Coupled Hartree-Fock calculations of 31P NMR shieldings in phosphorous fluorides and PO_4^{3-}

Ab initio coupled Hartree–Fock perturbation theory (CHFPT) calculations on PF3, PF ⁺ ₄ , PF5, PF ^- ₆, and PO^{3 -} ₄ using large polarized Gaussian bases satisfactorily reproduce observed 31P NMR chemical shifts. In PF3 the 31P NMR shielding constant sigma increases as the P–F bond distance or the F–P–F angle is decreased. The 31P chemical shift delta of PF ⁺ ₄ is predicted to be ~−15 ppm (vs 85% H3PO4). Although the average values of the isotropic NMR shieldings are overestimated by the calculations (e.g., sigma^P_av calc. 372.5 ppm vs expt. 259±20 ppm in PF3) the anisotropy in sigmaP is well reproduced (calc. 244.7 ppm vs expt. 228±2 ppm). The ~200 ppm smaller 31P NMR shielding of PF3 compared to PF5 arises partly from a diamagnetic contribution lower by ~100 ppm and partly from the more negative paramagnetic contribution arising from mixing of the 8a1 P lone pair MO with unoccupied orbitals of e symmetry in PF3.