Investigation of the feasibility of amplitude-modulated dipolar decouplingin 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 decoupli ng in solid-state nuclear magnetic resonance is addressed. The RF waveforms used are tailored to satisfy the theoretical criterion for decoupling whic h 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 me t by using the Floquet formalism to provide insight into the influence of a n arbitrary waveform on the dynamics of the irradiated spin-1/2 nuclei and invoking perturbation methods to design particular modulation functions whi ch impose the required cyclicity on the propagator. Simple RF modulations w hich are synchronized with the sample spinning are thus derived analyticall y Finally, the validity of the scheme is explored in simple test experiment s and the decoupling performance is compared with the traditional 'continuo us-wave' method and the recently developed technique of 'two-pulse phase mo dulation'.