Sensitivity-enhanced multiple-quantum MAS NMR for half-integer spin quadrupolar nuclei using WURST-amplitude shaped pulses

Jonas Koppe; Robert Knitsch; Sebastian Wegner; Michael Ryan Hansen

doi:10.1016/j.jmr.2020.106873

Sensitivity-enhanced multiple-quantum MAS NMR for half-integer spin quadrupolar nuclei using WURST-amplitude shaped pulses

J Magn Reson. 2020 Dec:321:106873. doi: 10.1016/j.jmr.2020.106873. Epub 2020 Nov 10.

Authors

Jonas Koppe¹, Robert Knitsch¹, Sebastian Wegner², Michael Ryan Hansen³

Affiliations

¹ Institute for Physical Chemistry, Westfälische Wilhelms-Universität, Corrensstr. 28/30, D-48149 Münster, Germany.
² Bruker Biospin, Silberstreifen 4, D-76278 Rheinstetten, Germany.
³ Institute for Physical Chemistry, Westfälische Wilhelms-Universität, Corrensstr. 28/30, D-48149 Münster, Germany; Center for Multiscale Theory and Computation (CMTC), Westfälische Wilhelms-Universität, Corrensstrasse 40, D-48149 Münster, Germany. Electronic address: mhansen@uni-muenster.de.

PMID: 33221668
DOI: 10.1016/j.jmr.2020.106873

Abstract

Two-dimensional multiple-quantum MAS (MQMAS) NMR spectroscopy is one of the most widely used solid-state NMR techniques for resolving multiple overlapping central-transition lineshapes for half-integer spin quadrupolar nuclei. In particular when relying on nutation-driven MQ coherence transfers, this technique suffers from low sensitivity that can only be improved by increasing the rf-amplitude of the involved radio-frequency (rf) pulses, which are therefore typically operated at the rf-limit. In such situations, frequently encountered for the three-pulse z-filtered and split-t₁ shifted-echo MQMAS NMR sequences, we introduce the advantages of rf-pulses with smoothly truncated amplitude profiles, which we have termed WURST-Amplitude Shaped Pulses (WASPs). When considering the NMR spectrometer hardware, we demonstrate that WASPs feature more suitable properties in comparison to conventional rectangular pulses, enabling a substantial reduction of voltage reflections and transient effects under identical rf-conditions. By employing extensive numerical simulations and experimental validation, we further show that WASPs intrinsically possess a higher potential for nutation-based 3Q excitation involving spin-3/2 and 3Q and 5Q excitation for spin-5/2 quadrupolar nuclei, specifically when large nutation frequencies are available. The concept of smoothly truncating rf-amplitudes is also extended to Fast Amplitude Modulation (FAM) pulses, normally incorporated for rotor-driven 1Q conversion. We additionally evaluate the potential of employing WASPs with peak rf-amplitudes beyond the rf-limit for conventional rectangular rf-pulses.

Keywords: Amplitude modulation; Half-integer spin quadrupolar nuclei; MAS; Multiple-quantum NMR; Shaped pulses; Solid-state NMR.