High-resolution 1H NMR in solids with frequency-switched multiple-pulse sequences.

A new pulse sequence for abundant-spin NMR in solids, called FSLG240W, is demonstrated. The sequence employs phase-coherent frequency-switching of the rf irradiation to induce opposite rotations around the magic-angle axis in the rotating frame. Three 4 pi/3 rotations in one sense are followed by three 4 pi/3 rotations in the opposite sense. Observation windows separate each rotation. The pulse sequence cancels out the average dipolar Hamiltonian, the first-order correction terms, and the most deliterious second-order terms. The pulse sequence has a short cycle time and a high scaling factor kappa = 1/square root of 3, but is sensitive to rf inhomogeneity. We demonstrate slightly better resolution than either MREV8 or BR24 on a small spherical sample of L-alanine.