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 10(3) 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 + H-2 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.