Spectral spin diffusion in polycrystalline solids under magic-angle spinning

Abstract

Spectral spin diffusion in ¹³C NMR of double ¹³C-labelled sodium acetate trihydrate (SAC), and in ³¹P NMR of zinc(II) bis(O,O′-diethyldithiophosphate)(ZNP) has been studied under magic-angle spinning conditions. Spin-diffusion time constants, T_SD, were determined from the intensities of the spinning sidebands in experiments using rotation-synchronized DANTE pulse sequences, at several different spinning frequencies. The theory of Suter and Ernst, developed for spectral spin diffusion in single crystals, was extended to the case of polycrystalline samples rotating under magic-angle spinning conditions. We considered two mechanisms for the spin diffusion, i.e. dipolar interaction and J-coupling. The spin-diffusion time constants, T_SD, were related to the zero-quantum lineshape functions in a manner similar to the theory of Suter and Ernst. The zero-quantum lineshape functions were estimated from the observed single-quantum lineshape functions. In the present studies the dependence of the experimental values for T_SD on the rotational frequency v_r are in good agreement with those calculated from the theory based on the dipolar interaction mechanism. The values of T_SD for SAC showed a deep minimum at Δω≈ 2ω_r, and a shallow minimum at Δω≈ 3ω_r. This phenomenon is rotational relaxation resonance.