Selected ion flow tube studies of air plasma cations reacting with alkylbenzenes

Rate constants and product branching fractions are reported for reactions of the air plasma cations NO⁺, O₂⁺, O⁺, N⁺, and N₂⁺ with several alkylbenzenes: toluene, ethylbenzene, n-propylbenzene, and m-xylene. The measurements were made using a selected ion flow tube (SIFT) apparatus at 300 K. All reactions were found to proceed at the collision rate. NO⁺ reactions yield exclusively nondissociative charge-transfer products. C₇H₇⁺ is the dominant product ion observed in all the O⁺, N⁺, and N₂⁺ reactions. Charge transfer and formation of C₇H₇^- are the major product channels in the O₂⁺ reactions. Product distributions were converted to crude breakdown diagrams, showing the relative abundance of each product ion as a function of the reactant ion recombination energy. The flow tube results exhibit a shift in the product ion threshold energies, an effect attributed to a kinetic shift resulting from slow fragmentation of the excited charge-transfer complex combined with collisional stabilization of the complex by the He buffer gas. Two isomeric forms of the C₇H₇⁺ product ion are produced in these reactions: the benzyl (Bz^-) and tropylium (Tr⁺) cations. The Bz⁺/Tr⁺ isomeric mixture ratio was quantified as a function of energy for all four alkylbenzenes. Changes in the Bz⁺/Tr⁺ mixture suggest that ethylbenzene has a relatively larger reverse activation barrier compared with toluene for forming Tr⁺ from the charge-transfer complex, while formation of Tr⁺ from the larger alkylbenzenes probably proceeds via a different mechanism altogether. For m-xylene, the formation of both Bz⁺ and Tr⁺ isomers likely proceeds via a different mechanism than for the n-alkylbenzenes.