Kinetic parameters for gas-phase reactions: Experimental and theoretical challenges

Abstract

This article aims to illustrate the added value provided to experimental kinetics investigations by complementary theoretical kinetics studies, using as examples (i) reactions of two major hydrocarbon flame radicals, HCCO and C(2)H, and (ii) reactions of several oxygenated organic compounds with hydroxyl radicals of interest to atmospheric chemistry. The first part, on HCCO and C(2)H kinetics, does not attempt to give an extensive literature review, but rather addresses some major experimental techniques, mainly specific ones, that have allowed a great part of the available reactivity databases on these two species to be established. For several key reactions, it is shown how potential energy surfaces and statistical rate predictions based thereon have provided insight into the molecular mechanisms and have allowed estimates of product distributions as well as reliable extrapolations of experimental rate coefficients and branching ratios to higher temperatures. The second part addresses current issues in atmospheric chemistry relating mainly to hydroxyl radical reactions with oxygenated organics, and focuses on the experimental characterization of the often unusual temperature dependence of their rate coefficients and on the theoretical rationalization thereof, through the formation of hydrogen-bonded pre-reactive complexes and resulting tunnelling-enhanced H-abstraction. Finally, the development of general structure-activity relationships for OH reactions with organics, H-abstractions as well as OH-additions for unsaturated compounds, is briefly discussed.

Towards cleaner combustion engines through groundbreaking detailed chemical kinetic models

“…These are used either directly or are converted to radicals by titration reactions (e.g. the reaction of propene with fluorine atoms to produce allyl radicals 125 , the reaction of acetylene with O-atoms to yield HCCO radicals 126 ).…”

Kinetics of the •OH-radical initiated reactions of acetic acid and its deuterated isomers

“…In the photooxidation reaction system, the CH 3 C(O)O • radical and its deuterated isomers undergo fast decomposition forming the methyl radical and CO 2 [2][3][4]. The further fate of the minor primary product • CH 2 C(O)OH (and its deuterated counterparts) has been examined in the present study by product analysis.…”

“…KIE is defined here as the rate constant ratio of the OH reactions of two acetic acid isomer molecules, one of which contains H-atom(s) while the other one D-atom(s) in the same position. This can be calculated for reactions (2) to (4) from the experimental values of k 1 to k 4 . KIE is found to be small for the hydrogen atoms in the methyl group, that is, k 1 /k 3 = 1.0 and k 2 /k 4 = 1.6, but large for the hydrogen atom in the carboxylic group: k 1 /k 2 = 5.2 and k 3 /k 4 = 8.6.…”

“…to correct for consumption in the • OH reaction). Calculation of the ratio of the concentration of GA formed and the concentration of AA reacted, both measured at the end of the experiment, provides the glyoxylic acid yields of 0.10 for reaction (1) and 0.15 for reaction (2). Taking into account the branching ratio of reaction (1) [4][5][6] and assuming channel (10a) to be negligible, GA yields of 0.22 to 0.36 are expected for the OH-initiated oxidation of CH 3 C(O)OH.…”

“…As a ubiquitous compound, it has been detected both in the gaseous and condensed phases in the troposphere and up to the Upper TroposphereLower Stratosphere [1]. The reaction kinetics of the reaction between CH 3 C(O)OH and the • OH radical has been investigated in substantial detail in the past few years as it has been discussed in a recent feature article [2]. Below _________________________ * Corresponding author.…”