Recoupling of heteronuclear dipolar interactions in solid-state NMR using symmetry-based pulse sequences

We apply symmetry theorems to the problem of heteronuclear dipolar recoupling in the presence of magic-angle spinning in solid-state NMR. Examples are shown in which the ¹³C NMR signal is acquired while rotor-synchronized pulse sequences with the symmetry R18₁⁷ or R18₂⁵ are applied to the ¹H spins. This allows recoupling of heteronuclear dipolar interactions combined with homonuclear decoupling of the irradiated ¹H spins. The structure of the ¹³C NMR spectrum is sensitive to bond lengths and bond angles. A two-dimensional procedure is described for applications to multiply isotopically labelled systems.