Zero-Field NMR J-Spectroscopy of Organophosphorus Compounds

Abstract

Organophosphorus compounds are a wide and diverse class of chemicals playing a crucial role in living organisms. This aspect has been often investigated using nuclear magnetic resonance (NMR), which provides information about molecular structure and function. In this paper, we report the results of theoretical and experimental studies on basic organophosphorus compounds using zero-field NMR, where spin dynamics are investigated in the absence of a magnetic field with the dominant heteronuclear J-coupling. We demonstrate that the zero-field NMR enables distinguishing the chemicals owing to their unique electronic environment even though their spin systems have the same alphabetic designation. Such information can be obtained just in a single measurement, while amplitudes and widths of observed low-field NMR resonances enable the study of processes affecting spin dynamics. An excellent agreement between simulations and measurements of the spectra, particularly in the largest frequency J-couplings range ever reported in zero-field NMR, is demonstrated.

Figure 1

(a) Structural formula and heteronuclear J-coupling interactions in trimethyl phosphate (left) and trimethyl phosphite (right). (b) Energy levels and observable transitions for a XA₉ spin system. The manifolds are grouped by the quantum number I_A, and each manifold is labeled by its quantum number F. For clarity, only a single magnetic sublevel is shown in a manifold and just one transition between two states is marked. (c) Experimental and (d) simulated J-spectra of trimethyl phosphate and trimethyl phosphite. The experimental spectra are the result of 128 averaged transients.

Figure 2

Structural formulas and heteronuclear J-couplings for (a) dichlorvos and (b) dimethyl phosphite investigated as examples of (XA_n)B_m systems.

Figure 3

Schematic energy level structure for (a) XA₆ to (b) (XA₆)B spin systems and related observable transitions. For a perturbed XA₆ spin system, v_0–6 denotes the high frequency (>3.5 Hz) ΔF_T = ±1 transitions. The manifolds are grouped by quantum number I_A, and each manifold is labeled by its quantum number F or F_T. Only a single sublevel in each manifold and a single transition at each frequency are shown for clarity. (c) Experimental spectra (blue solid line), transition frequencies (v_0–6) predicted by a first-order perturbation theory (black dashed lines), and the simulated zero-field spectra (solid yellow line) for dichlorvos. The experimental spectrum is the result of 128 averaged transients.

Figure 4

Schematic energy-level structure of the (a) XA and (b) (XA)B₆ spin system with observable transitions (dashed arrows). The high- and low-frequency transitions are denoted by v_1–10 and v_2–9^*, respectively. The manifolds are grouped by the quantum numbers I_A and I_B, and each manifold is labeled by its quantum number F or F_T. (c) Experimental spectra (solid blue line), transition frequencies predicted by a first-order perturbation theory (black dashed lines), and simulated zero-field spectra (solid yellow line) for dimethyl phosphite. The experimental spectrum is the result of 128 averaged transients.

Figure 5

Schematics of the experimental setup. After thermal prepolarization in a 1.8-T Halbach magnet, a sample is transferred to the magnetic shield, to the vicinity of the ⁸⁷Rb vapor cell. The Z-component of the magnetic field, originating from the sample, is measured by monitoring the intensity of light traversing a vapor cell.