Investigation of the feasibility of amplitude-modulated dipolardecoupling in magic-angle spinning solid-state nuclear magneticresonance

Abstract

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