A robust pulse sequence for the determination of small homonuclear dipolar couplings in magic-angle spinning NMR

We present a new solid-state NMR pulse sequence that accomplishes efficient broad-band dipolar recoupling in systems with small dipolar couplings and large chemical shift anisotropies. The method involves a supercycled symmetry-based recoupling sequence incorporated in a constant-interval data acquisition strategy. The supercycle removes destructive higher-order average Hamiltonian terms, and makes the method more robust at long time intervals. We demonstrate 38.4% double-quantum filtering efficiency on diammonium [1,4-13C2]-fumarate in which the internuclear distance, as estimated by X-ray diffraction, is 387.8 pm. The estimated 13C–13C dipolar coupling was −136.5 ± 5.1 Hz, corresponding to an internuclear distance of 382.5 ± 4 pm.