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 serie s, CH4-nCln, are reported. The spectra are compared with results from ab in itio calculations at the MP2 level using the Z+1 equivalent-cores approxima tion. Good agreement is generally obtained, and a detailed evaluation of th e contributing vibrational modes and the resulting geometry changes upon co re ionization is possible. We find that, for all of these molecules, bendin g modes contribute very little to the vibrational structure. Surprisingly, for both C-H and C-Cl bonds, the bond shortening upon core-ionization decre ases with addition of chlorine atoms. For this reason, an intensity model b ased on the linear-coupling model, which has been successfully applied in e arlier studies of simple hydrocarbons, is found to be of limited applicabil ity in predicting the vibrational structure of the chloromethane series. Ve rtical and adiabatic C1s binding energies have been extracted and compared to the number of chlorine atoms. A steady, but nonlinear increase in bindin g energy with addition of chlorine atoms is observed. (C) 1999 American Ins titute of Physics. [S0021-9606(99)00211-1].