Fluorinated acids and alcohols dissociate near their ionization threshold: understanding the difficulty in their mass spectrometric detection

The dissociative photoionization of 3,3,3-trifluoropropionic acid (1), 2,2,3,3,3-pentafluoropropionic acid (2), and 2,2,3,3,3-pentafluoropropanol (3) has been explored with imaging photoelectron photoion coincidence spectroscopy. Threshold photoelectron spectra and ion breakdown diagrams are presented along with CBS-QB3 level calculations of the observed ion chemistry. All three molecules dissociate upon ionization, with only 1 exhibiting a stable molecular ion close to the ionization onset. Experimental (11.12 ± 0.04, 11.6 ± 0.1, and 11.1 ± 0.1 eV) and CBS-QB3 calculated ionization energies (11.16, 11.63, and 11.09 eV) were established for 1, 2, and 3, respectively. In the studied energy range, ion 1 + dissociates to form m/z 64 (CF 2 CH 2 +• ) or 45 (CO 2 H + ) via a pathway in which the CO 2 H moiety migrates to interact with a fluorine atom of the CF 3 group. This intermediate complex dissociates to form m/z 45 or 64. Alternatively, it isomerizes to an ion–molecule complex of fluoroformic acid with a CF 2 CH 2 group, which also proceeds to form m/z 64. An analogous mechanism is responsible for the two main fragment ions observed from 2 + , m/z 100 (C 2 F 4 + ) and 45. The ground state ion 3 + is itself an ion–molecule complex between CF 3 CF 2 • and CH 2 OH + , and simple bond cleavage results in the only fragment ion formed, m/z 31 (CH 2 OH + ).