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

Abstract

The inherently quantitative nature of nuclear magnetic resonance (NMR) spectroscopy is one of the most attractive aspects of this analytical technique. Quantitative NMR analyses have typically been limited to high-field (>1 T) applications. The aspects for quantitation at low magnetic fields (<1 mT) have not been thoroughly investigated and are shown to be impacted by the complex signatures that arise at these fields from strong heteronuclear Jcouplings. This study investigates quantitation at Earth's magnetic field (∼50 μT) for a variety of samples in strongly, weakly, and uncoupled spin systems. To achieve accurate results in this regime, the instrumentation, experimental acquisition, processing, and theoretical aspects must be considered and reconciled. Of particular note is the constant field nuclear receptivity equation, which has been re-derived in this study to account for strong coupling and quality factor effects. The results demonstrate that the quantitation of homonuclear molecular groups, determination of heteronuclear pseudoempirical formulas, and mixture analysis are all feasible at Earth's magnetic field in a greatly simplified experimental system.

Reactivity of [(PNP)Mn(CO)2] with Organophosphates

“…The related resonance in the 19 F NMR spectrum was found as a doublet centered at δ −44.88 ( J P–F = 965 Hz). These shifts are similar to the spectrum of MF in C 6 D 6 …”

“…The chemicals KHMDS, NaHMDS, diisopropylfluorophosphate, dimethylphosphate, and diisopropylphosphate were ordered from Millipore Sigma and were used after degassing. The complexes ( i Pr PN H P)­Mn­(CO) 2 Br, ( i Pr PN H P)­Mn­(CO) 2 ( 1 ), ( i Pr PN H P)­Mn­(OH)­(CO) 2 ( 2 ), and monofluor were prepared as previously described. ( i Pr PN H P)­Mn­( 17 OH)­(CO) 2 ( 17 O-2 ) was prepared analogously to 2 using 17 OH 2 .…”

“…These shifts are similar to the spectrum of MF in C 6 D 6 . 58 Complex 1 displayed productive reactivity with both neutral P− OH moieties and an anionic P−O − substrate. This breadth of [( iPr PNP)Mn(CO) 2 ]-coordinated phosphate molecules (Table 1) provides a foundation to study similar reactivity with live agents.…”

Chemical Analysis of an Isotopically Labeled Molecule Using Two-Dimensional NMR Spectroscopy at 34 μT

“…In addition, the heteronuclear J -coupling constant can be measured with high precision, , and efficient hyperpolarization using breakthrough techniques is feasible, resulting in significant enhancement of sensitivity. − Despite such benefits, however, fundamental limitations remain: a lack of chemical shift resolution and the invisibility of homonuclear J -coupling have been major hurdles to the widespread use of low-field NMR spectroscopy for chemical analysis. One solution, although limited, is the use of isotopically labeled molecules such as 1- 13 C-ethanol (CH 3 13 CH 2 OH), as shown in Figure a. ,− The labeled nuclear spin lifts the degeneracy in the resonance frequency through a strong J -coupling effect. The one-dimensional (1D) NMR spectrum, then, becomes highly complicated but reflects all J -couplings, including homonuclear coupling, which is otherwise invisible in low magnetic fields. ,,,− Given the position of the labeled nucleus within the molecule, structural analysis using low-field NMR spectroscopy can be feasible.…”