Theoretical Study on the Kinetics of Secondary Oxygen Addition Reactions for N-Butyl Radicals

Chemical kinetics for second oxygen addition reactions
(·QOOH
+ O2) of long-chain alkanes are of great importance in
low-temperature combustion technologies. However, kinetic data for
key reactions of ·QOOH + O2 systems are often difficult
to obtain experimentally and are primarily estimated or calculated
by using theoretical methods. In this work, barrier heights (BHs),
reaction energies (ΔEs), and relative energies
(REs) of stationary points for key reactions of two representative
·QOOH + O2 systems in the low-temperature oxidation
of n-butyl as well as pressure-dependent rate constants
for the involved reactions are calculated with the high-level quantum
chemical method CCSD(T)-F12b/CBS. These results can be employed in
the development of low-temperature combustion mechanisms for n-butane and longer straight-chain alkanes. In addition,
the performance of some quantum chemistry methods with a lower computational
cost on BHs, ΔEs, and REs as well as rate constants
is also investigated. Our results indicate that the maximum error
on these energies with PNO-LCCSD(T)-F12a is within 1 kcal/mol, and
rate constants with this method are in the best agreement with reference
values, with a maximum relative error of about half the reference
values. Due to its low computational cost and memory requirements,
this method is strongly recommended for studying low-temperature combustion
reactions involving larger hydrocarbon fuels.