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

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 K_p =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.