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.