Threshold photoelectron photoion coincidence study of the fragmentation of valence states of CF3–CH3+ and CHF2–CH2F+ in the range 12–24 eV

Abstract

Using tunable vacuum-ultraviolet radiation from a synchrotron source, threshold photoelectron photoion coincidence spectroscopy has studied the unimolecular decay dynamics of the valence electronic states of CF₃–CH₃⁺ and CHF₂–CH₂F⁺. Threshold photoelectron spectra and fragment ion yield curves of CF₃–CH₃ and CHF₂–CH₂F have been recorded in the range 12–24 eV, electrons and ions being detected by a threshold electron analyser and a linear time-of-flight mass spectrometer, respectively. For the dissociation products of (CF₃–CH₃⁺)* and (CHF₂–CH₂F⁺)* formed via cleavage of a single covalent bond, the mean translation kinetic energy releases have been measured and compared with the predictions of statistical and impulsive mechanisms. Ab initio G2 calculations have determined the minimum energies of CF₃–CH₃ and CHF₂–CH₂F and their cations, with their geometries optimised at the MP2(full)/6-31G(d) level of theory. The nature of the valence orbitals of both neutral molecules has also been deduced. Enthalpies of formation of both titled molecules and all fragment ions and neutrals observed by dissociative photoionisation have also been calculated. Combining all experimental and theoretical data, the fragmentation mechanisms of the ground and excited states of CF₃–CH₃⁺ and CHF₂–CH₂F⁺ are discussed. The ground state of both ions, formed by electron removal from the C–C σ-bonding highest occupied molecular orbital, is stable only over a narrow range of energies in the Franck–Condon region; it dissociates by C–C bond cleavage with a small fractional translational energy release. Low-lying excited states of both ions, produced by electron removal from F 2pπ nonbonding orbitals, show some evidence for isolated-state behaviour, with impulsive dissociation by cleavage of a C–F bond and a larger fractional translational energy release into the two fragments. For energies above ca. 16 eV smaller fragment ions, often resulting from cleavage of multiple bonds and HF elimination, are observed; for both molecules with hν > 18 eV, CF–CH₂⁺ is the dominant fragment ion. New experimental values are determined for the enthalpy of formation at 298 K of CF₃–CH₃ (−751 ± 10 kJ mol⁻¹) and CHF₂–CH₂F (−671 ± 12 kJ mol⁻¹), with upper limits being determined for CF₂–CH₃⁺ (≤546 ± 11 kJ mol⁻¹) and CHF–CH₂F⁺ (≤663 ± 13 kJ mol⁻¹).