Theoretical investigations of free radicals and negative molecular ions and their calculated photoelectron spectra

Large-scale coupled cluster or coupled electron pair calculations have been carried out for the radicals H2CCCH, CF3 and CCl2F, the anions SiC4- and S i2C5- as well as the corresponding (N-1) electron systems. Accurate equilib rium structures are established in all cases. For the adiabatic ionization potential (IPad) of CF3, the calculations strongly favour the higher experi mental value (9.05+/-0.01 eV). The photoelectron (PE) spectrum of CCl2F is predicted to have a very complex vibrational structure; IPad (CCl2F) is pre dicted to be 8.22 eV. The PE spectrum of SiC4- shows some excitation in the nu(4) and nu(1) stretching vibrational modes and the PE spectrum of Si2C5- is dominated by the adiabatic peak. (C) 2000 Elsevier Science B.V. All rig hts reserved.