Homonuclear Decoupling of H-1 Dipolar Interactions in Solids by means of Heteronuclear Recoupling
Line narrowing has been traditionally achieved in solid-state H-1 NMR spectroscopy by applying pulse sequences that combine multiple-pulse operations with magic-angle spinning (MAS), to effectively average out the dipoledipole homonuclear Hamiltonian. The present study explores a new alternative that departs from the usual concept of directly acting on the strongly coupled spins with radiofrequency pulses; instead, we seek to achieve a net homonuclear dipolar decoupling in solids by exploring the reintroduction of MAS-averaged heteronuclear dipolar couplings between the H-1 nuclei and directly bonded C-13 or N-15 nuclei. This recouplinganti-recoupling (RaR) scheme thus relies on the recoupling of the dipolar interaction with heteronuclear spins, which, under fast MAS, will exceed the strength and will not commute with the homonuclear (HH)-H-1-H-1 coupling one is intending to average out. Subsequent removal (antiRecoupling) of these heteronuclear interactions can lead to narrowed H-1 resonances, without ever pulsing on the aforementioned channel. The line-narrowing properties of RaR are illustrated with numerical simulations and with experiments on model organic solids.