Theoretical Study Of The Water Effect On The Reaction Mechanism Of SO₂with Peroxy Radicals

The effect of a single water molecule on the reaction mechanism of SO2with peroxy radicals has been theoretically investigated by the density functional theory (DFT). The main job is as follows:1. Water effect on the reaction mechanism of SO2with HO2radicalThe influence of a single water molecule on the reaction mechanism of SO2with HO2radical has been investigated at B3LYP/aug-cc-pVTZ level by the reactions of SO2with the HO2radical and with the HO2H2O radical complex. The SO2+HO2reaction leads to the formation of the HSO4radical, with an energy barrier of9.86kcal mol-1. The addition of the single water molecule results in a much more complex potential energy surface. The SO2+HO2H2O reaction can give two different products, namely (a) HSO4+H2O, which has an energy barrier of15.23kcal mol-1that is5.37kcal mol-1higher than the HSO4reaction path of the naked reaction, and (b) H2SO3+HO2, with an energy barrier of9.87kcal mol-1that is5.36kcal mol-1lower than the HSO4+H2O reaction path. In addition, the H2SO3+HO2is the favorable reaction path of SO2+HO2H2O reaction. Water molecule acts as a reactant directly involved in the reaction to form H2SO3. However, the HO2radical acts as a catalyst of the water-splitting reaction, and reduces the energy barriers of H2SO3formation in SO2+H2O and SO2+2H20reactions with21.40and7.29kcal mol-1, respectively. The results suggest that, the catalytic power of HO2radical in the SO2+H2O reaction is stronger than that of H2O molecule. The rate constants (k) at298K of SO2+HO2H2O and SO2+H2O reaction to form H2SO3are2.57×10-26and3.54×10-36cm3molecules-1s-1, respectively. The reaction rate constant of the new route to form H2SO3investigated herein is ten orders higher than that of the SO2+H2O reaction. The result clearly shows that the HO2radical can significantly accelerate the SO2+H2O reaction under atmospheric conditions.2. Water effect on the reaction mechanism of SO2with Criegee intermediates (CH2O2and CHOCHO2)The effect of a single water molecule on the reactions of SO2+CH2O2and SO2+CHOCHO2has been theoretically investigated at B3LYP/aug-cc-pVTZ level. The SO2+CH2O2reaction can give two different products, one is the (a) SO3+HCHO path, which has an energy barrier of15.93kcal mol-1; and the other is the (b) SO2+HCOOH path, with an energy barrier of26.00kcal mol-1. The SO3+HCHO+H2O (aw) is the favorable reaction path of the water-asissted reaction, with an energy barrier of16.50kcal mol-1that is similar to the reaction path (a). The results suggest that the SO3+HCHO reaction path has no dependence on the water molecule. The other reaction path leads to form SO2+CO2+H2(bw), in which the water molecule acts as a catalyst which splits the CH2O2radical into CO2and H2, and the energy barrier of path (bw) is1.26kcal mol-1lower than path (b).The SO2+CHOCHO2reaction can give two different products:SO2+HCHO+CO2and SO2+CHOOCHO. The energy barrier of the reaction path to form SO2+HCHO+CO2is13.74kcal mol"’. However, there are two feasible ways (path (al) and (b)) to form SO2+CHOOCHO, it is noted that the path (al) involves two steps which have an energy barrier of13.74and49.09kcal mol-1, respectively, and path (b) can be achieved in a single step which has an energy barrier of8.45kcal mol-1. The reaction mechanism of the water-asissted reaction is similar to the SO2+CHOCHO2reaction. The reaction path of the water assisted-reaction leads to form SO2+HCHO+CO2+H2O with an energy barrier of15.31kcal mol-1, which is1.57kcal mol-1lower than that of path (a). There are also two feasible wavs (path (alw) and (bw)) to form SO2+CHOOCHO+H2O. The energy barrier of the two steps in path (alw) are15.38and46.33kcal mol-1, respectively, the addition of a single water molecule increases the energy barrier of the first step with1.38kcal mol-1and lowers the energv barrier of the second step witn2.76kcal mol-1. The energy barner of path (bw) is6.02kcal mol-1which is2.43kcal mol-1lower than that of path(b), it suggests that the H2O acts as a catalyst of the CHOCHO2radical rearrangement reaction.3. Water effect on the reaction mechanism of SO2with benzene-OH-00The reactions between SO2and the benzene-OH-00radical, without and with a single water molecule have been theoretically investigated at B3LYP/6-311++G(d,p) level.The SO2+benzene-OH-OO reaction can give two different products, namely, SO3and H2SO3. There are two feasible ways (path (al) and (a2)) to form SO3and the energy (?) barrier of the H2SO3path is36.55kcal·mol-1. The mechanism of water-asissted reaction is similar to the naked reaction.1he energy barrier of path (alw) is23.56kcal mol which is4.31kcal mol-1lower than that of path (al), the result suggests that the H2O acts as a positive catalyst of path (alw). The energy barriers of path (a2w) and path (bw) are23.74and36.55kcal mol-1, respectively. The results suggest that path (a2) and path (b) have no dependence on the water molecule.The research work indicates that the influence of a single water molecule on the reactions of SO2with different peroxy radicals show different characteristics. Firstly, the water molecule can act as a reactant directly involved in the SO2+HO2reaction to form H2SO3. Secondly, the water molecule can change the energy barrier of the reactions, it increases the energy barrier of the reaction path (a) in SO2+HO2reaction and (al)(a2) in SO2+CHOCHO2, and lowers the energy barrier of the reaction path (b) in SO2+CHOCHO2and (al) in SO2+benzene-OH-00reaction; Thirdly, the water molecule enhances the rearrangement reaction of SO2+CHOCHO2; Finally, the water molecule has an effect on the stability of the reactants or intermediates, it acts as a catalytic of CH2O2radical splitting reaction.