Spinning-frequency-dependent narrowband RF-driven dipolar recoupling

Dipolar recoupling techniques of homonuclear spin pairs are commonly used f or distance or orientation measurements in solids. Accurate measurements ar e interfered with by broadening mechanisms. In this publication narrowband RF-driven dipolar recoupling magnetization exchange experiments are perform ed as a function of the spinning frequency to reduce the effect of zero-qua ntum T-2 relaxation. To enhance the exchange of magnetization between the c oupled spins, a fixed number of rotor-synchronous pi-pulses are applied at spinning frequencies approaching the rotational resonance (R-2) conditions. The analysis of the powder averaged dipolar decay curves of the spin magne tizations as a function of the spinning frequency provides a quantitative m easure of the dipolar coupling. An effective Hamiltonian for this experimen t is derived, taking into account all chemical shift parameters of the spin s. The length of the nbRFDR mixing time and the number of rotor cycles per pi-pulse are optimized by numerical simulations for sensitive probing of th e dipolar coupling strength. The zero-quantum T-2 relaxation time can easil y be taken into account in the data analysis, because the overall exchange time is almost constant in these experiments. Spinning-frequency-dependent nbRFDR experiments near the m = 1 and m = 2 R-2 condition are shown for dou bly C-13-labeled hydroxybutyric acid. (C) 2000 Academic Press.