Efficient amplitude-modulated pulses for triple- to single-quantum coherence conversion in MQMAS NMR
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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".
Colaux , H , Dawson , D M & Ashbrook , S E 2014 , ' Efficient amplitude-modulated pulses for triple- to single-quantum coherence conversion in MQMAS NMR ' Journal of Physical Chemistry A , vol 118 , no. 31 , pp. 6018-25 . DOI: 10.1021/jp505752c
Journal of Physical Chemistry A
© 2014 American Chemical Society. This is an Open Access article distributed in accordance with the terms of the Creative Commons Attribution (CC-BY) license, which permits others to distribute, remix, adapt and build upon this work, for commercial use, provided the original work is properly cited
We thank EPSRC (EP/E041825/1 and EP/J501542/1) for support, for the award of a studentship to H.C. We also thank the ERC (EU FP7 Consolidator Grant 614290 “EXONMR”).
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