Double nutation cross-polarization between heteronuclear spins in solids

We study transfer of magnetization from one nuclear spin species to another in solid-state nuclear magnetic resonance by cross-polarization (CP) employing radiofrequency irradiation that causes simultaneous nutations around a pair of orthogonal axes. Under such DOuble NUTation (DONUT), polarization transfer proceeds in an unexplored arena of what we refer to as the nutation frame, which represents the interaction frame with respect to the Hamiltonian that drives nutation. The effect of DONUT is to develop either the zero-quantum or double-quantum secular component of the heteronuclear dipolar interaction, causing flip-flop or flop-flop exchange of the spin states. We demonstrate DONUT CP in polycrystalline adamantane, glycine, and histidine, also examining folding of the CP spectrum under magic-angle spinning as well as the buildup behavior of the magnetization in comparison with the conventional CP scheme. In addition, we put forth a concept of spin relaxation in the nutation frame, which is a straightforward extension of the well-known concept of spin relaxation in the rotating frame.