High-Pressure Rate Rules for Alkyl + O₂ Reactions. 1. The Dissociation, Concerted Elimination, and Isomerization Channels of the Alkyl Peroxy Radical

Abstract

The reactions of alkyl peroxy radicals (RO(2)) play a central role in the low-temperature oxidation of hydrocarbons. In this work, we present high-pressure rate estimation rules for the dissociation, concerted elimination, and isomerization reactions of RO(2). These 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. The rate constants for the dissociation reactions are obtained from calculated equilibrium constants and a literature review of experimental rate constants for the reverse association reactions. For the concerted elimination and isomerization channels, rate constants are calculated using canonical transition state theory. To determine if the high-pressure rate expressions from this work can directly be used in ignition models, we use the QRRK/MSC method to calculate apparent pressure and temperature dependent rate constants for representative reactions of small, medium, and large alkyl radicals with O(2). A comparison of concentration versus time profiles obtained using either the pressure dependent rate constants or the corresponding high-pressure values reveals 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).

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

“…For example, a primary radical at C1 in 2-methylhexane ( Figure 4) renders C 2 a chiral center as it is connected to four different groups, which requires the addition of the OI group value. It is also added for every OO and OOH group in peroxy and alkyl hydroperoxide species since they are considered as pseudo-chiral centers 11 . The OI group value adds a correction of Rln (2) to the entropy.…”

“…Villano et al 11 found that the activation energy of this elimination reaction is hardly affected by the thermochemistry or the bond dissociation energies of C-OO and C-H bonds. The reactivity of this pathway is only influenced by the chemical nature of the produced olefin.…”

“…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 .…”

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].…”