The electronic structure of the carbonyl dihalides, COF2, COCl2, COBr2, COClF, COBrF and COBrCl: a combined computational and experimental study

Abstract

The He-I, -II photoelectron (PE) spectra of the carbonyl dihalides, COF₂, COCl₂ and COBr₂, and the He I spectra of the mixed carbonyl halides COClF, COBrF and COBrCl have been recorded. The assignments of the photoelectron peaks are discussed with reference to ab initio electronic structure calculations in the Hartree–Fock, configuration interaction (Cl) and Green's function (OVGF and 2ph-TDA) formalisms and the preferred assignments tabulated. Koopmans' theorem results in a systematic overestimate of the ionisation energies whereas the two Green's function methods give ionisation energies in very good agreement with the spectra, with errors of as little as 0.3 eV for the outer valence orbitals. A long standing problem with the assignements of the photoelectron spectra of COCl₂ was resolved by the use of multi-reference Cl calculations on the low-lying states of the ion. The trends in the halogen-dependent ionisation energies are discussed in terms of the halide inductive effects and the energies of the halogen lone-pair orbitals relative to the π-bonding orbital of CO.