Experimental and Master Equation Study of the Kinetics of OH + C2H2:  Temperature Dependence of the Limiting High Pressure and Pressure Dependent Rate Coefficients

The kinetics of the reaction OH + C2H2 have been studied using laser flash photolysis at 248 nm to generate OH radicals and laser-induced fluorescence to monitor OH removal. An attempt was made to use the rate coefficients OH (v = 1,2) + C2H2 to determine the limiting high-pressure rate coefficient, k 1a ∞, over the temperature range of 195−823 K. This method is usually applicable if the reaction samples the potential energy well of the adduct, HOC2H2, and if intramolecular vibrational relaxation is fast. In the present case, however, the rate coefficients for loss of the vibrationally excited states by reaction with C2H2 also contain a substantial contribution from nonreactive vibrational relaxation, which occurs via a mechanism that does not sample the adduct potential energy well but involves, at least at low temperatures, collisions that access a shallower, longer range van der Waals well. The data were analyzed using a composite mechanism that incorporates both reactive and nonreactive energy transfer mechanisms, which allows the determination of k 1a ∞(T) for OH + C2H2 with satisfactory accuracy over the temperature range 195−823 K. The kinetics of the reaction OH (v = 0) + C2H2 were also studied in He over the range of conditions: 210−373 K and 5−760 Torr. A one-dimensional master equation (ME) analysis of the experimental data provided a further determination of k 1a ∞(T) and also 〈ΔE〉down for He. Combining the two sets of results gives a consistent dataset for k 1a ∞ and the Arrhenius parameters A 1a ∞ = 7.3 × 10-12 cm3 molecule-1 s-1and E 1a ∞ = 5.3 kJ mol-1, with 〈ΔE〉down = 150(T/300 K) cm-1. Additional experiments were conducted at room temperature in N2 and SF6 by laser flash photolysis with cavity ring down spectroscopy, and ME calculations were then optimized for the pressure falloff in N2 by varying the average downward energy transfer parameter (〈ΔE〉down). The ouput from the best fit ME was parametrized using a modified Troe expression to provide rate data for use in atmospheric modeling.