Ab initio coupled Hartree-Fock calculation of the 31P NMR shielding constants in P_4, P_2, and PN

Ab initio coupled Hartree-Fock perturbation theory using near-Hartree-Fock bases of gaugeless Gaussian orbitals is usedto calculate the ''P NMR shielding constant in P4. The isotropic shielding constant is calculated to be more positive in P4than in PH, by 288 ppm, compared to an experimental difference of 247 h 5 ppm between gaseous PH, and P4 dissolvedin CS,. The calculated shielding anisotropy is -357 ppm, compared to the most recent experimental value of -405 i 10ppm in solid white phosphorus. The isotropic shielding is a strong function of internuclear distance, increasing about 75ppm for a reduction in P-P distance of 0.1 A. In contrast to P4, the isotropic shielding for P2 is calculated to have a negatiuevalue (-299.3 ppm), so that its resonance will occur about 875 ppm upfield from PH,, and a shielding anisotropy of 1902ppm. For the valence isoelectronic molecule PN the calculated isotropic shielding is barely negative (-10 ppm) and the anisotropyis somewhat smaller (1463 ppm), in reasonable agreement with experimental values of +53 and 1376 ppm, respectively.The very large shielding in P4 is a consequence of the very small magnitude of the UP contributions from the highest occupiedorbitals of P4, 6t2 and 2e, which are predominantly P 3p bonding in character. By contrast, the 5ug bonding orbital of P2gives a very large negative contribution to up in directions perpendicular to the P-P bond, arising from CT* - rg* mixingwithin the magnetic field. The low energy of the T* orbital of P2 is thus responsible for its large antishielding.