Thermal Decomposition of Ethanol. 4. Ab Initio Chemical Kinetics for Reactions of H Atoms with CH3CH2O and CH3CHOH Radicals

Abstract

The potential energy surfaces of H-atom reactions with CH₃CH₂O and CH₃CHOH, two major radicals in the decomposition and oxidation of ethanol, have been studied at the CCSD(T)/6-311+G(3df,2p) level of theory with geometric optimization carried out at the BH&HLYP/6-311+G(3df,2p) level. The direct hydrogen abstraction channels and the indirect association/decomposition channels from the chemically activated ethanol molecule have been considered for both reactions. The rate constants for both reactions have been calculated at 100−3000 K and 10⁻⁴ Torr to 10³ atm Ar pressure by microcanonical VTST/RRKM theory with master equation solution for all accessible product channels. The results show that the major product channel of the CH₃CH₂O + H reaction is CH₃ + CH₂OH under atmospheric pressure conditions. Only at high pressure and low temperature, the rate constant for CH₃CH₂OH formation by collisonal deactivation becomes dominant. For CH₃CHOH + H, there are three major product channels; at high temperatures, CH₃+CH₂OH production predominates at low pressures (P < 100 Torr), while the formation of CH₃CH₂OH by collisional deactivation becomes competitive at high pressures and low temperatures (T < 500 K). At high temperatures, the direct hydrogen abstraction reaction producing CH₂CHOH + H₂ becomes dominant. Rate constants for all accessible product channels in both systems have been predicted and tabulated for modeling applications. The predicted value for CH₃CHOH + H at 295 K and 1 Torr pressure agrees closely with available experimental data. For practical modeling applications, the rate constants for the thermal unimolecular decomposition of ethanol giving key accessible products have been predicted; those for the two major product channels taking place by dehydration and C−C breaking agree closely with available literature data.