2-DIMENSIONAL MULTIPLE-QUANTUM MAS NMR OF QUADRUPOLAR NUCLEI - A COMPARISON OF METHODS

Multiple-quantum magic-angle-spinning (MQMAS) NMR experiments have rec ently been used to remove second-order broadening from the central tra nsition of half-integer quadrupolar nuclei. In this paper, the various methods that have been proposed for obtaining pure absorption-mode Li neshapes in MQMAS experiments are described and compared. The methods can be classified according to whether the data are amplitude-or phase -modulated as a function of the evolution period, t(1). Both classes o f experiment are usually performed in such a way that the inhomogeneou s 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 respe ct to the F-2 axis. This paper shows, however, that there are disadvan tages associated with recording the data in this fashion and demonstra tes, in particular, that a shearing transformation of the final two-di mensional spectrum can lead to distorted Lineshapes. Novel amplitude-a nd phase-modulated ''split-t(1)'' MQMAS experiments are introduced whi ch fully refocus the second-order broadening during the evolution peri od, t(1), thereby avoiding the need for a shearing transformation. The considerable practical advantages of these split-t(1) experiments are discussed, particularly with regard to ease of implementation and pro cessing. In general, the sensitivities achievable using the split-t(1) MQMAS experiments are predicted to be similar to those obtainable wit h other methods and, in the special case of the spin I = 3/2 phase-mod ulated experiments, are even shown to be slightly superior. (C) 1997 A cademic Press.