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

The methyl (^•CH3) +³O2 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+ O2 system. The mechanism for three reaction systems, i.e., for^•CH3 +³O2,^•CH3 +³O2 (+CO2) and^•CH3 +³O2 (+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 +³O2 reaction, the reactive complexes, intermediates, transition states and post complexes become thermodynamically more favorable. The calculated rate constant for the^•CH3 +³O2 (3 × 10⁻¹⁵ cm³ molecule⁻¹ s⁻¹ at 1000 K) is in good agreement with the previously reported experimentally measured values (~1 × 10⁻¹⁵ cm³ molecule⁻¹ s⁻¹ at 1000 K). The rate constant for the effect of CO2 (3 × 10⁻¹⁶ cm³ molecule⁻¹ s⁻¹ at 1000 K) and H2O (2 × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹ at 1000 K) is at least one–two-order magnitude smaller than the free reaction (3 × 10⁻¹⁵ cm³ molecule⁻¹ s⁻¹ at 1000 K). The effect of CO2 and H2O on^•CH3 +³O2 shows non-RRKM behavior, however, the effect on^•CH3 +³O2 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.