Threshold photoelectron photoion coincidence study of the fragmentation of valence states of CF 3 –CH 3+ and CHF 2 –CH 2 F + in the range 12–24 eV

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 CF\(_3\)-CH\(_3^+\) and CHF\(_2\)-CH\(_2\)F\(^+\). Threshold photoelectron spectra and fragment ion yield curves of CF\(_3\)-CH\(_3\) and CHF\(_2\)-CH\(_2\)F 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 (CF\(_3\)-CH\(_3^+\))* and (CHF\(_2\)-CH\(_2\)F\(^+\))* 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 CF\(_3\)-CH\(_3\) and CHF\(_2\)-CH\(_2\)F 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 photoionsiation have also been calculated. Combining all experimental and theoretical data, the fragmentation mechanisms of the ground and excited states of CF\(_3\)-CH\(_3^+\) and CHF\(_2\)-CH\(_2\)F\(^+\) are discussed. The ground state of both ions, formed by electron removal from the C-C \(\sigma\)-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 2p\(\pi\) 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 h\(v\)> 18 eV, CF-CH\(_2^+\) is the dominant fragment ion. New experimental values are determined for the enthalpy of formation at 298 K of CF\(_3\)-CH\(_3\) (-751 \(\pm\) 10 kJ mol\(^{-1}\)) and CHF\(_2\)-CH\(_2\)F (-671 \(\pm\) 12 kJ mol\(^{-1}\)), with upper limits being determined for CF\(_3\)-CH\(_3^+\) (\(\geq\) 546 \(\pm\) 11 kJ mol\(^{-1}\)) and CHF-CH\(_2\)F\(^+\) (\(\geq\) 663 \(\pm\) 13 kJ mol\(^{-1}\)).