Phosphorus-31 NMR spectroscopy (NMR stands for nuclear magnetic resonance

Nuclear magnetic resonance

Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...

) is an analytical technique. Solution ³¹P-NMR is one of the more routine NMR techniques because ³¹P has an isotopic abundance of 100% and a relatively high magnetogyric ratio

Magnetogyric ratio

In physics, the gyromagnetic ratio of a particle or system is the ratio of its magnetic dipole moment to its angular momentum, and it is often denoted by the symbol γ, gamma...

. The ³¹P nucleus also has a spin of ½, making spectra relatively easy to interpret. Phosphorus is commonly found in organic compound

Organic compound

An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...

s and coordination complexes (as phosphines), making it useful to measure ³¹P NMR spectra routinely.

Operational aspects

With a magnetogyric ratio 42.5% of that for ¹H, ³¹P NMR signals are observed near 212 MHz on a 11.7 kilogaus magnet (used for 500 MHz ¹H NMR measuements). Chemical shifts are referenced to 85% phosphoric acid

Phosphoric acid

Phosphoric acid, also known as orthophosphoric acid or phosphoric acid, is a mineral acid having the chemical formula H3PO4. Orthophosphoric acid molecules can combine with themselves to form a variety of compounds which are also referred to as phosphoric acids, but in a more general way...

, which is assigned the chemical shift of 0, with positive shifts to low field/high frequency. Due to the inconsistent nuclear Overhauser effect

Nuclear Overhauser effect

The Nuclear Overhauser Effect is the transfer of nuclear spin polarization from one nuclear spin population to another via cross-relaxation. It is a common phenomenon observed by nuclear magnetic resonance spectroscopy. The theoretical basis for the NOE was described and experimentally verified...

, integrations are not useful. Most often, spectra are recorded with protons decoupled.

Applications in chemistry

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.

Chemical shifts

The ordinary range of chemical shifts ranges from abut 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

Paramagnetism

Paramagnetism is a form of magnetism whereby the paramagnetic material is only attracted when in the presence of an externally applied magnetic field. In contrast with this, diamagnetic materials are repulsive when placed in a magnetic field...

). 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).

Coupling constants

One-bond coupling is illustrated by PH₃ where J(P,H) is 189 Hz. Two-bond couplings, e.g. PCH are an order of magnitude smaller. The situation for phosphorus-carbon couplings are more complicated since the two-bond couplings are often larger than one-bond couplings. The J(¹³C,³¹P) values for triphenylphosphine are = -12.5, 19.6, 6.8, 0.3 for one-, two-, three-, and four-bond couplings.

Biological applications

³¹P-NMR spectroscopy is widely used for studies of phospholipid bilayers and biological membranes in native conditions. The analysis of ³¹P-NMR spectra of lipids could provide a wide range of information about lipid bilayer packing, phase transitions (gel phase, physiological liquid crystal phase, ripple phases, non bilayer phases), lipid head group orientation/dynamics, and elastic properties of pure lipid bilayer and as a result of binding of proteins and other biomolecules.

In addition, a specific H-N...(O)-P experiment (INEPT transfer by scalar coupling ³J_H-P~5 Hz) could provide a direct information about formation of hydrogen bonds between amid protons of protein to phosphate of lipid headgroups, which is useful in studies of protein/membrane interactions.