Theoretical chemical kinetic study of the h-atom abstraction reactions from oxygenated species by different radicals
Mendes Ferreira, Jorge Raul
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This work presents an ab-initio and chemical kinetic study of the H-atom abstraction reactions of ethers and ketones + HO2 radicals, aldehydes and acids + H atoms, OH, HO2 and CH3 radicals and esters + OH and HO2 radicals. The second order Møller-Plesset method using the 6-311G(d,p) basis set has been used in the geometry optimization and in the frequency calculation of all of the species involved in the above reactions, as well as the hindrance potential description for reactants and transition states. Intrinsic reaction co-ordinate calculations were carried out to validate all of the connections between transition states and local minima. The CCSD(T)/cc-pVXZ method (X = D, T, Q) and corresponding extrapolation to the complete basis set (CBS) limit was used in order to calculate the relative electronic energies of the reaction mechanisms of dimethyl ether, methanal, dimethyl ketone, methanoic acid and methyl ethanoate and are reported in kcal/mol. The relative energies calculated with the computationally less expensive methods of G3 in esters + OH radicals and CCSD(T)/cc-pVTZ in the remaining reaction mechanisms were benchmarked against the energies calculated with the CCSD(T)/CBS method and are within 1 kcal/mol. Rate constants have been calculated for all of the reaction channels by conventional transition state theory with asymmetric Eckart tunneling corrections and 1-D hindered rotor approximations in the temperature range 500-2000 K. A branching ratio analysis has also been investigated for some of the reaction mechanisms studied in this work. The rate constant results have been compared to available experimental and theoretical data and they are generally in good agreement. A comparison of abstraction of a H-atom by OH and HO2 radicals at the different positions relative to the functional group of the studied oxygenated species has been performed. For both OH and HO2 radicals, abstraction of a H-atom at the alpha' or alpha positions of alcohols and ethers is faster than for alkanes and generally slower for aldehydes, ketones, acids and esters throughout the temperature range studied.