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

Abstract

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 (Δf H 0K) of CO2H+ and CF3, this yields a Δf H 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.

Valence Photoionization and Energetics of Vanillin, a Sustainable Feedstock Candidate

“…If the parent ion is metastable close to the dissociative photoionization threshold and the acceleration region is long in the mass spectrometer, slow dissociation rate constants in the 10 3 –10 7 s –1 range result in asymmetrical fragment ion peak shapes and can be extracted based on the ion optics parameters . PEPICO unveiled complex dissociation mechanisms with parallel and sequential steps and delivered accurate thermochemistry and energetics of elusive species. − It can even support ab initio simulations of mass spectra. , …”

Pyrolysis of Trifluoroacetic Acid and Trifluoroacetic Anhydride Studied with Mass Spectrometry and Synchrotron Radiation: Decomposition and Free Radical Formation

“…At low T, m/z 114 and 45 are observed, corresponding to the ionization of the intact TFA molecule and its dissociation product, CO 2 H + . [19] As T increases, these two ions decrease in abundance as the population of neutral TFA decreases (with increasing neutral decomposition). At low T, there are minor signals at m/z 97 (CF 3 CO + ), 86 (CF 3 OH + ), 70 (CF 3 H + ) and 69 (CF 3…”

“…) which also decrease in abundance with increasing T. However, none of these ions is a dissociation product of the TFA molecular ion below 12 eV, [19] and thus are either high energy ion dissociation products or the corresponding neutrals must be formed by thermolysis of TFA only at low T. CF 2 + (m/z 50) and CF 2 O + (m/z 66) grow in abundance with increasing T indicating that CF 2 and CF 2 O are pyrolysis products at these temperatures. In addition, since the choice of 13.4 eV was a compromise, pyrolysis products CO and CO 2 are not seen in Figure 1a, although they were present at 980 °C where the threshold photoelectron (TPES) spectra were recorded.…”

From Energetics to Intracluster Chemistry: Valence Photoionization of Trifluoromethylsulfur Pentafluoride (CF3SF5) by Double Velocity Map Imaging

“…28−30 Both contribute to better defined energetics as benchmarks, ancillary data, and anchors to be used with dissociative photoionization thresholds, as shown in the thermochemistry of the trifluoroacetic acid. 31 Perhalogenated molecules, such as CF 4 , 32,33 CCl 4 , 34,35 and SF 6 , 36,37 often ionize dissociatively because the geometry change between the neutral and the cation is large and the cation is formed on a repulsive part of the potential energy surface at the ionization onset. As observed in CCl 4 35 and CF 3 SF 5 38 and also indicated by the declining kinetic energy release in SF 5 + from SF 6 in the photon energy range of the cation excited states, 37 the dissociation may turn from impulsive to statistical at higher energies when the parent ion lifetime is enhanced by ionization to an excited electronic state.…”

“…When ions dissociate statistically, such as trans -1,3,3,3-tetrafluoropropene (HFO-1234ze), the BD can be modeled to extract accurate dissociative ionization thresholds, which may then yield proton affinities, enthalpies of formation, − and bond energies with unparalleled accuracy . In halogen-containing systems, there are synergies with novel quantum chemical approaches as well as with the Active Thermochemical Tables. − Both contribute to better defined energetics as benchmarks, ancillary data, and anchors to be used with dissociative photoionization thresholds, as shown in the thermochemistry of the trifluoroacetic acid …”