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) to selective homonuclear polarization transfer under magic angle spinning is proposed. It is shown that, for a spinning speed fast enough to remove the maximum homonuclear dipolar coupling constant omegaD involved, the flip-flop and flop-flop mechanisms are suitable for recoupling the spins with a chemical shift difference larger than omegaD and a difference comparable to or smaller than omegaD, respectively. It is also shown that, for fast polarization transfer, the off-resonance frequencies should be much higher than the RF intensity in the flip-flop condition, while for the flop-flop condition, the off-resonance frequencies should be much lower than the RF intensity. Some one- and two-dimensional experiments are proposed by utilizing the capability of the R2TR method to abruptly switch on and off the recoupling condition, and are demonstrated for triply 13C-enriched l-alanine. The mixing time required for population transfer was found to be ca. 0.5 ms for the methine and methyl 13C spins separated by 1.5 A and ca. 5 ms for the methyl and the carboxyl carbons separated by 2.5 A. The experimental results and theoretical simulations show that selective polarization transfer is achieved when the difference in the isotropic chemical shifts between the relevant pair of spins and a neighboring spin is more than 1000 Hz.