THE ELECTRONIC-STRUCTURE OF THE CARBONYL DIHALIDES, COF2, COCL2, COBR2, COCLF, COBRF AND COBRCL - A COMBINED COMPUTATIONAL AND EXPERIMENTAL-STUDY

The He-I, -II photoelectron (PE) spectra of the carbonyl dihalides, CO F2, COCl2 and COBr2, and the He I spectra of the mixed carbonyl halide s COCIF, COBrF and COBrCl have been recorded. The assignments of the p hotoelectron peaks are discussed with reference to ab initio electroni c structure calculations in the Hartree-Fock. configuration interactio n (CI) and Green's function (OVGF and 2ph-TDA) formalisms and the pref erred assignments tabulated. Koopmans' theorem results in a systematic overestimate of the ionisation energies whereas the two Green's funct ion methods give ionisation energies in very good agreement with the s pectra, with errors of as little as 0.3 eV for the outer valence orbit als. A long standing problem with the assignments of the photoelectron spectra of COCl2 was resolved by the use of multi-reference CI calcul ations on the low-lying states-of the ion. The trends. in the halogen- dependent ionisation energies are discussed in terms of the halide ind uctive effects and the energies of the halogen lone-pair orbitals rela tive to the pi-bonding orbital of CO.