First-principles dynamics along the reaction path of CH₃CH₂ + O₂ → H₂C=CH₂ + HOO: evidence for vibronic state mixing and neutral hydrogen transfer

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 → CH₂=CH₂ + HOO. The transition state connecting CH₃CH₂OO to CH₂=CH₂ 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 C_s 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.