A computational study on the kinetics of unimolecular reactions of ethoxyethylperoxy radicals employing CTST and VTST

Diethyl ether (DEE) has favorable properties as a diesel fuel component, including its outstanding cetane number. To utilize this promising fuel, more and more knowledge on the chemical kinetics of DEE oxidation will be required. For the present article, the rate constants of unimolecular reactions of ethoxyethylperoxy radicals, which are main intermediates in the oxidation of DEE under the engine relevant conditions, have been computationally investigated and compared with those of alkanes. Geometries, vibrational frequencies, and energies of reactants, products, and transition states with pronounced barrier were calculated according to the procedure of the CBS-QB3 method. The high-pressure limiting rate constants were calculated in the temperature range of 500−2000 K by using a conventional transition state theory with hindered rotor approximation for low frequency torsional mode. The oxygen dissociation reactions have been investigated by using a variational transition state theory based on the CASPT2(7,5)/aug-cc-pvdz single point calculations at UB3LYP/CBSB7 geometries and vibrational frequencies. It was found that the oxidation pathways are equal to those of alkane oxidations, however, the rate constants are significantly different from those of alkanes due to the oxygen vicinity. The rate constants of intramolecular hydrogen shift reactions are from 3 to 8 times larger at 700 K than those of alkylic peroxy radical when the abstracted hydrogen is in the β-position of the ether. The rate constant of β-scission reactions for 1,5-intramolecular hydrogen shift products of 1-ethoxyethylperoxy radial is 163 times larger at 700 K than that of alkylic hydroperoxy radical, and this reaction becomes a main reaction pathway, whereas cyclic-ether is a main product in alkane oxidation. These characteristic rate constants are given in three-parameter modified Arrhenius form for the refinement of predictive chemical kinetic models being developed.