Unraveling Interaction Chemistry between Saturated Methyl Esters and Nitrogen Dioxide: An <i>Ab Initio</i> Kinetic Study on H-Atom Abstraction Reactions

The kinetic interactions between fuel molecules and nitrogen dioxide (NO₂) play a central role in fuel ignition and combustion characteristics in advanced combustion engines equipped with exhaust gas recirculation (EGR). This study presents the first theoretical calculations to unravel the interaction chemistry between methyl esters and the methyl ethers and NO₂. High-level <i>ab initio</i> kinetic calculations were carried out to investigate H-atom abstraction reactions from C2-C5 saturated methyl esters by NO₂. Five small molecule esters, including methyl formate, methyl ethanoate, methyl propanoate, methyl butanoate, and methyl <i>iso</i>-butanoate, were considered, encompassing 42 distinct H-atom abstraction reaction channels. The CCSD(T)/CBS//M06-2<i>X</i>/6-311++G(d,p) level of theory was employed to calculate the potential energy surfaces. The conventional transition state theory was adopted to calculate the rate coefficients within a temperature range of 400-2000 K. The effect of fuel molecular structures was analyzed via a systematic comparison. The rate rules for H-atom abstraction reactions from different carbon sites of methyl esters were proposed. The rate coefficients reported in this study are beneficial for developing reliable combustion kinetic models for methyl esters or real biodiesel interactions with nitrogen oxides.