Thermodynamic and kinetic properties of the reaction Cl+O2+M=ClOO+M in the range 160–300 K and 1–1000 bar

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Abstract
The reaction Cl+O2+M■ClOO+M was studied by laser flash photolysis in the bath gases M=He, Ar, N2, and O2 over the temperature range 160–300 K and the pressure range 1–1000 bar. UV absorptions of ClOO were monitored, a maximum absorption cross section of σ(248 nm)=3.4×10−17 cm2 was determined. An expression for the equilibrium constant Kp =5.3×10−6 exp(+23.4 kJ mol−1/RT) bar−1 was derived between 180 and 300 K, which, by a third law analysis, yields ΔH○0 =−20.2±0,2 kJ mol−1. Limiting low pressure rate coefficients for Cl+O2 recombination of k0=[He]8.8×10−34(T/300 K)−3.0, k0=[O2]1.6×10−33(T/300 K)−2.9, k0=[N2]1.4×10−33(T/300 K)−3.9 cm6 s−1 were obtained over the range 160–260 K, as well as k0 (160 K)=[Ar]2.2×10−32 cm6 s−1. Rate constants for the reactions Cl+ClOO→Cl2+O2 or 2ClO, ClOO+ClOO→products, and ClOO+Cl2→Cl2O+ClO were also derived. The recombination Cl+O2(+M)→ClOO(+M) at pressures above 10 bar shows a transition to a high pressure plateau and, at pressures above 200 bar, to diffusion control. It is suggested that, like O+O2(+M)→O3(+M), the reaction is governed by a radical complex mechanism.

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