Didehydro radical anions from ketones via O•- chemical ionization

Gas-phase ion-molecule reactions of the atomic oxygen radical anion, O.- with selected ketones are investigated to explore a strategy to 1,3-distonic radical anions. These O.- chemical ionization reactions, especially that of cyclopentanone, are examined in detail under the well-defined thermal energy conditions uniquely available by using the flowing afterglow technique. To further understanding of the reaction mechanism, a series of related ketones (acetone, 3-pentanone, 2-butanone, 2,4-dimethyl-3-pentanone, cyclohexanone) are also examined in detail. Rate constants for each ketone reaction are measured and products from each ketone examined are identified and their branching ratios are reported. Based on these results, a detailed reaction scheme for each ketone reaction studied is proposed. The individual reaction schemes are summarized in a composite O.-/ketone reaction scheme and the specific reaction pathways are discussed. A strategy is developed to differentiate 1.1- and 1,3-H-2(.+)-abstraction products and to establish the absolute yields of 1,3-distonic radical anions from appropriate ketones. (C) 2004 Elsevier B.V. All rights reserved.