Resolution enhancement of magic-angle spinning NMR spectra for paramagnetic solids by zero-quantum NMR

Abstract

13C zero-quantum (ZQ) magic-angle spinning (MAS) NMR spectroscopy for paramagnetic solids is proposed to obtain both highly resolved isotropic peaks and accurate values of shift anisotropies by removing line broadening due to bulk magnetic susceptibility (BMS) shifts. Several pulse sequences for ZQ NMR experiments under fast MAS are presented which are based on homonuclear J couplings between a pair of 13C spins. By using a spin-locking pulse the two spin order is removed while the ZQ coherence is retained at half of the initial intensity. For determining paramagnetic shift anisotropies, ZQ spinning sideband (ssb) patterns are observed which are free from the distortion due to the BMS shifts. To enhance ZQ ssb intensities, a 180° pulse was inserted during the t 1 period of two-dimensional (2D) experiments. A selective excitation scheme is also developed which leads to the improvement of the signal-to-noise ratio of ZQ NMR spectra.