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Issue 36, 2020
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t1-Noise eliminated dipolar heteronuclear multiple-quantum coherence solid-state NMR spectroscopy

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Abstract

Heteronuclear correlation (HETCOR) spectroscopy is one of the key tools in the arsenal of the solid-state NMR spectroscopist to probe chemical and spatial proximities between two different nuclei and enhance spectral resolution. Dipolar heteronuclear multiple-quantum coherence (D-HMQC) is a powerful technique that can be potentially utilized to obtain 1H detected 2D HETCOR solid-state NMR spectra of any NMR active nucleus. A long-standing problem in 1H detected D-HMQC solid-state NMR experiments is the presence of t1-noise which reduces sensitivity and impedes spectral interpretation. In this contribution, we describe novel pulse sequences, termed t1-noise eliminated (TONE) D-HMQC, that minimize t1-noise and can provide higher sensitivity and resolution than conventional D-HMQC. Monte-Carlo and numerical simulations confirm that t1-noise in conventional D-HMQC primarily occurs because random MAS frequency fluctuations cause variations in the NMR signal amplitude from scan to scan, leading to imperfect cancellation of uncorrelated signals by phase cycling. The TONE D-HMQC sequence uses 1H π-pulses to refocus the evolution of 1H CSA across each SR421 recoupling block, improving the stability of the pulse sequence to random MAS frequency fluctuations. The 1H refocusing pulses also restore the orthogonality of in-phase and anti-phase magnetization for all crystallite orientations at the end of each recoupling block, enabling the use of 90° flip-back or LG spin-lock trim pulses to reduce the intensity of uncorrelated signals. We demonstrate the application of these methods to acquire 1H detected 2D 1H{35Cl} and 1H{13C} HETCOR spectra of histidine·HCl·H2O with reduced t1-noise. To show generality, we also apply these methods to obtain 2D 1H{17O} spectra of 20%-17O fmoc-alanine and for the first time at natural abundance, 2D 1H{25Mg} HETCOR spectra of magnesium hydroxide. The TONE D-HMQC sequences are also used to probe 1H–25Mg and 1H–27Al proximities in Mg–Al layered double hydroxides and confirm the even mixing of Mg and Al in these materials.

Graphical abstract: t 1-Noise eliminated dipolar heteronuclear multiple-quantum coherence solid-state NMR spectroscopy

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Supplementary files

Article information


Submitted
30 Jun 2020
Accepted
28 Aug 2020
First published
28 Aug 2020

This article is Open Access

Phys. Chem. Chem. Phys., 2020,22, 20815-20828
Article type
Paper

t 1-Noise eliminated dipolar heteronuclear multiple-quantum coherence solid-state NMR spectroscopy

A. Venkatesh, X. Luan, F. A. Perras, I. Hung, W. Huang and A. J. Rossini, Phys. Chem. Chem. Phys., 2020, 22, 20815
DOI: 10.1039/D0CP03511D

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