CN– and C3N– Reactivity with Formic and Acetic Acid, Acetaldehyde, and Methanol

Abstract

The anion-molecule reactivity of CN^– and C₃N^–, produced by dissociative electron attachment of the respective bromide precursors, with four oxygenated molecules has been investigated in a guided ion beam mass spectrometer, and absolute reaction cross-sections are derived as a function of collision energy. The four targets are formic acid, acetic acid, acetaldehyde, and methanol. Exothermic and endothermic proton transfer has been observed as the main reaction channel, with differences in cross-section between the two anions. Oxidation of the anions is also observed, forming OCN^– and OC₃N^–, for both anions and limited to the targets with endothermic proton transfer for CN^–. This reaction requires several rearrangements and, therefore, a long-lived complex to proceed. Other complex-mediated products are observed for C₃N^– but not for CN^–, interpreted as the ability to proceed through a long-lived complex because of less easy proton transfer for C₃N^–. Comparison between the present results at low-collision energy, models, and previous studies are producing a coherent picture. Several products observed with C₃N^– were missing formation enthalpies. Using the experimental exothermic behavior, it was possible to determine the upper limit values for enthalpies for the formation of OC₃N^– (0.39 ± 0.25 eV), [H₂OC₃N]⁻ (0.39 ± 0.25 eV), C₄N^– (0.65 ± 0.25 eV), and CH₃C₃N^– (3.52 ± 0.25 eV).