Magic angle spinning technique multiple-quantum effects

In order to remove the anisotropic effects of quadrupolar interactions which can obscure MAS spectra other techniques such as Double Rotation (DOR) [18, 19], Dynamic Angle Spinning (DAS) [18] or Multiple Quantum Magic Angle Spinning (MQMAS) have to be employed. Of these the most simple to apply practically is MQMAS, which can be conducted on a standard MAS-equipped NMR spectrometer. A number of texts and review papers discuss MQMAS in significant technical detail [20, 21]. 

Two Dimensional Multiple Quantum MAS. For non-integer spin quadrupolar nuclei a two dimensional (2D) Multiple Quantum Magic Angle Spiiming (2D MQMAS) technique emerged in 1995 that has had a profound effect on their study, and has meant that much higher resolution spectra can be produced from these nuclei in sol-gel materials. This experiment correlates the (m, -m) multiple quantum transition to the (1/2, -1/2) transition (Fig. 3-2(c)). The resolution enhancement stems from the fact that the quadrupole frequencies for both transitions are correlated. At specific times, the anisotropic parts of the quadrupole interaction are refocused and an echo forms. The amplitude of the echo is determined onlyhy isotropic interactions, so that the experiment is repeated at different echo times which forms the 2D data set where the echo amplitude is modulated only by isotropic effects. FT of the echo amplitude then produces an isotropic spectrum that... 

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