Investigation of the feasibility of amplitude-modulated dipolar decoupling in magic-angle spinning solid-state nuclear magnetic resonance

The feasibility of using periodic amplitude-modulated radio-frequency (RF) irradiation under `magic-angle spinning' for heteronuclear dipolar decoupling in solid-state nuclear magnetic resonance is addressed. The RF waveforms used are tailored to satisfy the theoretical criterion for decoupling which calls for an irradiated spin propagator which is cyclic in the sense that it equals the identity matrix irrespective of the strength and orientation of the chemical shift and dipolar coupling tensors. This requirement is met by using the Floquet formalism to provide insight into the influence of an arbitrary waveform on the dynamics of the irradiated spin-½ nuclei and invoking perturbation methods to design particular modulation functions which impose the required cyclicity on the propagator. Simple RF modulations which are synchronized with the sample spinning are thus derived analytically. Finally, the validity of the scheme is explored in simple test experiments and the decoupling performance is compared with the traditional `continuous-wave' method and the recently developed technique of `two-pulse phase modulation'.