Second-order recoupling of chemical-shielding and dipolar-coupling tensors under spin decoupling in solid-state NMR

Publisher Website
Google Scholar
Abstract
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.