Rotational resonance in solid state NMR

In magic angle spinning experiments, on samples containing dilute homonuclear dipolar-coupled spin pairs, rotational resonance occurs when the spinning speed is adjusted so that the condition ω_δ^iso=nω_r is satisfied. Here ω_δ^iso is the difference isotropic chemical shifts, ω_r is the spinning speed, and n is an integer. Under these conditions the normally sharp resonance lines broaden and split. In addition, a rapid oscillatory exchange of Zeeman order between the dipolar-coupled spins is observed. The time dependence of the exchange and the spectral lineshapes agree with numerical simulations. The method is potentially useful for estimating through-space dipolar couplings, and therefore internuclear distances in polycrystalline solids.