Thermal decomposition of ethanol. II. A computational study of the kinetics and mechanism for the H+C2H5OH reaction

The kinetics and mechanism for the H+C2H5OH reaction, a key chain-propagation step in the high temperature decomposition and combustion of ethanol, have been investigated with the modified GAUSSIAN -2 (G2M) method using the structures of the reactants, transition states and products optimized at the B3LYP/6-311+G(d,p) level of theory. Four transition states have been identified for the production of H2+CH3CHOH (TS1), H2+CH2CH2OH (TS2), H2+C2H5O (TS3), and H2O+C2H5 (TS4) with the corresponding barriers, 7.18, 13.30, 14.95, and 27.10 kcal/mol. The predicted rate constants and branching ratios for the three H-abstraction reactions have been calculated over the temperature range 300–3000 K using the conventional and variational transition state theory with quantum-mechanical tunneling corrections. The predicted total rate constant, kt=3.15×103T3.12 exp(−1508/T) cm3 mol−1 s−1, agrees reasonably with existing experimental data; in particular, the result at 423 K was found to agree quantitatively with an available experimental value. The small deviation between the predicted kt and another set of experimental data measured at 295–700 K has been examined by kinetic modeling; the deviation is attributable to insufficient corrections for contributions from secondary reactions.