Laser flash photolysis of Cl2/CO/M mixtures (M=N2, CO, Ar, CO2) has been employed in conjunction with Cl(2PJ) detection by time‐resolved resonance fluorescence spectroscopy to investigate equilibration kinetics in the reactions Cl(2PJ) +CO↔ClCO as a function of temperature (185–260 K) and pressure (14–200 Torr). The association and dissociation reactions are found to be in the low‐pressure limit over the range of experimental conditions investigated. In N2 and/or CO buffer gases, the temperature dependences of the ClCO formation and dissociation reaction rate constants are described by the Arrhenius expressions k1=(1.05±0.36)×1034 exp[(810±70)/T] cm6 molecule2 s1 and k1=(4.1±3.1)×1010 exp[(−2960±160)/T] cm3 molecule1 s1 (errors are 2σ). Second‐ and third‐law analyses of the temperature dependence of the equilibrium constant (k1/k1) lead to the following thermodynamic parameters for the association reaction: ΔH0298 =−7.7±0.6 kcal mol1, ΔH00 =−6.9±0.7 kcal mol1, ΔS0298 =−23.8±2.0 cal mole1 K1, ΔH0f,298 (ClCO)=5.2±0.6 kcal mol1 (errors are 2σ). The results reported in this study significantly reduce the uncertainties in all reported kinetic and thermodynamic parameters.

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