Flow tube studies of benzene charge transfer reactions from 250 to 1400 K

Temperature dependent rate constants and product branching fractions are re ported for reactions of the atmospheric plasma cations NO+, O-2(+), O+, N+, N-2(+), and N-4(+) with benzene, as measured from 250 to 500 K by the sele cted ion flow tube technique. For the reactions of O-2(+) and N-2(+), data have also been obtained between 500 and 1400 K in a high-temperature flowin g afterglow. These are among the first determinations of ion-molecule branc hing fractions above 600 K. Temperature dependent rate constants and produc t branching fractions are also reported for the reactions of benzene with K r+, Ar+, Ne+, and F+. All reactions were found to proceed at the collision rate at all temperatures studied. With increasing reactant ion recombinatio n energy, the mechanism changed from association and nondissociative charge transfer to dissociative charge transfer. Primary and secondary dissociati on products were observed. Some of the reactivity in the N+ and F+ reaction s is attributed to chemical channels. The temperature dependent branching f ractions are converted to product ion breakdown curves and compared to prev ious studies. The current results exhibit a kinetic shift, resulting from s low fragmentation of the C6H6+* complex, combined with collisional stabiliz ation of the complex by the He buffer gas. The pressure dependence of the N + reaction was examined from 0.35 to 0.8 Torr. The flow tube data provide t he first breakdown curve for the C5H3+ product and further indicate that C5 H3+ is relatively unreactive, consistent with it having the cyclic ethynyl cyclopropene ion structure. The C3H3+ product was shown to have a cyclic st ructure, while the C4H4+ product was found to be a mixture of linear and cy clic isomers. The isomeric mixture of C4H4+ products was quantified as a fu nction of the C6H6+* excess energy. A schematic reaction coordinate diagram representing the primary dissociation channels of C6H6+ is constructed fro m previous experimental and theoretical work. A possible reaction pathway f or the C5H3+ product is discussed.