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 H 3 O + and H 3 O + (H 2 O) with toluene (C 7 H 8 ), ethylbenzene (C 8 H 10 ), and n -propylbenzene (C 9 H 12 ) 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 H 3 O + (H 2 O) with these reactants. The H 3 O + + 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. H 3 O + reacts with toluene above 900 K to produce C 7 H 9 +, C 7 H 7 +, and C 6 H 5 + predominately, whereas only C 7 H 9 + is observed at lower temperatures. Proton transfer from H 3 O + to ethylbenzene produces C 8 H 11 + exclusively at 300 K, and by 800 K, C 6 H 7 + is the major product with only minor amounts of C 8 H 11 + and C 7 H 7 + being formed. Both nondissociative and dissociative proton-transfer products are observed at all temperatures for H 3 O + reacting with propylbenzene producing C 9 H 13 + as the major product at low temperatures and C 6 H 7 + and C 3 H 7 + as the major products at temperatures above 650 K, with minor amounts of C 7 H 7 + observed at all temperatures. For all three alkylbenzenes reacting with H 3 O + (H 2 O), nondissociative proton transfer dominates at low temperature; however, at 300 K, an association complex of all three alkylbenzenes with H 3 O + (H 2 O) is observed (14−18%). At higher temperatures, dissociative proton transfer is observed for H 3 O + (H 2 O) reacting with ethyl- and propylbenzene with similar product yields as observed in the H 3 O + reactions. All of the H 3 O + (H 2 O) + alkylbenzene proton-transfer reactions are fast even though the proton affinity of H 3 O + (H 2 O) is 140 kJ mol -1 less than that of H 3 O + making the H 3 O + (H 2 O) reactions endothermic. More specifically, the rate constant for the reaction of H 3 O + (H 2 O) 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 kJ mol -1 . Similarly, the reaction rate constants for H 3 O + (H 2 O) 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 H 3 O + (H 2 O) 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 where it is not thermodynamically feasible.