Publications

Reactions of α-hydroxyethyl (CH3CHOH) and β-hydroxyethyl (CH2CH2OH) radicals with oxygen are of key importance in ethanol combustion. High-level ab initio calculations of the potential energy surfaces of these two reactions were coupled with master equation methods to compute rate coefficients and product branching ratios for temperatures of 250-1000 K. The α-hydroxyethyl + O2 reaction is controlled by the barrierless entrance channel and shows negligible pressure dependence; in contrast, the reaction of the β isomer displays pronounced pressure dependence. The high pressure limit rate coefficients of both reactions are about the same at the temperatures investigated. Products of the reactions were monitored experimentally at 4 Torr and 300-600 K using tunable synchrotron photoionization mass spectrometry. Hydroxyethyl radicals were produced from the reaction of ethanol with chlorine atoms and the β isomer was also selectively produced by the addition reaction C2H4 + OH → CH2CH2OH. Formaldehyde, acetaldehyde, vinyl alcohol and H2O2 products were detected, in qualitative agreement with the theoretical predictions. © 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

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This work reports measurements of absolute rate coefficients and Rice-Ramsperger-Kassel-Marcus (RRKM) master equation simulations of the C 2H 3 + C 2H 4 reaction. Direct kinetic studies were performed over a temperature range of 300-700 K and pressures of 20 and 133 mbar. Vinyl radicals (H 2C=CH) were generated by laser photolysis of vinyl iodide (C 2H 3I) at 266 nm, and time-resolved absorption spectroscopy was used to probe vinyl radicals through absorption at 423.2 nm. Measurements at 20 mbar are in good agreement with previous determinations at higher temperature. A weighted three-parameter Arrhenius fit to the experimental rate constant at 133 mbar, with the temperature exponent fixed, gives k = (7 ±1) × 10 -14 cm 3 molecule -1 s -1 (T/298 K) 2 exp[-(1430 ± 70) K/T]. RRKM master equation simulations, based on G3 calculations of stationary points on the C 4H 7 potential energy surface, were carried out to predict rate coefficients and product branching fractions. The predicted branching to 1-methylallyl product is relatively small under the conditions of the present experiments but increases as the pressure is lowered. Analysis of end products of 248 nm photolysis of vinyl iodide/ethylene mixtures at total pressures between 27 and 933 mbar provides no direct evidence for participation of 1-methylallyl. © 2007 American Chemical Society.

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The implementation of variational transition state theory (VTST) for long-range asymptotic potential forms is considered, with particular emphasis on the energy and total angular momentum resolved (μJ -VTST) implementation. A long-range transition state approximation yields a remarkably simple and universal description of the kinetics of reactions governed by long-range interactions. The resulting (μJ -VTST) implementation is shown to yield capture-rate coefficients that compare favorably with those from trajectory simulations (deviating by less than 10%) for a wide variety of neutral and ionic long-range potential forms. Simple analytic results are derived for many of these cases. A brief comparison with a variety of low-temperature experimental studies illustrates the power of this approach as an analysis tool. The present VTST approach allows for a simple analysis of the applicability conditions for some related theoretical approaches. It also provides an estimate of the temperature or energy at which the "long-range transition state" moves to such short separations that short-range effects, such as chemical bonding, steric repulsion, and electronic state selectivity, must be considered. © 2005 American Institute of Physics.