Variablehyphen;anglehyphen;spinning nuclear magnetic resonance spectra of homonuclear spin pairs consisting of two crystallographically equivalent spins in the solid state

摘要

Variablehyphen;anglehyphen;spinning (VAS) nuclear magnetic resonance (NMR) spectra arising from a pair of crystallographically equivalent but magnetically nonequivalent spins have been investigated under conditions of rapid sample spinning. It is demonstrated that although the indirect spinndash;spin coupling,J, between two crystallographically equivalent spins is not observable in rapid magichyphen;anglehyphen;spinning (MAS) NMR studies, it can be reintroduced by spinning the sample off the magic angle (54.74deg;). ThisJhyphen;recoupling phenomenon is more general than the previously reportedJrecoupling via homonuclear dipolendash;dipole interactions and can be observed for anyJhyphen;coupled spin pair consisting of two crystallographically equivalent but magnetically nonequivalent nuclei. In addition, we demonstrate that VAS NMR spectra contain information concerning the relative orientation of two chemical shielding tensors which involve a pair ofJhyphen;coupled nuclei. In contrast to dipolarhyphen;chemical shift NMR techniques which depend on the magnitude and orientation of the homonuclear dipolar interaction, the VAS method does not depend on the dipolar interaction; instead, the method makes use of secondhyphen;order (AB) features present in highhyphen;resolution NMR spectra acquired by rapid sample spinning about an axis tilted slightly off the magic angle.