Effect of Water and Formic Acid on ∙OH + CH₄ Reaction: An Ab Initio/DFT Study

In this work, we used ab initio/DFT method coupled with statistical rate theory to answer the question of whether or not formic acid (HCOOH) and water molecules can catalyze the most important atmospheric and combustion prototype reaction, i.e.,^∙ OH (OH radical) + CH4. The potential energy surface for^∙OH + CH4 and^∙ OH + CH4 (+X) (X = HCOOH, H2O) reactions were calculated using the combination of hybrid‐density functional theory and coupled‐cluster theory with Pople basis set [(CCSD(T)/ 6‐311++G(3df,3pd)//M06‐2X/6‐311++G(3df,3pd)]. The results of this study show that the catalytic effect of HCOOH (FA) and water molecules on the^∙OH +CH4 reaction has a major impact when the concentration of FA and H2O is not included. In this situation the rate constants for the CH4 + HO∙∙∙HCOOH (3 × 10⁻⁹ cm³ molecule⁻¹ s⁻¹) reaction is ~10⁵ times and for CH4 + H2O∙∙∙HO reaction (3 × 10⁻¹⁴ cm³ molecule⁻¹ s⁻¹ at 300 K) is ~20 times higher than^∙ OH +CH4 (~6 × 10⁻¹⁵ cm³ molecule⁻¹ s⁻¹). However, the total effective rate constants, which include the concentration of both species in the kinetic calculation has no effect under atmospheric condition. As a result, the total effective reaction rate constants are smaller. The rate constants when taking the account of the FA and water for CH4 + HO∙∙∙HCOOH (4.1 × 10⁻²² cm³ molecule⁻¹ s⁻¹) is at least seven orders magnitude and for the CH4 + H2O∙∙∙HO (7.6 × 10⁻¹⁷ cm³ molecule⁻¹ s⁻¹) is two orders magnitude smaller than^∙ OH +CH4 reaction. These results are also consistent with previous experimental and theoretical studies on similar reaction systems. This study helps to understand how FA and water molecules change the reaction kinetic under atmospheric conditions for^∙OH +CH4 reaction.