A Gas-Phase Investigation of Oxygen-Hydrogen Exchange Reaction of O(³P) + C₂H₅→ H(²S) + C₂H₄O

Abstract

The gas-phase radical-radical reaction O( 3 P) + C 2 H 5 (ethyl) → H( 2 S) + CH 3 CHO (acetaldehyde) was investigated by applying a combination of vacuum-ultraviolet laser-induced fluorescence spectroscopy in a crossed beam configuration and ab initio calculations. The two radical reactants O( 3 P) and C 2 H 5 were respectively produced by photolysis of NO 2 and supersonic flash pyrolysis of the synthesized precursor azoethane. Doppler profile analysis of the nascent H-atom products in the Lyman-α region revealed that the average translational energy of the products and the average fraction of the total available energy released as translational energy were 5.01 ± 0.72 kcal mol −1 and 6.1%, respectively. The empirical data combined with CBS-QB3 level ab initio theory and statistical calculations demonstrated that the title exchange reaction is a major channel and proceeds via an addition-elimination mechanism through the formation of a short-lived, dynamical addition complex on the doublet potential energy surface. On the basis of systematic comparison with several exchange reactions of hydrocarbon radicals, the observed small kinetic energy release can be explained in terms of the loose transition state with a product-like geometry and a small reverse activation barrier along the reaction coordinate.

Theoretical Investigation of the Radical–Radical Reaction of O(3P) + C2H3 and Comparison with Gas-Phase Crossed-Beam Experiments

“…For closed-shell primary, secondary, and tertiary hydrocarbon molecules, as the reaction exothermicity increases, the barrier height gradually decreases from ∼7 to ∼3 kcal mol –1 , which stands in sharp contrast to the negligible activation barrier of the atom–radical reaction in this study. Analogous barrierless abstraction processes were also observed in the H-atom abstraction reactions of O­( 3 P) + hydrocarbon radicals such as allyl and t -butyl, as reported by the authors. − ,− …”

“…The T 1 diagnostic calculations were also performed to obtain a qualitative assessment of the significance of nondynamic correlations: the smaller the T 1 value, the more reliable the results of the single-reference wave function. The small T 1 diagnostic values obtained indicate that the single-reference method employed in this study was an efficient and reliable alternative to multireference calculations. , The calculated values of the heat of formation and reaction enthalpies were in good agreement, within 1.5 kcal mol –1 , with the previously reported experimental values. − …”

“…Donalson and coworkers studied the CO channel (2) by applying arrested relaxation FTIR spectroscopy and found that CO shows highly nonstatistical vibrational distrubutions . Recently, the authors performed a crossed-beam study utilizing high-resolution, vacuum–ultraviolet laser-induced fluorescence (LIF) spectroscopy and interrogated the kinetic energy release of the nascent H-atom products and the nascent rovibrational energy distributions of the OH products from exothermic reaction channels (1) and (10), respectively. − …”

“…This investigation was carried out as part of continuing efforts to deduce the general principles that explain the mechanistic and dynamic traits of atom-radical reactions. In recent years, the authors have reported gas-phase crossed-beam studies of the reaction dynamics of O­( 3 P) with saturated hydrocarbon radicals such as methyl (CH 3 ), ethyl (C 2 H 5 ), and t -butyl ( t -C 4 H 9 ) and unsaturated π-conjugated hydrocarbon radicals such as allyl (C 3 H 5 ) and propargyl (C 3 H 3 ). − A supersonic flash pyrolysis source was adopted to produce clean, jet-cooled radical beams without experiencing recombination and/or secondary dissociation. By means of ab initio simulations and statistical calculations, these experiments revealed that the atom-radical reactions followed microscopic mechanisms with unusual dynamic features.…”

Gas-phase formation of acetaldehyde: review and new theoretical computations

“…Specifically, reaction (2) was studied by combined cross-beam and computational studies. However, these studies were not focused on kinetics and they were carried out in the 295-600 K temperature range (Jung et al 2011;Jang et al 2014;Park et al 2010). Reaction (3) was studied computationally, using an ab initio and DFT composite method but no kinetic computations were carried out (Zhang et al 2002).…”