Features of the potential energy surface for the reaction of OH radical with acetone

Abstract

The mechanism of the reaction of OH with acetone has been studied by quantum chemical computations. 21 stationary points (among them reactant complexes, reaction transition states, intermediate complexes and product complexes) have been characterised on the potential energy surface of the reaction. The MP2 method with 6-31G(d,p) basis set was employed for geometry optimisation. Electronic energies were obtained at the CCSD(T)/6-311G(d,p) level of theory. Hydrogen abstraction was found to occur through two complex mechanisms; no transition state for direct abstraction could be located. Minimum energy path analyses have revealed two distinct pathways which lead to CH 3 (+CH 3 COOH) formation. One of them sets out the abstraction channel and proceeds via intermolecular complexes and the other one involves addition of OH to the carbonyl double bond and subsequent decomposition of the adduct hydroxy-alkoxy radical. The rate limiting steps involve large energy barriers and, consequently, these pathways do not explain the high methyl yields observed experimentally at and below room temperature. Characteristic for the reaction of OH with acetone is the existence of numerous hydrogen-bridged complexes on the potential energy surface that are stabilised by as much as 3.2-26.6 kJ mol À1 binding energy. Some properties of these complexes and their possible role in the molecular mechanism of the reaction are discussed.y Electronic supplementary information (ESI) available: Harmonic vibrational frequences and MP2 structural parameters for intermolecular complexes, reactants, transition states and reaction products. See

The reaction of OH with acetone and acetone-d6 from 298 to 832 K: Rate coefficients and mechanism

“…At room temperature k 3 is computed to be Ͻ0.1% of the measured k H and so the addition channel is not considered further. Alternative paths to CH 3 formation were noted by Henon et al 14 These are based on decomposition of acetonyl complexed with water to CH 3 ϩCH 2 CO. Very high barriers of around 90 kJ mol Ϫ1 were derived, 14 so these channels cannot contribute to the observed kinetics.…”

“…Henon et al recently argued for an entrance adduct in path ͑2͒, bound by about 8 kJ mol Ϫ1 . 14 We are unable to reproduce their given MP2/6-31G(d,p) structure for this complex ''MC1b,'' 14 and instead optimizations starting from this geometry led ultimately to their ''MC1'' structure. Our own B3LYP/6-311G(d,p) searches did not reveal a bound intermediate in this region.…”

“…14,26 It is important to note that K eq is small-at 200 K it is about 1.1ϫ10 Ϫ20 cm 3 molecule Ϫ1 -and K eq decreases as the temperature is raised. Aloisio and Francisco 26 previously derived DH 0 ϭ19.2 kJ mol Ϫ1 , with which we agree closely, but presented a much larger K eq value at 200 K that appears to be in error, due in part to an incorrect temperature dependence for DH.…”

“…In an initial presentation we argued for a similar set of three channels. 13 Very recently, Henon et al 14 have expanded their original potential energy diagram 6 to include two hydrogen abstraction pathways leading to water plus acetonyl formation, both with entrance adducts. Henon et al concluded with several open issues, including: can the measured lowtemperature activation energy which is close to zero be reconciled with the significant barriers computed for H-abstraction?…”

Features of the potential energy surface for the reaction of HO2 radical with acetone

“…The photolysis process is dominant at high altitude, while at lower altitude the dominant process is the reaction with OH 3, 4. Concerning this reaction, experimental and theoretical studies have shown that the major process is H‐abstraction 5–17. A recent experimental study 18 has shown that the reaction between acetone and HO 2 has no importance in the atmosphere.…”