Trifluoroacetic Acid and Trifluoroacetic Anhydride Radical Cations Dissociate near the Ionization Limit.
Lukas LesniakJuana SalasJake BurnerMalick DiedhiouMaxi Burgos PaciAndras BodiPaul M MayerPublished in: The journal of physical chemistry. A (2019)
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 (AE0K) 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 (ΔfH0K) of CO2H+ and CF3, this yields a ΔfH0K 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.
Keyphrases
- cystic fibrosis
- room temperature
- density functional theory
- molecular dynamics simulations
- ionic liquid
- electron transfer
- single molecule
- molecular dynamics
- high resolution
- gas chromatography
- electronic health record
- monte carlo
- air pollution
- machine learning
- liquid chromatography
- simultaneous determination
- energy transfer
- artificial intelligence
- data analysis