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

Susan T. Arnold, Itzhak Dotan, Skip Williams, A. A. Viggiano, Robert A. Morris

Research output: Contribution to journalArticlepeer-review


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.

Original languageEnglish
Pages (from-to)928-934
Number of pages7
JournalJournal of Physical Chemistry A
Issue number5
StatePublished - 10 Feb 2000


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