Fast radio-frequency amplitude modulation in multiple-quantum magic-angle-spinning nuclear magnetic resonance: Theory and experiments

Multiple-quantum magic-angle-spinning (MQMAS NMR) spectroscopy has become a routine method to obtain high-resolution spectra of quadrupolar nuclei. On e of the main problems in the performance of this experiment has been the p oor efficiency of the radio-frequency pulses used in converting multiple-qu antum coherences to the observable single-quantum signals. As the MQMAS exp eriment is basically an echo experiment this problem can be related to the efficiency with which continuous wave pulses can normally achieve the multi ple- to single-quantum conversion for different crystallites in a spinning powdered sample. In this paper we investigate various aspects involved in t his multiple-to-single quantum conversion, in the hope to facilitate the de vise of new experimental schemes that can lead to significant MQMAS signal enhancements. We examine in particular a recently suggested experiment for MQMAS spectroscopy which employs amplitude-modulated radio-frequency pulses , and which can yield substantial signal and even resolution enhancements o ver the commonly used pulse schemes in MQMAS experiments. The mechanisms of operation of continuous-wave and of amplitude-modulated pulses as applied to the selective manipulation of spin-3/2 coherence elements are examined i n detail, with the aid of the fictitious spin-1/2 formalism in combination with quadrupolar adiabaticity arguments. New insight into the nature of the MQMAS experiment is thus revealed, and the superior performance of suitabl e amplitude modulations toward the formation of MQMAS powder echoes is just ified. Experimental results highlighting the utility of this scheme in samp les possessing multiple quadrupolar sites with varying quadrupolar anisotro pies and chemical shift offsets are demonstrated, as is the relative insens itivity of the new signal-enhancement technique to the actual level of rf i rradiation. Further implications and uses of this new irradiation scheme ar e also briefly discussed. (C) 2000 American Institute of Physics. [S0021-96 06(00)00703-0].