On Bloch-Siegert effects in MAS NMR of spin-3/2 nuclei: quasi-isotropic 1D spectra

Numerical investigations of Bloch-Siegert effects in MAS NMR of I = 3/2 quadrupolar nuclei unveil a quasi-isotropic component of the satellite-transition single-quantum coherence. The resulting quasi-isotropic peaks allow to discern, in a 1D spectrum, different chemical environments that would otherwise be overlapped in their conventional central- and satellite-transition lineshapes. The site-specific residual inhomogeneity depends on the offset from the carrier and on the strength of the perturbing rf field, and may be completely masked by the typical homogeneous broadenings observed in MAS NMR. The experimental implementation of the method is described. Numerical simulations indicate that the quasi-isotropic component of the observable magnetization is in rf-driven exchange with unobservable double-quantum coherence. An approximate decoupling condition that relates the relevant parameters is given.