Homonuclear decoupling of ¹H dipolar interactions in solids by means of heteronuclear recoupling

Line narrowing has been traditionally achieved in solid-state ¹H NMR spectroscopy by applying pulse sequences that combine multiple-pulse operations with magicangle spinning (MAS), to effectively average out the dipole-dipole 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 ¹H nuclei and directly bonded ¹³C or ¹⁵N nuclei. This recoupling-anti-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 ¹H- ¹H coupling one is intending to average out. Subsequent removal ("antiRecoupling") of these heteronuclear interactions can lead to narrowed ¹H 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.