Mechanistic insight into the reactions of O [rad]− with aromatic compounds and the synthesis of didehydroaromatic anions in the gas phase

Abstract

The gas-phase ion/molecule reactions of O − with benzene, pyridine, furan, thiophene, pyrrole, and 1,3-cyclopentadiene have been quantitatively characterized under the thermally equilibrated (298 ± 3 K) conditions attainable with the flowing afterglow technique. All atomic oxygen radical anion reactions examined were found to be fast, with reaction occurring on at least three out of four collisions. While reaction rate coefficients are the same for all aromatic compounds, the product distributions for the O − reactions are extremely sensitive to the structure and properties of the neutral. In general, four reaction channels need to be considered: H + 2 -transfer, oxide anion formation (i.e. addition of an oxygen atom and loss of a hydrogen atom), H-atom transfer (i.e. formation of HO −), and H +-transfer. Formation of the didehydroaromatic radical anion was detected for each neutral while H-atom transfer was a minor pathway and was only found for pyrrole and 1,3-cyclopentadiene. Oxide anion formation was a major pathway for the six-membered aromatic rings. For the five-membered rings, only thiophene produced the corresponding oxide anion (as a trace product) upon reaction with O − . Proton transfer was always important where thermochemically allowed (thiophene, pyrrole, and 1,3-cyclopentadiene). The observations are interpreted with the aid of a reaction coordinate diagram detailing a series of sequential steps for O − with benzene, starting with formation of an ion/neutral complex followed by partitioning into the four reaction channels. The data are supplemented by measurements made for benzene- d 6 and pyridine- d 5, as well as for HO − reacting with each of the neutrals. In addition to demonstrating the synthetic utility of didehydroaromatic radical anion generation by this method, this work suggests that O − and HO − can be complementary to each other as negative-ion chemical ionization mass spectrometric reagents for analysis of mixtures of aromatic compounds by mass spectrometry.