The favorable routes for the hydrolysis of CH2OO with (H2O)n (n = 1-4) investigated by global minimum searching combined with quantum chemical methods.

The hydrolysis reaction of CH2OO with water and water clusters is believed to be a dominant sink for the CH2OO intermediate in the atmosphere. However, the favorable route for the hydrolysis of CH2OO with water clusters is still unclear. Here global minimum searching using the Tsinghua Global Minimum program has been introduced to find the most stable geometry of the CH2OO(H2O)n (n = 1-4) complex firstly. Then, based on these stable complexes, favorable hydrolysis of CH2OO with (H2O)n (n = 1-4) has been investigated using the quantum chemical method of CCSD(T)-F12a/cc-pVDZ-F12//B3LYP/6-311+G(2d,2p) and canonical variational transition state theory with small curvature tunneling. The calculated results have revealed that, although the contribution of CH2OO + (H2O)2 is the most obvious in the hydrolysis of CH2OO with (H2O)n (n = 1-4), the hydrolysis of CH2OO with (H2O)3 is not negligible in atmospheric gas-phase chemistry as its rate is close to the rate of the CH2OO + H2O reaction. The calculated results also show that, in a clean atmosphere, the CH2OO + (H2O)n (n = 1-2) reaction competes well with the CH2OO + SO2 reaction at 298 K when the concentrations of (H2O)n (n = 1-2) range from 20% relative humidity (RH) to 100% RH, and SO2 is 2.46 × 1011 molecules per cm3. Meanwhile, when the RH is higher than 40%, it is a new prediction that the CH2OO + (H2O)3 reaction can also compete well with the CH2OO + SO2 reaction at 298 K. Besides, Born-Oppenheimer molecular dynamics simulation results show that all the favorable channels of the CH2OO + (H2O)n (n = 1-3) reaction cannot react on a time scale of 100 ps in the NVT simulation. However, the NVE simulation results show that the CH2OO + (H2O)3 reaction can be finished well at 8.5 ps, indicating that the gas phase reaction of CH2OO + (H2O)3 is not negligible in the atmosphere. Overall, the present results have provided a definitive example of how the favorable hydrolysis of important atmospheric species with (H2O)n (n = 1-4) takes place, which will stimulate one to consider the favorable hydrolysis of water and water clusters with other Criegee intermediates and other important atmospheric species.

The catalytic effect of water, water dimers and water trimers on H2S + (3)O2 formation by the HO2 + HS reaction under tropospheric conditions.

In this article, the reaction mechanisms of H2S + (3)O2 formation by the HO2 + HS reaction without and with catalyst X (X = H2O, (H2O)2 and (H2O)3) have been investigated theoretically at the CCSD(T)/6-311++G(3df,2pd)//B3LYP/6-311+G(2df,2p) level of theory, coupled with rate constant calculations by using conventional transition state theory. Our results show that in the presence of catalyst X (X = H2O, (H2O)2 and (H2O)3) into the channel of H2S + (3)O2 formation, the reactions between the SH radical and HO2(H2O)n (n = 1-3) complexes are more favorable than the corresponding reactions of the HO2 radical with HS(H2O)n (n = 1-3) complexes due to the lower barrier of the former reactions and the higher concentrations of HO2(H2O)n (n = 1-3) complexes. Meanwhile, the catalytic effect of water, water dimers and water trimers is mainly taken from the contribution of a single water vapor molecule, since the total effective rate constant of HO2H2O + HS and H2OHO2 + HS reactions was, respectively, larger by 7-9 and 9-12 orders of magnitude than that of SH + HO2(H2O)2 and SH + HO2(H2O)3 reactions. Besides, the enhancement factor of water vapor is only 0.37% at 240 K, while at high temperatures, such as 425 K, the positive water vapor effect is enhanced up to 38.00%, indicating that at high temperatures the positive water effect is obvious under atmospheric conditions. Overall, these results show how water and water clusters catalyze the gas phase reactions under atmospheric conditions.

Effect of NH3 and HCOOH on the H2O2 + HO → HO2 + H2O reaction in the troposphere: competition between the one-step and stepwise mechanisms.

The H2O2 + HO → HO2 + H2O reaction is an important reservoir for both radicals of HO and HO2 catalyzing the destruction of O3. Here, this reaction assisted by NH3 and HCOOH catalysts was explored using the CCSD(T)-F12a/cc-pVDZ-F12//M06-2X/aug-cc-pVTZ method and canonical variational transition state theory with small curvature tunneling. Two possible sets of mechanisms, (i) one-step routes and (ii) stepwise processes, are possible. Our results show that in the presence of both NH3 and HCOOH catalysts under relevant atmospheric temperature, mechanism (i) is favored both energetically and kinetically than the corresponding mechanism (ii). At 298 K, the relative rate for mechanism (i) in the presence of NH3 (10, 2900 ppbv) and HCOOH (10 ppbv) is respectively 3-5 and 2-4 orders of magnitude lower than that of the water-catalyzed reaction. This is due to a comparatively lower concentration of NH3 and HCOOH than H2O which indicates the positive water effect under atmospheric conditions. Although NH3 and HCOOH catalysts play a negligible role in the reservoir for both radicals of HO and HO2 catalyzing the destruction of O3, the current study provides a comprehensive example of how acidic and basic catalysts assisted the gas-phase reactions.