Kinetics and mechanism of the reactions of CH3CO and CH3C(O)CH2 radicals with O2. Low-pressure discharge flow experiments and quantum chemical computations

Abstract

The reactions CH(3)CO + O(2)--> products (1), CH(3)CO + O(2)--> OH +other products (1b) and CH(3)C(O)CH(2) + O(2)--> products (2) have been studied in isothermal discharge flow reactors with laser induced fluorescence monitoring of OH and CH(3)C(O)CH(2) radicals. The experiments have been performed at overall pressures between 1.33 and 10.91 mbar of helium and 298 +/- 1 K reaction temperature. OH formation has been found to be the dominant reaction channel for CH(3)CO + O(2): the branching ratio, Gamma(1b) = k(1b)/k(1), is close to unity at around 1 mbar, but decreases rapidly with increasing pressure. The rate constant of the overall reaction, k(2), has been found to be pressure dependent: the fall-off behaviour has been analysed in comparison with reported data. Electronic structure calculations have confirmed that at room temperature the reaction of CH(3)C(O)CH(2) with O(2) is essentially a recombination-type process. At high temperatures, the further reactions of the acetonyl-peroxyl adduct may yield OH radicals, but the most probable channel seems to be the O(2)-catalysed keto-enol transformation of acetonyl. Implications of the results for atmospheric modelling studies have been discussed.

Kinetics and mechanism of the reaction of acetonyl radical, CH3C(O)CH2, with Br2

“…The current experimental and theoretical study confirms our previous conclusion [2][3][4] that acetonyl reacts like a carbon-centered free radical and not like an alkoxyl radical in its elementary reactions. Specifically, BrCH 2 C(=O)CH 3 is formed in the reaction with Br 2 , while alkoxyl radicals apparently do not enter reaction with molecular bromine [19].…”

“…They have found the CH 3 + Br 2 reaction to proceed through a shallow pre-reaction complex (with ~4 kJ mol -1 stabilization energy) in the entrance channel, followed by a low barrier (~1 kJ mol -1 ) transition state structure, but unlike in our case, they have also mapped a post-reaction complex, CH 3 Br…Br, in the exit channel formed with a substantial amount of excess energy [15]. The microcanonical RRKM and QCT computations have supplied rate coefficients in good agreement with experiment [16].…”

“…The radical site in acetonyl is partially delocalized over the carbonyl double bond, which has been found to give rise to unusual kinetic behavior in some of the elementary reactions of this free radical [2,3]. As a continuation of our previous experimental and theoretical work [2][3][4] on the structurereactivity relations of acetonyl, we have studied the reaction of CH 3 C(O)CH 2 radical with Br 2 molecule (1).…”

“…The radical site in acetonyl is partially delocalized over the carbonyl double bond, which has been found to give rise to unusual kinetic behavior in some of the elementary reactions of this free radical [2,3]. As a continuation of our previous experimental and theoretical work [2][3][4] on the structurereactivity relations of acetonyl, we have studied the reaction of CH 3 C(O)CH 2 radical with Br 2 molecule (1). This reaction has relevance also for the enthalpy of formation of the acetonyl radical: in the classical photobromination study of acetone by King et al [5], reaction (1) was assumed to have small positive activation energy at deriving ∆ f H°2 98 (CH 3 C(O)CH 2 ), but no method was available at the time of the investigations to confirm the assumption of the authors.…”

“…We have applied the direct reaction kinetic method of low pressure fast discharge flow (DF) coupled with laser induced fluorescence (LIF) monitoring of CH 3 …”