Second-order Moller-Plesset perturbation theory for computing molecular-field splitting: application to the S2p(3/2) level in C2H2n+1SF5, n=0, 1, and2

High-resolution molecular X-ray photoelectron spectra of second-row atoms m ay reveal broadening or even splitting of the 2p(3/2) peak as compared to t he 2p(1/2) component of the spectrum. This splitting reflects the lifted de generacy of atomic 2p orbitals at sites of less than cubic symmetry and is referred to as molecular-field splitting (MFS). The prospect of using secon d-order Moller-Plesset theory (MP2) for ab initio calculation of the MFS fo r sulfur 2p,I, levels is examined. This method is subsequently applied to c ompute the MFS in ethynyl, ethenyl, and ethyl sulfur pentafluoride, resulti ng in values of 80, 116 and 121 meV, respectively. The initial-state contri bution to the splitting is analyzed in terms of asymmetry in the electron d ensity at sulfur, on the one hand, and in terms of the sulfur-ligand overla p density and electron density at the groups bonded to sulfur, on the other . At the Koopmans' theorem level of theory, the main source of splitting in the title compounds is found to be s-d hybridization of sulfur. At post-SC F levels of theory, core-valence electron correlation contributes substanti ally to the MFS, in proportion to the occupational asymmetry of the S3p she ll. The realization that mixing of sulfur atomic orbitals of the same parit y may add important contributions to the electric-field gradient at the cor e, resolves the observed diversity in the relative importance of core-valen ce electron correlation between different classes of sulfur compounds. (C) 2001 Elsevier Science B.V. All rights reserved.