Pressure-dependent rate rules for intramolecular H-migration reactions of normal-alkyl cyclohexylperoxy radicals

Abstract

For hydrocarbon fuels, reliable chemical mechanisms play an essential role towards improving the simulations of combustion process. However, in most of the previous modeling studies for alkylcycloalkanes combustion, the kinetic parameters are taken from acyclic alkanes based on the similarities between the reaction classes of alkylcycloalkanes and acyclic alkanes. Among the reaction families for alkylcycloalkanes, the intramolecular H-migration reactions of normal-alkyl cyclohexylperoxy radicals are one of the most important reaction families in the low temperature oxidation mechanisms. High-pressure limit rate rules and pressure-dependent rate rules for this reaction family have been developed based on quantum chemical computations at the CBS-QB3 level in combination with the transition state theory (TST) and Rice–Ramsberger–Kassel–Marcus/Master-Equation (RRKM/ME) theory. The reactions in this family involve a bicyclic transition state and can be divided into classes depending upon the cycle size of the newly formed cycle in the transition states and the types of the carbons from which the H atoms are migrated and if the H-migration occurs on the alkyl side chain, or on the cycle, or from the side chain to the cycle, or from the cycle to the side chain. For each reaction class, a representative set of reactions are chosen from methylcyclohexane to n-butylcyclohexane and the energy barriers of all chosen reactions are calculated and the average value for each class is obtained and compared with the value of the similar reaction class in alkanes. The high-pressure limit rate constants and the pressure-dependent rate constants of all reactions at pressures varying from 0.01 to 100 atm are calculated and the high-pressure limit rate rules and pressure-dependent rate rules for each class are derived from the average rate constants of reactions within each class. All calculated rate constants are fitted by a nonlinear least-squares method to the form of a modified Arrhenius rate expression.It is shown that there are large differences of the rate constants between alkylcycloalkanes and alkanes no matter if the H-migration occurs on the alkyl side chain, on the cycle or between the side chain and the cycle. Therefore, the construction of rate rules for alkylcycloalkanes, instead of taking values from similar reactions in alkanes, is necessary and significant to the low-temperature combustion modeling of alkylcyclohexanes.