Sensitivity enhancement of the central transition NMR signal of quadrupolar nuclei under magic-angle spinning

An approach for enhancing the NMR sensitivity of the central transition of spin-3/2 nuclei is presented. Through selective excitation of the satellite transitions using a fast 180 degrees phase alternating pulse train during magic-angle spinning a selectively excited state is prepared where the populations of all eigenstates \ m > with the same sign of m are equal, resulting in an enhanced central m = -1/2 --> 1/2 transition polarization. Numerical simulations predict enhancements up to a factor of 2 and values of 1.7 and 1.9 have been obtained experimentally for Na-23 in Na2C2O4 and Rb-87 in RbClO4, respectively. We observe no significant anisotropic lineshape distortion. The conditions for optimum enhancement are discussed. (C) 2000 Published by Elsevier Science B.V.