Phosphorus Compounds and their 31P Chemical Shifts

In the largest and best-known group of phosphorus compounds, the phosphorus exhibits four nearest-neighbor atoms arranged in a roughly tetrahedral configuration, while in the next more common group, there are three nearest-neighbor atoms and an unshared pair of electrons, with the three substituents and the unshared pair again disposed approximately tetrahedrally. A phosphate may be denned as a molecular structure in which each phosphorus is surrounded by an approximately tetrahedral array of four oxygen atoms, with one or more P—O—P linkages per such PO₄ group being formed by sharing of an oxygen between two different PO₄ groups. Phosphorus has only one stable isotype, P; this isotope exhibits a nuclear spin of ½, with a magnetic moment of 1.1305, and no nuclear quadrupole moment. Therefore, phosphorus is an ideal atom for nuclear magnetic resonance determinations. The generally accepted primary reference for P chemical shifts is 85% phosphoric acid, for which the chemical shift is arbitrarily set to zero.