The reaction of Criegee radical CH₂O₂ with H₂O was investigated by density functional theory (DFT) at the B3LYP/6-311G＋(2d,2p) level. Three channels were identified: CH₂O₂＋H₂O®HOCH₂OOH (R1); CH₂O₂＋H₂O®HCO＋OH＋H₂O (R2); CH₂O₂＋H₂O®HCHO＋H₂O₂ (R3). The energy barriers are 43.35, 85.30 and 125.85 kJ/mol for (R1), (R2) and (R3), respectively. The rate constants of reaction (R1) are both 2.47×10^－17 cm³•molecule^－1•s^－1 for classic transitional state theory (TST) k^TST and canonical variational transition state theory (CVT) k^CVT. The rate constant with small-curvature tunneling (SCT) correction k^CVT/SCT is 5.22×10^－17 cm³•molecule^－1•s^－1. The dependence of the rate constants on temperature is given with a three parameters Arrhenius equation within the temperature range 200～2000 K.

Key words: Criegee radical CH₂O₂, H₂O, mechanism, rate constant