Fragmentation of valence electronic states of CF3–CH2F+ and CHF2–CHF2+ in the range 12–25 eV

Abstract

Tunable vacuum-ultraviolet radiation from a synchrotron source and threshold photoelectron–photoion coincidence spectroscopy have been used to study the decay dynamics of the valence electronic states of CF₃–CH₂F⁺ and CHF₂–CHF₂⁺. The threshold photoelectron spectra, fragment ion yield curves, and breakdown diagrams of CF₃–CH₂F and CHF₂–CHF₂ have been obtained in the photon energy range 12–25 eV, the electrons and fragment ions being detected by a threshold electron analyser and a linear time-of-flight mass spectrometer, respectively. For the dissociation products of (CF₃–CH₂F⁺)* and (CHF₂–CHF₂⁺)* formed via a single-bond cleavage, the mean translational kinetic energy releases have been measured and compared with the predictions of statistical and pure-impulsive mechanisms. Ab initio G2 calculations have determined the minimum-energy geometries of CF₃–CH₂F and CHF₂–CHF₂ and their cations, and deduced the nature of the high-lying valence orbitals of both neutral molecules. Furthermore, enthalpies of formation at 298 K of both neutral molecules, and all the neutral and fragment ions observed by dissociative photoionisation have been calculated. Combining all experimental and theoretical data, the decay mechanisms of the ground and excited valence states of CF₃–CH₂F⁺ and CHF₂–CHF₂⁺ are discussed. The first and second excited states of both ions show some evidence for isolated-state behaviour, with fast dissociation by cleavage of a C–F or C–H bond and a relatively large translational energy released in the two fragments. The ground state of both ions dissociate by cleavage of the central C–C bond, with a much smaller translational energy release. Several fragment ions are observed which form via H-atom migration across the C–C bond; for hν > 18 eV, CH₂F⁺ is even the dominant ion from dissociative photoionisation of CHF₂–CHF₂. New experimental values are determined for the enthalpy of formation at 298 K of CF₃–CH₂F (−905 ± 5 kJ mol⁻¹) and CHF₂–CHF₂ (−861 ± 5 kJ mol⁻¹), with upper limits being obtained for CF₂–CH₂F⁺ (⩽485 ± 7 kJ mol⁻¹), CF₂–CHF₂⁺ (⩽324 ± 7 kJ mol⁻¹) and CHF–CHF₂⁺ (⩽469 ± 7 kJ mol⁻¹).