Catalytic effect of (H(2)O)(n) (n = 1–3) clusters on the HO(2) + SO(2) → HOSO + (3)O(2) reaction under tropospheric conditions

The HO(2) + SO(2) → HOSO + (3)O(2) reaction, both without a catalyst and with (H(2)O)(n) (n = 1–3) as a catalyst, has been investigated using CCSD(T)/CBS//M06-2X/aug-cc-pVTZ methods, and canonical variational transition state theory with small curvature tunneling (CVT/SCT). The calculated results show that H(2)O exerts the strongest catalytic role in the hydrogen atom transfer processes of HO(2) + SO(2) → HOSO + (3)O(2) as compared with (H(2)O)(2) and (H(2)O)(3). In the atmosphere at 0 km altitude within the temperature range of 280.0–320.0 K, the reaction with H(2)O is dominant, compared with the reaction without a catalyst, with an effective rate constant 2–3 orders of magnitude larger. In addition, at 0 km, it is worth mentioning that the relevance of the HO(2) + SO(2) → HOSO + (3)O(2) reaction with H(2)O depends heavily on its ability to compete with the primary loss mechanism of HO(2) radicals (such as the HO(2) + HO(2) and HO(2) + NO(3) reactions) and SO(2) (such as the SO(2) + HO reaction). The calculated results show that the HO(2) + SO(2) → HOSO + (3)O(2) reaction with H(2)O cannot be neglected in the primary loss mechanism of the HO(2) radical and SO(2). The calculated results also show that for the formation of HOSO and (3)O(2), the contribution of H(2)O decreases from 99.98% to 27.27% with an increase in altitude from 0 km to 15 km, due to the lower relative concentration of water. With the altitude increase, the HO(2) + SO(2) → HOSO + (3)O(2) reaction with H(2)O cannot compete with the primary loss mechanism of HO(2) radicals. The present results provide new insight into (H(2)O)(n) (n = 1–3) catalysts, showing that they not only affect energy barriers, but also have an influence on loss mechanisms. The present findings should have broad implications in computational chemistry and atmospheric chemistry.