The catalytic effects of H<sub>2</sub>CO<sub>3</sub>, CH<sub>3</sub>COOH, HCOOH and H<sub>2</sub>O on the addition reaction of CH<sub>2</sub>OO + H<sub>2</sub>O → CH<sub>2</sub>(OH)OOH
<p>The addition reaction of CH<sub>2</sub>OO + H<sub>2</sub>O <b>→</b> CH<sub>2</sub>(OH)OOH without and with X (X = H<sub>2</sub>CO<sub>3</sub>, CH<sub>3</sub>COOH and HCOOH) and H<sub>2</sub>O was studied at CCSD(T)/6-311+ G(3df,2dp)//B3LYP/6-311+G(2d,2p) level of theory. Our results show that X can catalyse CH<sub>2</sub>OO + H<sub>2</sub>O → CH<sub>2</sub>(OH)OOH reaction both by increasing the number of rings, and by adding the size of the ring in which ring enlargement by COOH moiety of X inserting into CH<sub>2</sub>OO···H<sub>2</sub>O is favourable one. Water-assisted CH<sub>2</sub>OO + H<sub>2</sub>O → CH<sub>2</sub>(OH)OOH can occur by H<sub>2</sub>O moiety of (H<sub>2</sub>O)<sub>2</sub> or the whole (H<sub>2</sub>O)<sub>2</sub> forming cyclic structure with CH<sub>2</sub>OO, where the latter form is more favourable. Because the concentration of H<sub>2</sub>CO<sub>3</sub> is unknown, the influence of CH<sub>3</sub>COOH, HCOOH and H<sub>2</sub>O were calculated within 0–30 km altitude of the Earth's atmosphere. The results calculated within 0–5 km altitude show that H<sub>2</sub>O and HCOOH have obvious effect on enhancing the rate with the enhancement factors are, respectively, 62.47%–77.26% and 0.04%–1.76%. Within 5–30 km altitude, HCOOH has obvious effect on enhancing the title rate with the enhancement factor of 2.69%–98.28%. However, compared with the reaction of CH<sub>2</sub>OO + HCOOH, the rate of CH<sub>2</sub>OO···H<sub>2</sub>O + HCOOH is much slower.</p