Probing the kinetic energy-release dynamics of H-atom products from the gas-phase reaction of O(³P) with vinyl radical C₂H₃

Abstract

The gas-phase radical-radical reaction dynamics of ground-state atomic oxygen O((3)P) with vinyl radicals C2H3 has been studied by combining the results of vacuum-ultraviolet laser-induced fluorescence spectroscopy in a crossed beam configuration with ab initio calculations. The two radical reactants O((3)P) and C2H3 were produced by photolysis of NO2 and supersonic flash pyrolysis of C2H3I, respectively. Doppler profile analysis of the kinetic energy release of the nascent H-atom products from the title reaction O((3)P) + C2H3→ H((2)S) + CH2CO (ketene) revealed that the average translational energy of the products and the average fraction of the total available energy were 7.03 ± 0.30 kcal mol(-1) and 7.2%. The empirical data combined with CBS-QB3 level ab initio theory and statistical calculations demonstrated that the title oxygen-hydrogen exchange reaction is a major reaction channel, through an addition-elimination mechanism involving the formation of a short-lived, dynamical complex on the doublet potential energy surface. On the basis of systematic comparison with several exchange reactions of hydrocarbon radicals, the observed kinetic energy release can be explained in terms of the weak impulse at the moment of decomposition in the loose transition state with a product-like geometry and a small reverse barrier along the exit channel.

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. − …”

“…The direct abstraction reaction mechanism results from the dynamics of a direct H-atom transfer via a collinear transition state. Such mechanisms have been generally observed in the reactions of O­( 3 P) with saturated, closed-shell hydrocarbon molecules. − ,− Herein, the respective addition reaction mechanisms for reactions – are first described through energy-sufficient intermediates vINT1–vINT4. Afterward, the H-atom abstraction pathways for the production of OH in reactions – are described.…”

“…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.…”