Unsaturated C6 fatty acid methyl esters as reference molecules for biodiesel: kinetics of H-atom abstraction and addition

Abstract

This study reports accurate thermal rate constants for hydrogen abstraction and addition reactions by atomic hydrogen involving a series of C₆ unsaturated fatty acid methyl esters (FAMEs), namely methyl (Z)- and (E)-hex-2-enoate, methyl (Z)- and (E)-hex-3-enoate, methyl (Z)- and (E)-hex-4-enoate, and methyl hex-5-enoate. Rate constants were computed using multistructural canonical variational transition state theory combined with small-curvature tunneling (MS-CVT/SCT). Conformational searches were performed using a dual-level protocol based on HF/3-21G and MPWB1K/6-31+G(d,p) calculations. Conformational flexibility and torsional anharmonicity were rigorously treated by evaluating rovibrational partition functions using the multistructural torsional method with coupled torsional potentials [MS-T(CD)]. Among the species studied, (E)-3mhex shows the highest overall rate constants under conditions relevant to combustion. Multistructural effects significantly affect kinetic predictions, especially for hydrogen abstraction at the methyl group and the ε positions. In addition to the detailed rate constants, this study provides insights into the mechanisms involved, emphasizing the importance of stereochemistry and multistructural effects at both low and high temperatures. This information enhances the accuracy of kinetic models for the oxidation of unsaturated fatty acid methyl esters.