Kinetics of Hydrogen Abstraction Reactions of Methyl Palmitate and Octadecane by Hydrogen Atoms.

Hydrogen abstractions play a crucial role in the consumption of fuel molecules during fuel pyrolysis and combustion processes. In this study, a generalized energy-based fragmentation approach was used to obtain CCSD(T)-F12a/cc-pVTZ energy barriers of hydrogen abstraction reactions by hydrogen atoms from methyl palmitate (C15H31COOCH3), a key component of biodiesel. The accuracy of M06-2X/6-311++G(d,p) for obtaining the energy barriers was evaluated against the CCSD(T) results. Based on the quantum chemical results, the high-pressure-limit rate constants for C15H31COOCH3 + H were calculated and compared with those of octadecane ( n-C18H38) reacting with H. The treatment of hindered internal rotations for such long-chain molecules was discussed and the rate rules for different abstraction sites were summarized. The results show that in the C15H31COOCH3 + H system, the α hydrogen abstraction no longer plays a dominant role as in small methyl esters, and the hydrogen atoms of CH2 groups far away from the ester group are more easily abstracted than those near the ester group.