High-Pressure Rate Rules for Alkyl + O₂ Reactions. 2. The Isomerization, Cyclic Ether Formation, and β-Scission Reactions of Hydroperoxy Alkyl Radicals

Abstract

The unimolecular reactions of hydroperoxy alkyl radicals (QOOH) play a central role in the low-temperature oxidation of hydrocarbons as they compete with the addition of a second O(2) molecule, which is known to provide chain-branching. In this work we present high-pressure rate estimation rules for the most important unimolecular reactions of the β-, γ-, and δ-QOOH radicals: isomerization to RO(2), cyclic ether formation, and selected β-scission reactions. These rate rules are derived from high-pressure rate constants for a series of reactions of a given reaction class. The individual rate expressions are determined from CBS-QB3 electronic structure calculations combined with canonical transition state theory calculations. Next we use the rate rules, along with previously published rate estimation rules for the reactions of alkyl peroxy radicals (RO(2)), to investigate the potential impact of falloff effects in combustion/ignition kinetic modeling. Pressure effects are examined for the reaction of n-butyl radical with O(2) by comparison of concentration versus time profiles that were obtained using two mechanisms at 10 atm: one that contains pressure-dependent rate constants that are obtained from a QRRK/MSC analysis and another that only contains high-pressure rate expressions. These simulations reveal that under most conditions relevant to combustion/ignition problems, the high-pressure rate rules can be used directly to describe the reactions of RO(2) and QOOH. For the same conditions, we also address whether the various isomers equilibrate during reaction. These results indicate that equilibrium is established between the alkyl, RO(2), and γ- and δ-QOOH radicals.

Modeling Ignition of a Heptane Isomer: Improved Thermodynamics, Reaction Pathways, Kinetics, and Rate Rule Optimizations for 2-Methylhexane

“…Villano et al 14 calculated the high-pressure-limit rate coefficients of these reactions, in the reverse, exothermic direction, for C 2 -C 5 and selected C 6 and C 7 olefins at the same level of theory used in their previous work 11,13 .…”

“…Bugler et al 5 revisited the chemical kinetic models of the pentane isomers originally developed by Healy et al 9 . In their work, they investigated the effect of implementing thermodynamic and reaction rate updates [10][11][12][13][14][15] , as well as the addition of alternative OOQOOH isomerization pathways, on the combustion properties of pentane isomers. Finally, n-hexane sub-mechanism in Curran et al's 8 heptane model was updated by Zhang et al 6 .…”

An experimental and kinetic modeling study of n -hexane oxidation

“…A series of quantum chemical calculations have been reported on the low-temperature oxidation reactions of alkanes [46][47][48][49][50]. However, few of these calculations have been performed for n-hexane or larger fuels.…”

“…Villano et al [49] developed the reaction rate rules for the formation of cyclic ethers and the betascission reactions of H radicals as linear functions of the heat of reaction. Villano et al also reported that the uncertainty in the A-factors is a factor of two, while the uncertainty in the activation energy is ±1 kcal mol -1 .…”

Kinetics of the hydrogen abstraction ·C2H5 + alkane → C2H6 + alkyl reaction class: an application of the reaction class transition state theory

“…This approach was successfully applied by Sumathi et al [11,12], Allen et al [13], Sabbe et al [14][15][16][17], and Wang et al [18,19]. Another approach, where rate rules are derived from a systematic investigation of sets of reactions within a given reaction class using electronic structure calculations performed at the CBS-QB3 level of theory, was reported by Villano et al [20,21]. The survey of the different rate estimation rules, together with their applications, was recently reported by Carstensen and Dean [22].…”