Catalytic Effect of CO2 and H2O Molecules on •CH3 + 3O2 Reaction

The methyl ((CH3)-C-center dot) + O-3(2) radical is an important reaction in both atmospheric and combustion processes. We investigated potential energy surfaces for the effect of CO2 and H2O molecules on a (CH3)-C-center dot+ O-2 system. The mechanism for three reaction systems, i.e., for (CH3)-C-center dot + O-3(2), (CH3)-C-center dot + O-3(2) (+CO2) and (CH3)-C-center dot + O-3(2) (+H2O), were explored using ab initio/DFT methods [CCSD(T)//M062X/6-311++G(3df,3pd)] in combination with a Rice-Ramsperger-Kassel-Marcus (RRKM)/master-equation (ME) simulation between a temperature range of 500 to 1500 K and a pressure range of 0.0001 to 10 atm. When a CO2 and H2O molecule is introduced in a (CH3)-C-center dot + O-3(2) reaction, the reactive complexes, intermediates, transition states and post complexes become thermodynamically more favorable. The calculated rate constant for the (CH3)-C-center dot + O-3(2) (3 x 10(-15) cm(3) molecule(-1) s(-1) at 1000 K) is in good agreement with the previously reported experimentally measured values (similar to 1 x 10(-15) cm(3) molecule(-1) s(-1) at 1000 K). The rate constant for the effect of CO2 (3 x 10(-16) cm(3) molecule(-1) s(-1) at 1000 K) and H2O (2 x 10(-17) cm(3) molecule(-1) s(-1) at 1000 K) is at least one-two-order magnitude smaller than the free reaction (3 x 10(-15) cm(3) molecule(-1) s(-1) at 1000 K). The effect of CO2 and H2O on (CH3)-C-center dot + O-3(2) shows non-RRKM behavior, however, the effect on (CH3)-C-center dot + O-3(2) shows RRKM behavior. Our results also demonstrate that a single CO2 and H2O molecule has the potential to accelerate a gas-phase reaction at temperature higher than >1300 K and slow the reaction at a lower temperature. The result is unique and observed for the first time.