Theoretical Study on Abstraction and Addition Reaction Kinetics for a Medium-Size Unsaturated Methyl Ester: Methyl-3-hexenoate + H/OH Radicals
Bo Liu, Zihao Zhou, Zhenpeng Zhang, Hongbo Ning
Abstract
Combustion chemistry of methyl esters with long alkyl chains and different degrees of unsaturation has been of particular interest for biodiesel combustion. Methyl-3-hexenoate (mhx3d) with a medium-size unsaturated aliphatic chain is regarded as a valuable surrogate of biodiesel components. Here, the abstraction and addition reaction kinetics of mhx3d + H/OH radicals are comprehensively investigated. An accurate and efficient exchange-correlation density functional M06-2X/ma-TZVP is validated by the DLPNO-CCSD(T)/CBS(T-Q) benchmark calculations and is used for the multistructural search, geometry optimization, potential energy profiles, and direct dynamic calculations. The multistructural torsional (MS-T) anharmonicity, variational effects, and tunneling effects including one-dimensional Winger tunneling, multidimensional zero-curvature tunneling (ZCT), and small-curvature tunneling (SCT) are also evaluated in rate coefficient calculations. The existence of reactant complexes of the abstraction and addition reactions is neglected for mhx3d + H due to weak van der Waals interaction but is considered for mhx3d + OH. Therefore, a pre-equilibrium model is employed to obtain the rate coefficients for mhx3d + OH. The calculation results show that the MS-T factor ranges from 0.29 to 11.41, the tunneling transmission coefficient calculated by SCT is in the range of 1.0-4.0, and the recrossing transmission coefficient is between 0.65 and 1.0. Moreover, the two OH-addition reactions exhibit negative temperature coefficient behavior. The branching ratios show that the H/OH-addition reactions are dominant at lower temperature, especially for mhx3d + H. Rate coefficients of the title reactions are fitted in terms of a modified three-parameter Arrhenius expression.