RESEARCH PAPER

PCCP

Se´bastien Canneaux,a Nathalie Sokolowski-Gomez,a Eric Henon,a Fre´de´ric Bohr*a and Sa´ndor Do´be´b

www.rsc.org/pccp

Theoretical study of the reaction OH þ acetone: a possible kinetic effect of the presence of water?

a

UMR CNRS 6089, Equipe Chimie The´orique, Universite´ de Reims-Champagne-Ardenne, UFR Sciences, Moulin de la Housse, BP 1039, 51687 REIMS Cedex 2, France. E-mail: [email protected] b Chemical Research Center, Hungarian Academy of Sciences, Pusztaszeri u. 59-67, H-1025 Budapest, Hungary. E-mail: [email protected] Received 29th June 2004, Accepted 8th September 2004 First published as an Advance Article on the web 23rd September 2004

The effect of water on the molecular mechanism of the reaction of the OH radical with acetone in the homogeneous gas-phase has been studied by quantum chemical computations. The three-molecular reaction system of OH þ acetone þ H2O has been characterised using molecular parameters, electronic energies and Gibbs free energies computed for the stationary points of the potential energy surface. The MP2 method with a 6-31G(d,p) basis set was employed for geometry optimisation. The electronic energies were obtained at the MP4 and the CCSD(T) level of theory using the 6-311G(d,p) basis set. We have found that the presence of a water molecule changes significantly both the energy profile and free energy profiles of the reaction. A ‘‘water-assisted’’ reaction mechanism has been established in which both the H-abstraction channel and the CO-addition channel occur via intermolecular complexes and transition state structures that involve the water molecule. The activation free energy for the out-of-plane abstraction channel at low temperatures has been found to be significantly smaller than that for the ‘‘water-free’’ system indicating a possible catalytic rate enhancement effect. Abstraction is the predominant reaction route also for the water-assisted reaction as shown by the much larger activation free energy computed for the addition channel. In order to estimate atmospheric concentrations of some intermolecular complexes, we have validated our employed level of theory by computing the equilibrium constant of HO2 þ H2O $ HO2


H2O at three temperatures and compared them to the values derived from experiments available in the literature. Then, using our theoretical results, we have estimated the tropospheric concentration of OH


acetone


H2O complexes to be very small, but they are probably detectable under laboratory conditions.

DOI: 10.1039/b409900a

1. Introduction

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Acetone, CH3C(O)CH3, is one of the most abundant organics in the atmosphere providing an important source of HOx radicals (HO and HO2) in particular in the upper troposphere and lower stratosphere.1,2 HOx radicals are formed in the oxidative chain degradation of acetone that is initiated by photolysis and through the reaction with OH. In recent years, a great number of experimental and theoretical studies have been devoted to the OH þ acetone reaction owing to its atmospheric importance and because of its interest from a fundamental chemical kinetics point of view.3–21 Water is a major player in Earth’s atmosphere, which basically influences the radiation balance of Earth, dynamic processes and chemistry of the atmosphere. At ground level, the water concentration of saturated air is about 5  1017 molecule cm3 which decreases rapidly with altitude. Although the water vapour concentration at the tropopause is three orders of magnitude less than at ground level, it remains large throughout the troposphere. A recent field measurement in the near-infrared has revealed a surprisingly high concentration of 6  1014 molecule cm3 of water dimers in the atmosphere.22 This figure has been reproduced very well by a recent theoretical study23 predicting also that cyclic water trimers, tetramers and pentamers should be directly observable in the troposphere. Several papers, published in recent years suggest that gas-phase reaction chemistry could be catalyzed by water molecules and water clusters.22–38 For example, Loerting and Phys. Chem. Chem. Phys., 2004, 6, 5172–5177

Klaus25 have shown that larger hydrates of SO3 with water reduce the activation energy for conversion of SO3 to H2SO4. The suggested new role of water in gas-phase reactions, besides being of great theoretical interest, may have far-reaching consequences also for atmospheric chemistry given the high (H2O)x concentration in the atmosphere. H2O affects the reactions via hydrogen bonding. The role and importance of hydrogen bonded molecular complexes on the kinetics and dynamics of gas-phase free radical reactions have been the focus of interest for both experimentalists and theoreticians as presented in very recent review papers on the subject.36–38 The new findings in this field have been ‘‘. . .changing our perspectives on the molecular mechanism of radical–molecule reactions and their impact on atmospheric chemistry’’.38 In view of its academic interest and the possible atmospheric and laboratory kinetics implications, we have examined the effect of H2O on the reaction of OH with CH3C(O)CH3 by theoretical means. As a result of extensive theoretical work performed by several groups in the field over the past 3–4 years, the main features of the molecular mechanism of the reaction of OH radical with acetone (without H2O) have been clarified;9,11,13,14,18,20 disagreement among the different groups has remained concerning only minor details of the mechanism. We proposed in ref. 14 the following mechanism: OH þ CH3C(O)CH3 - MC1a - TS1a CH3C(O)CH2 þ H2O

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