Exploring the kinetics and thermochemistry effects on C2-C6 alkene combustion chemistry by ȮH radical; Implications for Combustion Modeling and Simulation

Abstract

A systematic theoretical study of C2 to C6 alkenes reacting with ȮH radical to explore the kinetics and thermochemistry effects are carried out in this work. The geometries and frequencies of 15 reactants, 15 pre-reaction complexes, 93 transition states, and 93 products are optimized at M06-2X/6-311++G(d,p) level of theory. The hindrance potentials for low-frequency torsional modes are treated at M06-2X/6–31 G level of theory. Single-point energies for all species involved in this work are calculated at spin-unrestricted CCSD(T)/CBS level of theory with basis-set correction from MP2/aug-cc-pVnZ (n = T and Q). High-pressure-limit rate constants for H-atom abstraction reaction channels and pressure-dependent rate constants for addition reaction channels are calculated by solving the Rice-Ramsperger Kassel-Marcus/Master Equation (RRKM/ME) implemented in the Master Equation System Solver (MESS) program, coupled with variational transition-state theory (VTST). Temperature-dependent thermochemical properties for all species and related radicals have been computed using a combination of composite methods to provide adequate accuracy results for modeling application. For straight-chain alkenes, the temperature range, in which the addition channel dominates, shifts to lower temperatures as the alkene gets longer. Hydrogen-atom abstractions from the allylic site of alkenes are the dominant reaction channels at high temperatures. The H-atom abstraction reactions from secondary alkylic and primary alkylic CH sites become more important as substituent groups become larger. High-pressure-limit and pressure-dependent rate constants calculated in this work have been extensively compared with the experimental and theoretical results available in the literature, and they show overall good agreement. The simulation results show that the calculated rate constants in this work have positive influences to the accuracy of simulation results at most reaction conditions. However, at some of the reaction conditions, thermochemical data have much more important influence to the simulation results at low temperature ranges. Rate constants obtained in this work and detailed branching ratio for each reaction can serve as a guidance in alkene combustion model development.