the various methods that have been proposed for obtaining pure absorption-mode lineshapes in MQMAS experiments are described and compared. The methods can be classified according to whether the data is amplitude- or phase-modulated as a function of the evolution period, t₁. Both classes of experiment are usually performed in such a way that the inhomogeneous quadrupolar broadening is spread out along a ridge which, for spins I = 3/2 and 5/2, respectively, has a slope of -7/9 or 19/12 with respect to the F₂ axis. This paper shows, however, that there are disadvantages associated with recording the data in this fashion and demonstrates, in particular, that a shearing transformation of the final two-dimensional spectrum can lead to distorted lineshapes. Novel amplitude- and phase-modulated "split-t₁" MQMAS experiments are introduced which fully refocus the second-order broadening during the evolution period, t₁, thereby avoiding the need for a shearing transformation. The considerable practical advantages of these split-t₁ experiments are discussed, particularly with regard to ease of implementation and processing. In general, the sensitivities achievable using the split-t₁ MQMAS experiments are predicted to be broadly similar to those obtainable with other methods and, in the special case of the spin I = 3/2 phase-modulated experiments, are even shown to be slightly superior.