Abstract
Chemical analysis via nuclear magnetic resonance (NMR) spectroscopy using permanent magnets, rather than superconducting magnets, is a rapidly developing field. Performing the NMR measurement in the strong heteronuclear J-coupling regime has shown considerable promise for the chemical analysis of small molecules. Typically, the condition for the strong heteronuclear J-coupling regime is satisfied at µT magnetic field strengths and enables high resolution J-coupled spectra (JCS) to be acquired. However, the JCS response to systematic chemical structural changes has largely not been investigated. In this report, we investigate the JCS of C6H6−xFx (x = 0, 1, 2, …, 6) fluorobenzene compounds via simultaneous excitation and detection of 19F and 1H at 51.5 µT. The results demonstrate that JCS are quantitative, and the common NMR observables, including Larmor frequency, heteronuclear and homonuclear J-couplings, relative signs of the J-coupling, chemical shift, and relaxation, are all measurable and are differentiable between molecules at low magnetic fields. The results, corroborated by ab initio calculations, provide new insights into the impact of chemical structure and their corresponding spin systems on JCS. In several instances, the JCS provided more chemical information than traditional high field NMR, demonstrating that JCS can be used for robust chemical analysis.
Highlights
One of the most utilitarian applications of nuclear magnetic resonance (NMR) spectroscopy is toAppl
We have recently developed an low field (LF) NMR instrument that is capable of detecting small sample volumes (200 μL) in a laboratory environment with minimal B0 inhomogeneity (≈35 ppm, 0.07 Hz) over the sample [43]
The results presented indicate that LF NMR should be considered an emerging technique for the chemical analysis of small molecules
Summary
One of the most utilitarian applications of nuclear magnetic resonance (NMR) spectroscopy is toAppl. Sci. 2020, 10, x FOR PEER REVIEW elucidate molecular structure, which can be leveraged to study the structure of small molecules, peptides, proteins, inorganic solids, and glasses [1,2,3]. NMR is most routinely used to determine the determine the structure of low molecular weight organic molecules. Chemical analysis of small structure of low molecular weight organic molecules. Chemical analysis of small molecules uses three molecules uses three important NMR parameters to deduce structure: the Larmor frequency, the important parameters to deduce structure: the Larmor frequency, the chemical and the chemical NMR shift, and the J-couplings. These parameters manifest as the observable peakshift, frequencies
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