Anisotropy of the nuclear spin coupling in PH_2-, PH_3, and PH_4+ molecules.

The anisotropy of the nuclear spin–spin coupling tensor, relative to P, H and H, H interacting nuclei, has been investigated by means of extended ab initio calculations in the molecules PH ^- ₂, PH3, and PH ⁺ ₄. The coupled Hartree–Fock perturbed scheme has been employed retaining the nonrelativistic Ramsey Hamiltonians. The most important contributions to the anisotropy in the P, H coupling arise from the Fermi contact-spin-dipolar cross term, which is partially counterbalanced by the spin-orbit effect. The discrepancies emerging between computed and experimental J(P,H) in phosphine and phosphonium are interpreted in terms of neglected electron correlation contributions. A dramatic sensitivity of the Fermi contact term of the molecular geometry has been found for PH₂ ^- . The calculations show that the P–H coupling is more anisotropic than N–H, which is seemingly due to the enhanced nonsphericity of electron distribution.