Reactions of H3O+(H2O)0,1 with alkylbenzenes from 298 to 1200 K

Rate constants and branching fractions have been measured for the reactions of H3O+ and H3O+(H2O) with toluene (C7H8), ethylbenzene (C8H10), and n-propylbenzene (C9H12) as a function of temperature using a variable temperature-selected ion flow tube (VT-SIFT) and a high-temperature flowing afterglow (HTFA). The reactions have been studied up to 1200, 1000, and 900 K for toluene, ethylbenzene, and n-propylbenzene, respectively. The measurements are the first for H3O+(H2O) with these reactants. The H3O+ + alkylbenzene reaction rate constants are equal to the collision rate constants given by the Su-Chesnavich equation at all temperatures. Both nondissociative and dissociative proton-transfer products are observed. H3O+ reacts with toluene above 900 K to produce C7H9+, C7H7+, and C6H5+ predominately, whereas only C7H9+ is observed at lower temperatures. Proton transfer from H3O+ to ethylbenzene produces C8H11+ exclusively at 300 K, and by 800 K, C6H7+ is the major product with only minor amounts of C8H11+ and C7H7+ being formed. Both nondissociative and dissociative proton-transfer products are observed at all temperatures for H3O+ reacting with propylbenzene producing C9H13+ as the major product at low temperatures and C6H7+ and C3H7+ as the major products at temperatures above 650 K, with minor amounts Of C7H7 + observed at all temperatures. For all three alkylbenzenes reacting with H3O+(H2O), nondissociative proton transfer dominates at low temperature; however, at 300 K, an association complex of all three alkylbenzenes with H3O(H2O) is observed (14-18%). At higher temperatures, dissociative proton transfer is observed for H3O+(H20) reacting with ethyl- and propylbenzene with similar product yields as observed in the H3O+ reactions. All of the H3O+(H20) + alkylbenzene proton-transfer reactions are fast even though the proton affinity of H3O+(H20) is 140 kJ mol(-1) less than that of H3O+ making the H3O+(H2O) reactions endothermic. More specifically, the rate constant for the reaction of H3O(H2O) With toluene is ca. half the collision rate constant at 300 K and equal to the collision rate constant at temperatures above 300 K even though the reaction is endothermic by 25 U mol(-1). Similarly, the reaction rate constants for H3O(H2O) reacting with ethyl- and propylbenzene are equal to the collision rate constant at all temperatures despite being endothermic by 21 and 19 kJ mol(-1), respectively. Therefore, predictions based simply on reaction energetics would severely underestimate the reactivity of H3O+(H2O) with these alkylbenzenes. The possibilities of neutral water dimer production, ligand switching, uncertainties in the thermochemical data, and decomposition of the ionic products are considered to rationalize the observation of proton transfer wh