Journal of Chemical Physics, Vol.105, No.9, 3387-3397, 1996
2nd-Order Recoupling of Chemical-Shielding and Dipolar-Coupling Tensors Under Spin Decoupling in Solid-State NMR
The source of the residual line broadening in continuous-wave (cw) decoupled spectra under magic-angle sample spinning conditions is reexamined. It is shown that an important contribution to the line broadening comes from a second-order recoupling of the heteronuclear dipolar-coupling tensor and the chemical-shielding tensor of the irradiated spin. Such an interference between the two tensors leads to a sum of a zeroth-rank, a second-rank, and a fourth-rank tensor component in the Hamiltonian. The zeroth-rank and the fourth-rank tensor components are not averaged out under magic-angle sample spinning (MAS) conditions, requiring the use of higher-order averaging such as double rotation (DOR) for obtaining narrow lines. This broadening is distinctly different from off-resonance decoupling effects which transform as a second-rank tensor and can be averaged out by MAS. The properties of this second-order recoupling as a source of structural information are explored, and the conditions for removing the broadening in systems with weak homonuclear dipolar-coupling networks are discussed.
Keywords:NUCLEAR-MAGNETIC-RESONANCE;HIGH-RESOLUTION NMR;MAGIC-ANGLE;ROTATING SOLIDS;C-13 NMR;FIELD;SPECTROSCOPY;ENHANCEMENT;SEQUENCE;WALTZ-16