NMR polarization transfer by second-order resonant recoupling: RESORT

We describe a resonant second-order dipolar recoupling condition for magic-angle spinning (MAS) solid-state NMR where the active spins are irradiated by continuous-wave irradiation and the passive spins by a phase-alternating sequence. The phase-modulation frequency is matched to an integer multiple of the MAS frequency to produce a second-order homonuclear dipolar coupling Hamiltonian that promotes broadband homonuclear zero-quantum polarization transfer on the spin-locked active spins. The recoupling sequence is based on second-order cross terms between two heteronuclear dipolar couplings. Similarities and differences to the proton-assisted recoupling sequence are discussed.