Ab initio studies of ClOx reactions. IX. Combination and disproportionation reactions of ClO and s-ClO3 radicals

Abstract

The mechanism for the reaction ClO+ClO3 on both singlet and triplet state potential surfaces has been investigated with the modified Gaussian-2 method based on the B3LYP/6-311+G(3df ) optimized stationary-point geometries. The result shows that the barrierless association reaction producing ClOClO3 and two lower barrier O-atom abstraction reactions take place primarily on the singlet state potential surface; they are energetically more favorable than those occurring on the triplet state surface. Rate constants calculated by variational transition state and Rice–Ramsperger–Kassel–Marcus theories suggest that the major products are ClOClO3 at low temperatures (<700 K) and OClO+ClOO at high temperatures. The following rate constants are recommended for atmospheric chemistry and combustion applications: k1∞(ClOClO3)=1.43×10−10 T0.094exp(−82/T) cm3 molecule−1 s−1 (200–3000 K) and k10(ClOClO3)=0.14 T−10.19exp(−1597/T) cm6 molecule−2 s−1 (200–800 K) for He as the third-body. In addition, a potential high-temperature reverse reaction involving Cl and ClO4 has been predicted to yield exclusively the ClO and ClO3 products; its rate constant is predicted to be: k4(ClO+ClO3)=8.05×10−11 T0.158 exp(−49/T) cm3 molecule−1 s−1 (200–3000 K). The heats of formation at 0 K for s-ClO3, ClO4, and ClOClO3 have been predicted to be 46.7, 59.4, and 38.3 kcal/mol with about 1 kcal/mol uncertainty using the new heat of formation of OClO, 24.1±0.1 kcal/mol, based on the most recent bond dissociation energy of O–ClO reported in the literature.