Chemical Analysis of Fluorobenzenes via Multinuclear Detection in the Strong Heteronuclear J-Coupling Regime

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.

Quantitation of Nuclear Magnetic Resonance Spectra at Earth’s Magnetic Field

“…Finally, the authors would like to correct the previously reported simulated errors on a system of fluorobenzene compounds simulations. 23 The quantitative results in the previous study produced larger errors than observed in the present study, even for simulated systems after accounting for the Ξ differences between 19 F and 1 H. In this case, normalizing the peaks by Ξ local was insufficient to rectify the errors in the simulation. The errors in quantitating the simulated spectra are on the order of 4−7% below the predicted stoichiometry and always tend to larger 19 F concentrations.…”

“…A previous report had demonstrated quantitation of 19 F and 1 H, two spin-1/2 nuclei separated by only ∼140 Hz. 23 We have extended this to single spectrum detection of 7 Li and 1 H, in which the Larmor frequencies are separated by ∼1.2 kHz and contain a quadrupolar nucleus. The 7 Li and 1 H spin systems are uncoupled, and the JCS of the mixture is shown in Figure 2.…”

“…22−24 While chemical shifts for other nuclei such as 19 F and 129 Xe have been observed, it is unlikely that quantitation at Earth's magnetic field via chemical shift differences will be ubiquitous for all nuclei and closely related molecules. 23,25,26 Instead of chemical shifts, heteronuclear J-couplings break the magnetic equivalence of the nuclear spins at these μT magnetic fields and result in a unique spectral signature: the J-coupled spectrum (JCS). 27 Recent studies have demonstrated unique JCS between small molecules with similar chemical structures in the absence of discernible chemical shifts.…”

“…27 Recent studies have demonstrated unique JCS between small molecules with similar chemical structures in the absence of discernible chemical shifts. [22][23][24]28,29 Quantitation of the JCS can only proceed by understanding the spin systems in the molecule and their corresponding resonances.…”

Editorial on Special Issue “Applications of Low Field Magnetic Resonance”

“…In [8], advances in the development of a Field Programmable Gate Array (FPGA) based spectrometers for NMR relaxation is described along with applications in NMR cryoporometry. Performing the NMR measurement in the strong heteronuclear J-coupling regime is described in [9] and shows promise for the chemical analysis of small molecules. The results demonstrate that high resolution J-coupled spectra 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 times, are all measurable and are differentiable between molecules at low magnetic fields.…”