Coupled Hartree–Fock calculations of 31P NMR shieldings in phosphorous fluorides and PO3−4

Abstract

A b initio coupled Hartree–Fock perturbation theory (CHFPT) calculations on PF3, PF+4 , PF5, PF−6, and PO3−4 using large polarized Gaussian bases satisfactorily reproduce observed 31P NMR chemical shifts. In PF3 the 31P NMR shielding constant σ increases as the P–F bond distance or the F–P–F angle is decreased. The 31P chemical shift δ 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., σPav calc. 372.5 ppm vs expt. 259±20 ppm in PF3) the anisotropy in σP 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.