Enhanced double-quantum nuclear magnetic resonance in spinning solids at rotational resonance

Abstract

The excitation efficiency of double‐quantum (2Q) coherences between pairs of spin‐1/2 nuclei in rotating powdered solids is greatly enhanced at rotational resonance, where an integer multiple of the spinning speed matches the difference in isotropic chemical shifts. In powdered samples it is possible to transfer up to 50% of the single‐quantum magnetization through the 2Q transitions, an order of magnitude more than is achievable in magic‐angle‐spinning experiments performed off rotational resonance. The effect is simulated numerically by integration of the homogeneous evolution propagator and described analytically using first‐order average Hamiltonian theory. Rotational resonance enhanced 2Q filtering is demonstrated experimentally for doubly 1 3C‐labeled samples of zinc acetate and diammonium oxalate monohydrate.