High-Pressure-Limit and Pressure-Dependent Rate Rules for Unimolecular Reactions Related to Hydroperoxy Alkyl Radicals in Normal-Alkyl Cyclohexane Combustion. 2. Cyclization Reaction Class

The
hydroperoxy alkyl radicals are important intermediates in the
low-temperature combustion for normal-alkyl cycloalkanes, and the
cyclization reactions of hydroperoxy alkyl radicals to form cyclic
ethers are responsible for a major fraction of the OH formation, which
has the potential to promote ignition. In most of the previous modeling
studies for normal-alkyl cycloalkane combustion, the kinetic data
of the cyclization reactions in the detailed combustion mechanism
were mainly taken from the analogous reactions in cyclohexane, methyl
cyclohexane, and alkanes in published literature studies. In this
work, the kinetics of the cyclization reaction class of hydroperoxy
alkyl radicals in normal-alkyl cycloalkanes is studied, where the
reaction class is divided into subclasses depending upon the ring
size of the transition states, the types of the carbons on which the
−OOH site is located and the types of the carbons on which
the radical site is located, and the positions of the cyclization
(on the alkyl side chain, on the cycle, or between the alkyl side
chain and the cycle). Energy barriers and high-pressure-limit site
rate constants and pressure-dependent rates for reactions in all subclasses
are calculated, and rate rules for all subclasses are developed. The
high-pressure-limit rate constants are determined from CBS-QB3 electronic
structure calculations combined with canonical transition-state theory
calculations, and pressure-dependent rate constants are calculated
by using the Rice–Ramsberger–Kassel–Marcus/Master
Equation theory at pressures varying from 0.01 to 100 atm. Comparisons
of the rate constants for cyclization reactions of hydroperoxy alkyl
cyclohexylperoxy radicals calculated in this work with the values
of the corresponding reactions in some of the popular combustion mechanisms
show that it is unreasonable to use the kinetic data of analogous
reactions in alkanes, cyclohexanes, or smaller normal-alkyl cyclohexanes.
Therefore, the accurate kinetic calculations and the construction
of rate rules for normal-alkyl cycloalkanes are necessary and significant
for the reliable modeling of the low-temperature combustion of normal-alkyl
cyclohexanes.