Efficient Amplitude-Modulated Pulses for Triple- to Single-Quantum Coherence Conversion in MQMAS NMR

The conversion between multiple-
and single-quantum coherences
is integral to many nuclear magnetic resonance (NMR) experiments of
quadrupolar nuclei. This conversion is relatively inefficient when
effected by a single pulse, and many composite pulse schemes have
been developed to improve this efficiency. To provide the maximum
improvement, such schemes typically require time-consuming experimental
optimization. Here, we demonstrate an approach for generating amplitude-modulated
pulses to enhance the efficiency of the triple- to single-quantum
conversion. The optimization is performed using the SIMPSON and MATLAB
packages and results in efficient pulses that can be used without
experimental reoptimisation. Most significant signal enhancements
are obtained when good estimates of the inherent radio-frequency nutation
rate and the magnitude of the quadrupolar coupling are used as input
to the optimization, but the pulses appear robust to reasonable variations
in either parameter, producing significant enhancements compared to
a single-pulse conversion, and also comparable or improved efficiency
over other commonly used approaches. In all cases, the ease of implementation
of our method is advantageous, particularly for cases with low sensitivity,
where the improvement is most needed (e.g., low gyromagnetic ratio
or high quadrupolar coupling). Our approach offers the potential to
routinely improve the sensitivity of high-resolution NMR spectra of
nuclei and systems that would, perhaps, otherwise be deemed “too
challenging”.