A theoretical study on the β-C-H fission of ethoxy radical

We present a direct ab initio dynamics study of the thermal gas-phase rate constants of the unimolecular decomposition reaction of C₂H ₅O→CH₃CHO+H. Both MPW1K/6-31+G(d,p) and QCISD/6-31+G(d,p) methods were employed to optimize the geometries of all stationary points and to calculate the minimum energy path (MEP). The energies of all the stationary points were refined at a series of multi-coefficient and multi-level methods. Among all methods, the QCISD(T)/aug-cc-pVTZ energies are in good agreement with the available experimental data. The rate constants were evaluated based on the energetics from the QCISD(T)/aug-cc-pVTZ//MPW1K/6-31+G(d, p) level of theory using both microcanonical variational transition (mVT) state theory and canonical variational transition (CVT) state theory with the Eckart tunneling correction in the temperature range of 200-2500 K. The calculated rate constants at the QCISD(T)/aug-cc-pVTZ//MPW1K/6-31+G(d,p) level of theory are in good consistent with the theoretical data from Caralp et al. The fitted Arrhenius expression in the temperature range 200-2500 K is k=1.30×10 ⁹T^1.42e^{(-1.03×104/T)} s^-1. The falloff curves of pressure-dependent rate constants are performed using master-equation method within the temperature range of 391-471 K.