Efficient dipolar recoupling in the NMR of rotating solids. A sevenfold symmetric radiofrequency pulse sequence

A new radiofrequency pulse sequence is introduced for the efficient reintroduction of magnetic dipolar couplings in the magic-angle-spinning NMR of solids. The sequence involves seven phase-shifted radiofrequency pulse cycles, timed to span two rotational periods of the sample. Double-quantum coherences are excited with high efficiency in a rotating powder sample of zinc acetate-13C2.