Selected ion flow tube studies of the atomic oxygen radical cation reactions with ethylene and other alkenes

The chemical ionization reaction of the atomic oxygen radical cation with ethylene have been investigated extensively at 300 K in 0.5 Torr of helium in a selected ion flow tube (SIFT). To help understand the ethylene data, five additional terminal alkenes (propene, isobutene, isoprene, styrene, and vinylidene chloride) were also examined. Considerable care was taken to account for the extremely high reactivity of O(.+) (i.e. correction for reaction with trace levels of impurity in the helium) and the possibility of generation of electronically excited reactant ions. Ethylene was found to react on 93% of encounters, and to give 26% of its parent radical cation, 18% of the vinyl cation, 47% of acetylene radical cation, and 9% of protonated carbon monoxide. A mechanistic proposal for how these ions arise is presented. Extension of the mechanistic proposal for ethylene accounts for the reactivity observed for the other alkenes as well. Correlation of the yield of charge transfer product from each alkene with ionization energy (IE) suggests that primary event in the reaction is charge transfer. One exception is styrene: It has the lowest IE of the alkenes examined but the highest yield of molecular radical cation, leading to the suggestion that styrene may yield electronically excited ion products. (C) 1999 Elsevier Science B.V.