First-Principles Dynamics along the Reaction Path of CH3CH2 + O2 → H2C=CH2 + HOO: Evidence for Vibronic State Mixing and Neutral Hydrogen Transfer

Abstract

We employ Born−Oppenheimer molecular dynamics (BOMD), with forces derived from spin-polarized density functional theory using the B3LYP hybrid exchange-correlation functional, to explore the dynamics of oxidation of ethyl radical to produce ethylene, along the concerted-elimination path CH3CH2 + O2 → CH3CH2OO → CH2CH2 + HOO. The transition state connecting CH3CH2OO to CH2CH2 and HOO has a planar, five-membered-ring structure ···C−C−H−O−O··· known as TS1. The electronic nature of this saddle point has been the subject of controversy. Recent ab initio calculations have indicated that TS1 has a 2A‘ ‘ electronic ground state within Cs symmetry. In this state, intramolecular neutral hydrogen transfer from the methyl group of the intermediate ethylperoxy radical (CH3CH2OO·) to the terminal oxygen is hindered by the lack of overlap between the 1s orbital of the in-plane hydrogen atom and the singly-occupied 2p (a‘ ‘) orbital of the terminal oxygen. Previous explanations invoked proton transfer, a rather unpalatable process for an alkylperoxy radical. Two other possibilities that both facilitate neutral H-transfer are explored in the present work, namely: (i) an O2 π*-resonance mechanism and (ii) 2A‘−2A‘ ‘ state mixing. First, we show that the structure of TS1 is a “late,” loose transition state, consistent with a loosely coupled O2 that can shift π*-electrons to aid neutral hydrogen atom transfer. Second, our BOMD trajectories reveal that torsional motion in the ethylperoxy radical and at the transition state causes symmetry-breaking and 2A‘−2A‘ ‘ state mixing. The low-lying 2A‘ excited state, with its in-plane, singly occupied oxygen 2p orbital, can easily transfer a neutral H atom. Not only is vibrationally-induced symmetry-breaking present near (and after crossing) TS1, but also in the CH3CH2 and O2 entrance channel, which again exhibits torsional motion that allows both the 2A‘ ‘ ground state and the excited 2A‘ state to be accessed while forming the ethylperoxy radical. Thus we propose that vibronic state mixing is a key feature of the reaction dynamics of ethane combustion, helping to facilitate dehydrogenation.