Ab initio rate coefficients for reactions of 2,5-dimethylhexyl isomers with O2: temperature- and pressure-dependent branching ratios

Effect of a single water molecule on ˙CH2OH + 3O2 reaction under atmospheric and combustion conditions

The hydroxymethyl (˙CH₂OH) radical is an important intermediate species in both atmosphere and combustion reaction systems. The rate coefficients for ˙CH₂OH + ³O₂ and (˙CH₂OH + ³O₂ (+H₂O)) reactions were calculated using the Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation (ME) simulation and canonical variational transition state theory (CVT) between the temperature range of 200 to 1500 K based on the potential energy surface constructed using CCSD(T)//ωB97XD/6-311++G(3df,3pd). The results show that ˙CH₂OH + ³O₂ leads to the formation of CH₂O and HO₂ at temperatures below 800 K, and goes back to reactants at high temperature (>1000 K). When a water molecule is added to the reaction, the formation of CH₂O and HO₂ is favored at all temperatures. The calculated rate coefficient for the ˙CH₂OH + ³O₂ (2.8 × 10^-11 cm³ molecule^-1 s^-1 at 298 K) is in good agreement with the previous experimental values (∼1 × 10^-11 cm³ molecule^-1 s^-1 at 298 K). The rate coefficients for the water-assisted reaction (2.4 × 10^-16 cm³ molecule^-1 s^-1 at 1000 K) is at least 3-4 orders of magnitude smaller than the water-free reaction (6.2 × 10^-12 cm³ molecule^-1 s^-1 at 1000 K). This result is consistent with the similar types of reaction system. Our calculations also predict that the effect of a single water molecule favors the formation of CH₂O in the combustion condition. However, the water-free reaction favors the formation of CH₂O in the atmospheric condition. The current study helps to understand how a single water molecule changes the reaction mechanism and chemical kinetic behaviour under atmospheric and combustion conditions.