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

Temperature dependent rate constants and product branching fractions are reported 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 selected 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 flowing afterglow. These are among the first determinations of ion-molecule branching fractions above 600 K. Temperature dependent rate constants and product branching fractions are also reported for the reactions of benzene with Kr+, Ar+, Ne+, and F+. All reactions were found to proceed at the collision rate at all temperatures studied. With increasing reactant ion recombination energy, the mechanism changed from association and nondissociative charge transfer to dissociative charge transfer. Primary and secondary dissociation products were observed. Some of the reactivity in the N+ and F+ reactions is attributed to chemical channels. The temperature dependent branching fractions are converted to product ion breakdown curves and compared to previous studies. The current results exhibit a kinetic shift, resulting from slow fragmentation of the C6H6+* complex, combined with collisional stabilization 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 the first breakdown curve for the C5H3+ product and further indicate that C5H3+ is relatively unreactive, consistent with it having the cyclic ethynyl cyclopropene ion structure. The C3H3+ product was shown to have a cyclic structure, while the C4H4+ product was found to be a mixture of linear and cyclic isomers. The isomeric mixture of C4H4+ products was quantified as a function of the C6H6+* excess energy. A schematic reaction coordinate diagram representing the primary dissociation channels of C6H6+ is constructed from previous experimental and theoretical work. A possible reaction pathway for the C5H3+ product is discussed.