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

LESNIAK, LUKAS; SALAS, JUANA; BURNER, JAKE; DIEDHIOU, MALICK; BURGOS PACI, MAXI A; BODI, ANDRAS; MAYER, PAUL M

JOURNAL OF PHYSICAL CHEMISTRY A

AMER CHEMICAL SOC

Lugar: Washington; Año: 2019

1089-5639

The threshold photoelectron spectra (TPES) and iondissociation breakdown curves for trifluoroacetic acid (TFA) andtrifluoroacetic anhydride (TFAN) were measured by imaging photo-electron photoion coincidence spectroscopy employing both effusiveroom-temperature samples and samples introduced in a seededmolecular beam. The fine structure in the breakdown diagram of TFAmirroring the vibrational progression in the TPES suggests that directionization to the X ̃ + state leads to parent ions with a lower ?effectivetemperature? than nonresonant ionization in between the vibrationalprogression. Composite W1U, CBS-QB3, CBS-APNO, G3, and G4calculations yielded an average ionization energy (IE) of 11.69 ± 0.06eV, consistent with the experimental value of 11.64 ± 0.01 eV, based on Franck−Condon modeling of the TPES. The measured0 K appearance energies (AE 0K ) for the reaction forming CO 2 H + + CF 3 from TFA were 11.92 for effusive data and 11.94 ± 0.01eV for molecular beam data, consistent with the calculated composite method 0 K reaction energy of 11.95 ± 0.08 eV. Togetherwith the 0 K heats of formation (Δ f H 0K ) of CO 2 H + and CF 3 , this yields a Δ f 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 signalwas observed when rovibrationally cold species were probed in a molecular beam. The two observed dissociation channels wereCF 3 C(O)OC(O) + + CF 3 and the dominant, sequential reaction CF 3 CO + + CF 3 + CO 2 . Calculations revealed a low-energyisomer of ionized TFAN, incorporating the three moieties CF 3 CO + , CF 3 , and CO 2 joined in a noncovalent complex, mediatingits unimolecular dissociation.