THERMODYNAMIC AND KINETIC-PROPERTIES OF THE REACTION CL+O2+M-REVERSIBLE-CLOO+M IN THE RANGE 160-300-K AND 1-1000 BAR

The reaction Cl + O2 + M half-arrow-right-over-half-arrow-left 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 sigma(248 nm) = 3.4 x 10(-17) cm2 was determined. An expression for the equilibrium constant K(p) = 5.3 x 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 DELTA-H0-degrees = -20.2 +/- 0,2 kJ mol-1. Limiting low pressure rate coefficients for Cl + O2 recombination of k0 = [He]8.8 x 10(-34) (T/300 K)-3.0, k0 = [O2] 1.6 x 10(-33) (T/300 K)-2.9, k0 = [N2]1.4 x 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 x 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.