MIRROR recoupling and its application to spin diffusion under fast magic-angle spinning

We introduce a second-order dipolar recoupling condition for magic-angle spinning NMR where the isotropic chemical-shift difference of two homonuclear S spins is matched by the rf-irradiation amplitude, or, alternatively, phase-modulation frequency, on a heteronuclear spin. The experiment can be looked at as a mixed rotational and rotary-resonance condition (MIRROR). The effective MIRROR Hamiltonian promotes homonuclear zero-quantum polarization transfer and heteronuclear zero-quantum and double-quantum polarization transfer. The MIRROR recoupling condition is applied to obtain enhanced polarization transfer through spin diffusion (MIRROR-SD) and to establish through-space neighborhood of spins at high MAS frequencies.