A Dual-Level Ab Initio and Hybrid Density Functional Theory Dynamics Study on the Unimolecular Decomposition Reaction C₂H₅O → CH₂O + CH₃

We present a direct ab initio and hybrid density functional theory dynamics study of the thermal rate constants of the unimolecular decomposition reaction of C₂H₅O → CH₂O + CH₃ at a high-pressure limit. MPW1K/6-31+G(d,p), MP2/6-31+G(d,p), and MP2(full)/6-31G(d) 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 multicoefficient and multilevel 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 state theory (μVT) and RRKM theory with the Eckart tunneling correction in the temperature range of 300-2500 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 experimental data. The fitted three-parameter Arrhenius expression from the μVT/Eckart rate constants in the temperature range 200-2500 K is k = 2.52 × 10¹²T ^0.41e^(-8894.0/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. The calculated results are in good agreement with the available experimental data.