M. Ernst et al., 2ND-ORDER RECOUPLING OF CHEMICAL-SHIELDING AND DIPOLAR-COUPLING TENSORS UNDER SPIN DECOUPLING IN SOLID-STATE NMR, The Journal of chemical physics, 105(9), 1996, pp. 3387-3397
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.