Electron-triggered processes in halogenated carboxylates: dissociation pathways in CF3COCl and its clusters

Abstract

Trifluoroacetyl chloride, CF₃COCl, is produced in the Earth's atmosphere by photooxidative degradation of hydrochlorofluorocarbons, and represents a potential source of highly reactive halogen radicals. Despite considerable insight into photochemistry of CF₃COCl, its reactivity towards electrons has not been addressed so far. We investigate the electron ionization and attachment in isolated CF₃COCl molecules and (CF₃COCl)_N, max. N ≥ 10, clusters using a molecular beam experiment in combination with quantum chemical calculations. The ionization of the molecule at 70 eV electron energy leads to strong fragmentation: weakening of the C–C bond yields the CF₃⁺ and COCl⁺ ions, while the fission of the C–Cl bond produces the major CF₃CO⁺ fragment ion. The cluster spectra are dominated by M_n·COCl⁺ and M_n·CF₃CO⁺ ions (M = CF₃COCl). The electron attachment at energies between 1.5 and 11 eV also leads to the dissociation of the molecule breaking either the C–Cl bond at low energies below 3 eV yielding mainly Cl⁻ ions, or dissociating the C–C bond at higher energies above 4 eV leading mainly to CF₃⁻ ions. In the clusters, the intact M_n⁻ ions are stabilized after electron attachment at low energies with contribution of M_n·Cl⁻ fragment ions. At higher energies, the M_n·Cl⁻ fragments dominate the spectra, and C–C bond dissociation occurs as well yielding M_n·CF₃⁻. Interestingly, M_n·Cl₂⁻ ions appear in the spectra at higher energies. We briefly discuss possible atmospheric implications.