Vibrational structure of the chloromethane series, CH4-nCln, studied by core photoelectron spectroscopy and ab initio calculations

Vibrationally resolved C1s photoelectron spectra of the chloromethane series, CH4-nCln, are reported. The spectra are compared with results from ab initio calculations at the MP2 level using the Z+1 equivalent-cores approximation. Good agreement is generally obtained, and a detailed evaluation of the contributing vibrational modes and the resulting geometry changes upon core ionization is possible. We find that, for all of these molecules, bending modes contribute very little to the vibrational structure. Surprisingly, for both C-H and C-Cl bonds, the bond shortening upon core-ionization decreases with addition of chlorine atoms. For this reason, an intensity model based on the linear-coupling model, which has been successfully applied in earlier studies of simple hydrocarbons, is found to be of limited applicability in predicting the vibrational structure of the chloromethane series. Vertical and adiabatic C1s binding energies have been extracted and compared to the number of chlorine atoms. A steady, but nonlinear increase in binding energy with addition of chlorine atoms is observed. (C) 1999 American Institute of Physics. [S0021-9606(99)00211-1].