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° 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 23Na in Na2C2O4 and 87Rb in RbClO4, respectively. We observe no significant anisotropic lineshape distortion. The conditions for optimum enhancement are discussed.