Rotary resonance in multiple-quantum magic-angle spinning

The theory of rotary resonance in multiple-quantum magic-angle spinning (MQMAS) experiment is presented. The rotary resonance effect can enhance the efficiency of MQMAS experiment for high-resolution NMR spectra of half-integer quadrupolar nuclei [J. Chem. Phys. 114 (2001) 4618]. Using a spin-1/2 formalism and Floquet theorem, the spin dynamics of a spin-3/2 under rf irradiation and magic-angle sample spinning is solved analytically in a doubly rotating frame. The results show an oscillatory behavior of MQ excitation with nulls at x1 nxr=2, where x1 represents the rf field strength and xr is the spinning frequency. Efficient MQ excitation occurs between these nulls and MQ conversion peaks at x1 nxr. The origin of the rotary resonance phenomenon is related to frequency shift of a general two-level system under a randomly modulated periodic perturbation. 2002 Elsevier Science B.V. All rights reserved. 1