Intramolecular Hydrogen Migration in Alkylperoxy and Hydroperoxyalkylperoxy Radicals: Accurate Treatment of Hindered Rotors

Abstract

We have calculated the thermochemistry and rate coefficients for stable molecules and reactions in the title reaction families using CBS-QB3 and B3LYP/CBSB7 methods. The accurate treatment of hindered rotors for molecules having multiple internal rotors with potentials that are not independent of each other can be problematic, and a simplified scheme is suggested to treat them. This is particularly important for hydroperoxyalkylperoxy radicals (HOOQOO). Two new thermochemical group values are suggested in this paper, and with these values, the group additivity method for calculation of enthalpy as implemented in reaction mechanism generator (RMG) gives good agreement with CBS-QB3 predictions. The barrier heights follow the Evans-Polanyi relationship for each type of intramolecular hydrogen migration reaction studied.

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

“…The weak O-OH bond in the ketohydroperoxide then breaks to form OH and an alkoxy radical. This sequence of chain branching reactions is responsible for low-temperature reactivity 10 .…”

“…However, Sharma et al 10 found that the difference between the activation energies of RO 2 isomerization and OOQOOH isomerization varies with the transition state's ring size. They estimated corrections of 8.6, 2.2 and 0 kcal/mol for 5-, 6-and 7-membered ring migrations, respectively.…”

“…They showed that the use of Sharma's rate coefficients gives very good agreement with experimental ignition delay data, better than when using Miyoshi's rate coefficients. Therefore, values from Sharma et al 10 were used in this work. These values are determined computationally at the CBS-QB3//B3LYP/6-31G(d) level of theory using coupled internal rotor treatment.…”

“…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 .…”

Advanced Biofuels and Beyond: Chemistry Solutions for Propulsion and Production

“…[140] Examples for typical reaction classes are hydrogen abstraction from the fuel molecule by different radicals or O 2 addition to the different fuel radicals,and each reaction class typically leads to an umber of elementary reactions.T hese reaction classes are applied consistently to species of different size,a nd Arrhenius parameters for the reaction rate coefficients are then assigned by so-called rate rules that depend on the reaction class and the specific sites at which the reaction occurs.Useful and accurate sets of reaction classes and associated rate rules have been published in the Angewandte Chemie Reviews recent literature. [153][154][155][156] Ab initio calculations to determine reliable bond dissociation energies,r eaction rate parameters for the most decisive reactions such as the initial hydrogen abstraction from the fuel, transition states,a nd branching ratios towards specific products are valuable complements to such efforts.…”