Spinning-Frequency-Dependent Narrowband RF-Driven Dipolar Recoupling

Dipolar recoupling techniques of homonuclear spin pairs are commonly used for distance or orientation measurements in solids. Accurate measurements are interfered with by broadening mechanisms. In this publication narrowband RF-driven dipolar recoupling magnetization exchange experiments are performed as a function of the spinning frequency to reduce the effect of zero-quantum T₂ relaxation. To enhance the exchange of magnetization between the coupled spins, a fixed number of rotor-synchronous π-pulses are applied at spinning frequencies approaching the rotational resonance (R²) conditions. The analysis of the powder averaged dipolar decay curves of the spin magnetizations as a function of the spinning frequency provides a quantitative measure of the dipolar coupling. An effective Hamiltonian for this experiment is derived, taking into account all chemical shift parameters of the spins. The length of the nbRFDR mixing time and the number of rotor cycles per π-pulse are optimized by numerical simulations for sensitive probing of the dipolar coupling strength. The zero-quantum T₂ relaxation time can easily be taken into account in the data analysis, because the overall exchange time is almost constant in these experiments. Spinning-frequency-depen-dent nbRFDR experiments near the m = 1 and m = 2 R² condition are shown for doubly ¹³C-labeled hydroxybutyric acid.