Kinetic Study of OH Radical Reactions with Cyclopentenone Derivatives

We investigated the reactions of the hydroxyl radical (OH) with cyclopentenone derivatives and cyclopentanone in a quasi-static reaction cell at 4 Torr across a 300-500 K temperature range. The OH radicals were generated using pulsed laser photolysis of hydrogen peroxide vapors, and the ketone reactants were introduced in excess. The relative concentrations of the radicals were monitored as a function of reaction time using laser-induced fluorescence. At room temperature, the reaction rate coefficients were measured to be 1.2(±0.1) × 10^-11 cm³ s^-1 for reaction with 2-cyclopenten-1-one (R1); 1.7(±0.2) × 10^-11 cm³ s^-1 for reaction with 2-methyl-2-cyclopenten-1-one (R2); and 4.4(±0. 7) × 10^-12 cm³ s^-1 for reaction with 3-methyl-2-cyclopenten-1-one (R3). Over the experimental temperature range, the rate coefficients can be fitted with the modified Arrhenius expressions k₁(T) = 1.2 × 10^-11 (T/300)^0.26 exp (6.7 kJ mol^-1/R {1/T - 1/300}) cm³ s^-1, k₂(T) = 1.7 × 10^-11 (T/300)^6.4 exp (27.6 kJ mol^-1/R {1/T - 1/300}) cm³ s^-1, k₃(T) = 4.4 × 10^-12 (T/300)^17.8 exp (57.8 kJ mol^-1/R {1/T - 1/300}) cm³ s^-1. In the cases of 2-cyclopenten-1-one and 2-methyl-2-cyclopenten-1-one, the temperature dependence of the rate coefficients is similar to that calculated or measured for noncyclic conjugated ketones. We also found that the reaction with 3-methyl-2-cyclopenten-1-one was slower, with rate coefficients similar to those measured for the reaction with the saturated cyclic ketone cyclopentanone. To discuss the experimental data, we use potential energy surfaces (PES) calculated at the CCSD(T)/cc-pVTZ//M06-2X/6-311+G** level of theory. RRKM-based Master equation calculations were also performed to infer the most likely reaction products over a wide range of temperatures and pressures. We suggest that both abstraction and addition mechanisms contribute to the overall OH removal, forming radical products stabilized by resonance. We also discuss the relevance for combustion and atmospheric chemistry.