Temperature dependence of the HO2+ClOreaction. 1. Reaction kinetics by pulsed photolysis-ultraviolet absorption and ab initio studies of the potential surface

The kinetics of the HO2 + ClO reaction was studied using the flash photolys is/ultraviolet absorption technique over the temperature range 203-364 K an d pressure range 50-700 Torr of NS In contrast to previous work, the temper ature dependence displayed linear Arrhenius behavior over the entire temper ature range with the rate constant being described by the expression k(T) = 2.84 x 10(-12) exp{(312 +/- 60)/T} cm(3) molecule(-1) s(-1) Ab initio calc ulations of intermediates and transition states have been carried our: on t he singlet and triplet potential energy surfaces. These calculations show t hat the reaction proceeds mainly through the ClO-HO2 complex on the triplet surface; however, collisionally stabilized HOOOCl formed on the singlet su rface will possess an appreciable lifetime due to large barriers toward dec omposition to HCl and HOCl. Termolecular rate calculations using ab initio parameters lead to a strong collision rate constant of similar to 5 X 10(-3 2) cm(6) molecule(-2) s(-1) for HOOOCl formation. This intermediate may be important under both laboratory and atmospheric conditions.