SELECTIVE HOMONUCLEAR POLARIZATION TRANSFER IN THE TILTED ROTATING-FRAME UNDER MAGIC-ANGLE-SPINNING IN SOLIDS

An application of the R2TR method (1995, Chem. Phys. Lett. 232, 424) t o selective homonuclear polarization transfer under magic angle spinni ng is proposed. It is shown that, for a spinning speed fast enough to remove the maximum homonuclear dipolar coupling constant omega(D) invo lved, the flip-flop and flop-flop mechanisms are suitable for recoupli ng the spins with a chemical shift difference larger than omega(D) and a difference comparable to or smaller than omega(D), respectively, It is also shown that, for fast polarization transfer, the off-resonance frequencies should be much higher than the RF intensity in the hip-ho p condition, while for the flop-hop condition, the off-resonance frequ encies should be much lower than the RF intensity. Some one- and two-d imensional experiments are proposed by utilizing the capability of the R2TR method to abruptly switch on and off the recoupling condition, a nd are demonstrated for triply C-13-enriched L-alanine, The mixing tim e required for population transfer was found to be ca. 0.5 ms for the methine and methyl C-13 spins separated by 1.5 Angstrom and ca. 5 ms f or the methyl and the carboxyl carbons separated by 2.5 Angstrom. The experimental results and theoretical simulations show that selective p olarization transfer is achieved when the difference in the isotropic chemical shifts between the relevant pair of spins and a neighboring s pin is more than 1000 Hz. (C) 1997 Academic Press.