HO + OClO Reaction System: Featuring a Barrierless Entrance Channel with Two Transition States

Chlorine-containing compounds play a significant role in the troposphere and are key players in the stratosphere. The free radical compound OClO reacts with HO free radicals, but the existing experimental kinetics data are limited and uncertain. In the present theoretical investigation, the reaction mechanism, rate constants, and product branching ratios for the HO + OClO reaction system were computed over wide temperature and pressure ranges and compared with the existing experimental data. Stationary points on the singlet potential energy surface (PES) were calculated at high levels of theory, and the kinetics parameters were computed using several methods, including variational transition state theory (VTST) and RRKM/master equation techniques. The computed PES is in reasonable agreement with previous calculations, and the computed rate constants and branching ratio are in good agreement with the recent experiments. The results are used as the basis for recommendations for atmospheric chemistry modeling. The PES along the reaction path forming the peroxy bond has a steplike structure and only a very weakly bound prereactive complex, and yet it still supports two transition states along the reaction path. This feature may also be present in other reactions in which electrostatic forces align the approaching reactants in an unfavorable orientation at long distances, thus requiring a dramatic geometry change before reaction can take place.