Ab initio calculations and mechanism of two proton migration reactions of ethoxy radical

We present a direct ab initio and density functional theory dynamics study of the thermal rate constants of the two H-migration reactions of C2H5O radical. MPW1K/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 the QCISD(T)/aug-cc-pVTZ level of theory. The thermal gas phase rate constants were evaluated based on the energetics from the QCISD(T)/aug-cc-pVTZH MPW1K/6-31+G(d,p) level of theory using both microcanonical variational transition state theory (muNT) and canonical variational transition state theory (CVT) with the Eckart tunneling correction in the temperature range of 200-2500 K. The extended Arrhenius expression fitted from the muVT/Eckart rate constants of 1,2 H-shift and 1,3 H-shift reactions of C2H5O radical in the temperature range of 200-2500 K are k = 3.90 x 10(-31) T(12.4)e((-2.13 x 103/T)) and k = 2.83 x 10(-29)T(11.9)e((-2.24) (x 103/T)) s(-1), respectively. The two isomerization rate constants exhibited positive temperature dependence in the calculated temperature region. The variational effects for the two isomerizations of ethoxy radical are small and the tunneling effects are important in the low temperature range. The titled reactions are minor and not essential compared to the decomposition pathways of ethoxy radical. (C) 2004 Elsevier B.V. All rights reserved.