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-C-13(2)]-fumarate in which the internuclear distance, as estimated by X-ray diffraction, is 387.8 pm. The estimated C-13-C-13 dipolar coupling was -136.5+/-5.1 Hz, corresponding to an internuclear distance of 382.5+/-4 pm. (C) 2004 Elsevier B.V. All rights reserved.