Solid-state NMR heteronuclear dipolar recoupling using off-resonance symmetry-based pulse sequences

A new approach to heteronuclear dipolar recoupling in solid-state NMR which combines symmetry-based, gamma-encoded dipolar recoupling with off-resonance rf irradiation is described. By irradiating the two spin species with offsets of equal magnitude but opposite sign, the dipolar scaling factor may be increased while using lower rf field strengths than under on-resonance conditions. Larger scaling makes the experiment less susceptible to relaxation, while released rf field requirements facilitate combination with fast magic-angle spinning while maintaining efficient H-1 decoupling. Furthermore, homonuclear dipolar couplings may be suppressed by adjusting the off-resonance irradiation to the Lee-Goldburg condition. (C) 2003 Elsevier Science B.V. All rights reserved.