Trifluoroacetic Acid and Trifluoroacetic Anhydride Radical Cations Dissociate near the Ionization Limit

The threshold photoelectron spectra (TPES) and ion dissociation breakdown curves for trifluoroacetic acid (TFA) and trifluoroacetic anhydride (TFAN) were measured by imaging photoelectron photoion coincidence spectroscopy employing both effusive room-temperature samples and samples introduced in a seeded molecular beam. The fine structure in the breakdown diagram of TFA mirroring the vibrational progression in the TPES suggests that direct ionization to the X̃⁺ state leads to parent ions with a lower "effective temperature" than nonresonant ionization in between the vibrational progression. Composite W1U, CBS-QB3, CBS-APNO, G3, and G4 calculations yielded an average ionization energy (IE) of 11.69 ± 0.06 eV, consistent with the experimental value of 11.64 ± 0.01 eV, based on Franck-Condon modeling of the TPES. The measured 0 K appearance energies (AE_0K) for the reaction forming CO₂H⁺ + CF₃ from TFA were 11.92 for effusive data and 11.94 ± 0.01 eV for molecular beam data, consistent with the calculated composite method 0 K reaction energy of 11.95 ± 0.08 eV. Together with the 0 K heats of formation (Δ_fH_0K) of CO₂H⁺ and CF₃, this yields a Δ_fH_0K of neutral TFA of -1016.6 ± 1.5 kJ mol^-1 (-1028.3 ± 1.5 kJ mol^-1 at 298 K). TFAN did not exhibit a molecular ion at room-temperature conditions, but a small signal was observed when rovibrationally cold species were probed in a molecular beam. The two observed dissociation channels were CF₃C(O)OC(O)⁺ + CF₃ and the dominant, sequential reaction CF₃CO⁺ + CF₃ + CO₂. Calculations revealed a low-energy isomer of ionized TFAN, incorporating the three moieties CF₃CO⁺, CF₃, and CO₂ joined in a noncovalent complex, mediating its unimolecular dissociation.