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Vibrational structures of the C1s and N1s photoelectron spectra of gas-phase HCN have been investigated using monochromated third-generation synchrotron radiation. Both spectra exhibit resolved fine structure associated with several vibrationally excited states. In the C1s spectrum a single vibrational progression is observed, while the N1s spectrum is more complex. High-level ab initio calculations were performed to simulate the spectra and the agreement with the experimental results is good. Based on the calculations, the C1s ionisation is found to induce vibrations solely in the CN stretching mode with an energy of 280 meV, while the N1s ionisation generates vibrations also in the CH stretching mode with an energy of about 387 meV, as well as combinations of these two modes.

High-resolution molecular X-ray photoelectron spectra of second-row atoms may reveal broadening or even splitting of the 2p 3/2 peak as compared to the 2p 1/2 component of the spectrum. This splitting reflects the lifted degeneracy of atomic 2p orbitals at sites of less than cubic symmetry and is referred to as molecular-field splitting (MFS). The prospect of using second-order Moller–Plesset theory (MP2) for ab initio calculation of the MFS for sulfur 2p 3/2 levels is examined. This method is subsequently applied to compute the MFS in ethynyl, ethenyl, and ethyl sulfur pentafluoride, resulting in values of 80, 116 and 121 meV, respectively. The initial-state contribution to the splitting is analyzed in terms of asymmetry in the electron density at sulfur, on the one hand, and in terms of the sulfur-ligand overlap 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-SCF levels of theory, core-valence electron correlation contributes substantially to the MFS, in proportion to the occupational asymmetry of the S3p shell. The realization that mixing of sulfur atomic orbitals of the same parity may add important contributions to the electric-field gradient at the core, resolves the observed diversity in the relative importance of core-valence electron correlation between different classes of sulfur compounds.

Hartree—Fock effective core potential calculations have been performed for the neutral ground state ( 1 A 1 ) and the first three low-lying ionic states ( 2 B 1 , 2 A 1 and 2 B 2 ) of H 2 S, H 2 Se and H 2 Te. The computational method enabled us to use the same, relatively large, basis for all three molecules. The calculations include potential energy curves for the three ionic states, equilibrium geometries, vibrational frequencies, Franck—Condon factors, and ionization potentials. The results show satisfactory agreement between calculated and experimental values when comparison is possible, and indicate that molecules and ions containing heavy elements may be studied at a satisfactory level with the effective core potential method.

1,6,6a-trithiapentalene and its 2,5-dimethyl derivative have been studied in the gas phase by means of ESCA in order to elucidate the unusual type of SS bonding in these molecules. The high resolution spectra show very wide S 2p lines from the terminal sulfur atoms due to vibrational broadening. The effect of core ionization on the SS bond lengths in trithiapentalene has been estimated using the equivalent core approximation and the CNDO/2 method. Ground and final state potential curves have been calculated and explain qualitatively the differences in line widths for the terminal and central sulfur atoms. The observed chemical shifts agree with predictions from CNDO/2 and ab initio calculations.