Internuclear distance measurements in solid state nuclear magnetic resonance: Dipolar recoupling via rotor synchronized spin locking

Abstract

The dipolar coupling in rotating solids, averaged to zero when the rotation is about the magic angle, can be reintroduced in the spin-locking interaction frame by rotor-synchronized 90° phase shifts of an applied spin-locking field. This new technique is an interaction frame analogue of previously proposed experiments. Because the spin-locking fields both truncate and play the role of the chemical shift terms in the Hamiltonian, this technique is insensitive to the chemical shifts as well as chemical shift anisotropies of the coupled spins. Through numerical simulations of the magnetization exchange trajectory (e.g., the mixing time dependence of the cross-peak intensity connecting two coupled spins), the through-space distance between two spin nuclei can be estimated. This technique can also be applied to obtain two-dimensional correlation spectra, or alternatively it can be used as a double-quantum filter. Preliminary experiments demonstrate that the cross peaks in the two-dimensional correlation spectra obtained with this technique have a ‘‘negative–positive–negative’’ pattern to the first, second, etc., coupled neighbor. This spectral feature is useful in the assignment of complex magic angle spinning spectra.