Abstract
Although a popular choice for obtaining high-resolution solid-state NMR spectra of quadrupolar nuclei, the inherently low sensitivity of the multiple-quantum magic-angle spinning (MQMAS) experiment has limited its application for nuclei with low receptivity or when the available sample volume is limited. A number of methods have been introduced in the literature to attempt to address this problem. Recently, we have introduced an alternative, automated approach, based on numerical simulations, for generating amplitude-modulated pulses (termed FAM-N pulses) to enhance the efficiency of the triple- to single-quantum conversion step within MQMAS. This results in efficient pulses that can be used without experimental reoptimisation, ensuring that this method is particularly suitable for challenging nuclei and systems. In this work, we investigate the applicability of FAM-N pulses to a wider variety of systems, and their robustness under more challenging experimental conditions. These include experiments performed under fast MAS, nuclei with higher spin quantum numbers, samples with multiple distinct sites, low-γ nuclei and nuclei subject to large quadrupolar interactions.
Highlights
Many of the nuclei of interest in inorganic chemistry, mineralogy and materials science, and ~75% of the magnetically-active nuclides in the Periodic Table, are quadrupolar, i.e., possess a nuclear spin quantum number I > 1/2 [1,2,3]
The effect of the FAM-N pulse is similar to a saturation of the transitions, while for the region of higher signal intensity described above, the FAM-N pulse is closer to an inversion
In contrast to most previous methods, there are no restrictions on the pulse lengths that are considered in subsequent units, and the penultimate pulse is automatically reoptimised upon addition of the enabling a much wider variety of pulse trains to be considered
Summary
Many of the nuclei of interest in inorganic chemistry, mineralogy and materials science, and ~75% of the magnetically-active nuclides in the Periodic Table, are quadrupolar, i.e., possess a nuclear spin quantum number I > 1/2 [1,2,3]. There have been a number of investigations into the optimum pulse durations for multiple-quantum filtration, and how these are affected by a variety of experimental conditions [7,11,12,13] The efficiency of this filtration increases with the radiofrequency (rf) field strength of the pulses; this is limited for low-γ nuclei [13]. We extend the investigation to include the application of FAM-N pulses to nuclei with high spin quantum number, materials with multiple distinct (and very different) sites, nuclei with low γ or low abundance and nuclei subject to a significant quadrupolar interaction. The aim is to demonstrate that this conceptually simple approach can be applied to “real” and challenging systems, where sensitivity enhancement is a necessity rather than just a welcome gain
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