Variable-angle-spinning nuclear magnetic resonance spectra of homonuclear spin pairs consisting of two crystallographically equivalent spins in the solid state

Abstract

Variable-angle-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 spin–spin coupling, J, between two crystallographically equivalent spins is not observable in rapid magic-angle-spinning (MAS) NMR studies, it can be reintroduced by spinning the sample off the magic angle (54.74°). This J-recoupling phenomenon is more general than the previously reported J recoupling via homonuclear dipole–dipole interactions and can be observed for any J-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 of J-coupled nuclei. In contrast to dipolar-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 second-order (AB) features present in high-resolution NMR spectra acquired by rapid sample spinning about an axis tilted slightly off the magic angle.