Phosphorus-31 NMR

Phosphorus-31 NMR spectroscopy is an analytical chemistry technique that uses nuclear magnetic resonance (NMR) to study chemical compounds that contain phosphorus. Phosphorus is commonly found in organic compounds and coordination complexes (as phosphines), making it useful to measure ³¹P NMR spectra routinely. Solution ³¹P-NMR is one of the more routine NMR techniques because ³¹P has an isotopic abundance of 100% and a relatively high gyromagnetic ratio. The ³¹P nucleus also has a spin of ½, making spectra relatively easy to interpret. The only other highly sensitive NMR-active nuclei spin ½ that are monoisotopic (or nearly so) are ¹H and ¹⁹F.

With a gyromagnetic ratio 40.5% of that for ¹H, ³¹P NMR signals are observed near 202 MHz on an 11.7 Tesla magnet (used for 500 MHz ¹H NMR measurements). Chemical shifts are referenced to 85% phosphoric acid, which is assigned the chemical shift of 0, with positive shifts to low field/high frequency. Due to the inconsistent nuclear Overhauser effect, integrations are not useful. Most often, spectra are recorded with protons decoupled.

³¹P-NMR spectroscopy is useful to assay purity and to assign structures of phosphorus-containing compounds because these signals are well resolved and often occur at characteristic frequencies. Chemical shifts and coupling constants span a large range but sometimes are not readily predictable. The Gutmann-Beckett method uses Et₃PO in conjunction with ³¹P NMR-spectroscopy to assess the Lewis acidity of molecular species.

The ordinary range of chemical shifts ranges from about δ250 to -δ250, which is much wider than typical for ¹H NMR. Unlike ¹H NMR spectroscopy, ³¹P NMR shifts are primarily not determined by the magnitude of the diamagnetic shielding, but are dominated by the so-called paramagnetic shielding tensor (unrelated to paramagnetism). The paramagnetic shielding tensor, σ_p, includes terms that describe the radial expansion (related to charge), energies of excited states, and bond overlap. Illustrative of the effects lead to big changes in chemical shifts, the chemical shifts of the two phosphate esters (MeO)₃PO (δ2.1) and (t-BuO)₃PO (δ-13.3). More dramatic are the shifts for phosphine derivatives H₃P (δ-240), (CH₃)₃P (δ-62), (i-Pr)₃P (δ20), and (t-Bu)₃P (δ61.9).

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