On the effects of quadrupolar relaxation in Earth's field NMR spectra

Adam R Altenhof; Derrick C Kaseman; Harris E Mason; Marc A Alvarez; Michael W Malone; Robert F Williams

doi:10.1016/j.jmr.2023.107540

On the effects of quadrupolar relaxation in Earth's field NMR spectra

J Magn Reson. 2023 Oct:355:107540. doi: 10.1016/j.jmr.2023.107540. Epub 2023 Aug 18.

Authors

Adam R Altenhof¹, Derrick C Kaseman², Harris E Mason³, Marc A Alvarez⁴, Michael W Malone⁵, Robert F Williams⁴

Affiliations

¹ MPA-Q, Los Alamos National Laboratory, Los Alamos, NM 87545, USA. Electronic address: altenhof@lanl.gov.
² NMR Facility, University of California, Davis, Davis, CA 95616.
³ C-IIAC, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
⁴ B-TEK, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.
⁵ MPA-Q, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.

PMID: 37722217
DOI: 10.1016/j.jmr.2023.107540

Abstract

There is growing interest in using low-field magnetic resonance experiments for routine chemical characterization. Earth's field NMR is one such technique that can garner structural information and enable sample differentiation with low cost and highly portable designs. The resulting NMR spectra are primarily influenced by J-couplings, resulting in so-called J-coupled spectra (JCS). Many small molecules include atoms with NMR-active nuclei that are quadrupolar either at natural abundance or are often isotopically enriched (e.g.,²H, ⁶Li, ¹¹B, ¹⁴N, ¹⁷O, etc.) where the effects of quadrupolar J-couplings and relaxation on JCS of strongly- and weakly-coupled spin systems have not been explored to date. Herein, using a set of seven fluoropyridine samples with unique substitution and J-couplings, we demonstrate that the ¹⁴N relaxation rates can induce drastic line-broadening in the JCS. This includes a previously unexplored unique line broadening mechanism enabled by strongly coupled spins at low-field. Numerical simulations are used to model and refine the magnitudes and signs of J-couplings, as well as indirectly determine the ¹⁴N relaxation rates in a single 1D experiment that has a higher fidelity than observed in high-field NMR experiments.

Published by Elsevier Inc.