Heteronuclear dipolar recoupling in solid-state nuclear magnetic resonance by amplitude-,phase-,and frequency-modulated Lee-Goldburg cross-polarization

摘要

This paper presents a theoretical,numerical,and experimental study of phase- and frequency-switched Lee-Goldburg cross-polarization (FSLG-CP) under magic-angle spinning conditions.It is shown that a well-defined amplitude modulation of one of the two radio-frequency (rf) fields in the FSLG-CP sequence results in highly efficient heteronuclear dipolar recoupling.The recoupled dipolar interaction is gamma-encoded and,under ideal conditions,the effective spin Hamiltonian is equivalent to that in continuous-wave Lee-Goldburg CP.In practice,however,FSLG-CP is less susceptible to rf field mismatch and inhomogeneity,and provides better suppression of ~1H spin diffusion.The performance of FSLG-CP is experimentally demonstrated on liquid-crystalline samples exhibiting motionally averaged dipolar couplings.