Brown, S.P., and Wimperis, S. (1997) Two-dimensional multiple-quantum MAS NMR of quadrupolar nuclei: a comparison of methods. Journal of Magnetic Resonance, 128 (1). pp. 42-61. ISSN 1090-7807 (doi:10.1006/jmre.1997.1217)
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Publisher's URL: http://dx.doi.org/10.1006/jmre.1997.1217
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 are 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 spinsI= 3/2 and 5/2, respectively, has a slope of −7/9 or 19/12 with respect to theF2axis. 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-t1experiments are discussed, particularly with regard to ease of implementation and processing. In general, the sensitivities achievable using the split-t1MQMAS experiments are predicted to be similar to those obtainable with other methods and, in the special case of the spinI= 3/2 phase-modulated experiments, are even shown to be slightly superior.
|Glasgow Author(s):||Wimperis, Prof Stephen|
|Authors:||Brown, S.P., and Wimperis, S.|
|College/School:||College of Science and Engineering > School of Chemistry|
|Journal Name:||Journal of Magnetic Resonance|
|Published Online:||15 April 2002|