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) dec oupled spectra under magic-angle sample spinning conditions is reexami ned. It is shown that an important contribution to the line broadening comes from a second-order recoupling of the heteronuclear dipolar-cou pling tensor and the chemical-shielding tensor of the irradiated spin. Such an interference between the two tensors leads to a sum of a zero th-rank, a second-rank, and a fourth-rank tensor component in the Hami ltonian. The zeroth-rank and the fourth-rank tensor components are not averaged out under magic-angle sample spinning (MAS) conditions, requ iring the use of higher-order averaging such as double rotation (DOR) for obtaining narrow lines. This broadening is distinctly different fr om off-resonance decoupling effects which transform as a second-rank t ensor and can be averaged out by MAS. The properties of this second-or der recoupling as a source of structural information are explored, and the conditions for removing the broadening in systems with weak homon uclear dipolar-coupling networks are discussed. (C) 1996 American Inst itute of Physics.