Jointly published by Akadémiai Kiadó, Budapest and Kluwer Academic Publishers, Dordrecht

React.Kinet.Catal.Lett. Vol. 82, No. 1, 105-110 (2004)

RKCL4439 THEORETICAL STUDY ON THE MECHANISM OF THE REACTION ACETONE + OH: CHANNELS TOWARDS THE FORMATION OF METHANOL Sébastien Canneaux, Eric Henon* and Frédéric Bohr Equipe de Chimie Théorique, U.M.R. CNRS 6089, Université de Reims Champagne-Ardenne, U.F.R. Sciences Exactes et Naturelles, Moulin de la Housse, BP 1039 - 51687 Reims, Cedex 2, France Received October 27, 2003 Accepted December 2, 2003

Abstract Two possible pathways for the acetone + OH reaction towards the formation of methanol have been examined theoretically. Our results show that both channels are characterized by a substantial activation barrier and reject the possibility of a significant CH3CO + CH3OH channel. Keywords: Acetone, methanol, ab initio, transition state

INTRODUCTION One of the important recent developments in atmospheric chemistry is the discovery that acetone provides a large primary source of HOx radicals (HO and HO2) in the atmosphere, in particular in the upper troposphere [1,2]. Acetone comes from the oxidation of non-methane hydrocarbons and biomass burning [3]. The main loss processes of acetone are photodecomposition [4] and thermal reaction with the OH radical [5] : CH3COCH3 + OH

Products

(1)

________________________ *Corresponding author. E-mail: [email protected] 0133-1736/2004/US$ 20.00. © Akadémiai Kiadó, Budapest. All rights reserved.

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Many experimental and theoretical studies [5-13] have been devoted to reaction (1). Experimental studies have shown that the corresponding rate constant k1 exhibits a non-Arrhenius behavior at temperatures below 250 K [5]. It was first suggested by Wollenhaupt et al. [7] that, above 250 K, the main channel was H-abstraction (1a) : CH3COCH3 + OH

CH3C(O)CH2 + H2O

(1a)

while, at lower temperature, the competing loss process (viz. addition-CH3 elimination) (1b) : CH3COCH3 + OH

CH3COOH + CH3

(1b)

might explain the observed non-Arrhenius behavior. Theoretical studies [8, 10, 11] have revealed that this channel (1b) was not favorable. Very recently, Talukdar et al. [13] have presented experimental evidence to show that the acetic acid yield in reaction (1) is negligible between 237 and 353 K. In their discussion, they mentioned another exothermic channel as a possible competing pathway : CH3COCH3 + OH

CH3OH + CH3CO

(1c)

Though detection of CH3OH was not carried out in their system, they concluded, based on the large yield of the CH3C(O)CH2 radical (1a), that the yield of other possible products via channel (1c) is small. Thus, in order to confirm this conclusion of Talukdar et al. [13], it was interesting theoretically to examine reaction (1c). COMPUTATIONAL DETAILS Ab initio calculations were performed using the GAUSSIAN 98 software package [14]. Reactants and transition state structures were fully optimized using the analytical gradients at the MP2 [15] (Møller-Plesset perturbation theory at the second order) level of theory using the 6-31G(d,p) basis set. Transition states have been characterized by one imaginary frequency (firstorder saddle point) on the Potential Energy Surface (PES). Intrinsic Reaction Coordinate (IRC) [16] analysis was performed to determine the Minimum Energy Pathways (MEPs) at the MP2/6-31G(d,p) level of theory. Transition states were found to connect proper reactants and products. Vibrational frequencies were calculated within the harmonic approximation at the same level of theory as for geometries.

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Single-point energy calculations were carried out at the MP4(SDTQ) [17] and CCSD(T) [18] levels of theory using the 6-311G(d,p) basis set. The ''frozen-core'' approximation was used both in Møller-Plesset and CCSD(T) calculations. RESULTS AND DISCUSSION The formation of methanol can occur through two different pathways: - two-step channel: addition of the OH radical to the C=O group of acetone, followed by elimination of the CH3OH molecule. - direct channel: elimination of the methyl group by the OH radical to give methanol and an acyl radical. The results from our recent work [11] rule out the two-step mechanism. Actually, the addition of the OH radical to the acetone molecule was found to occur through a relatively high energy barrier (33.9 kJ mol-1) compared to the H-abstraction one (16.7 kJ mol-1). Consequently, the formation of methanol cannot occur through the addition-elimination channel. Thus, in this work, the OH-addition channel will not be considered.

Fig. 1. MP2/6-31G(d,p) optimized structure of the transition state for the one-step mechanism

The optimized geometry of the transition state for the one step mechanism is shown in Fig. 1. The optimized geometrical parameters for the reactants, transition state and products are presented in Table 1.

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Table 1 Optimized MP2/6-31G(d,p) bond distances (in Å) for the reactants, transition and products, for the one-step mechanism Acetone C1-C2 C2-C3 C1-H1 C1-H2 C1-H3 C3-H4 C3-H5 C3-H6 C2-O1 O2-H7 O2-C3

OH

1.513 1.513 1.086 1.090 1.090 1.086 1.090 1.090 1.227 0.972

TS 1.510 1.833 1.087 1.088 1.089 1.078 1.083 1.079 1.195 0.969 1.785

Methanol

CH3CO 1.514 1.089 1.088 1.088

1.086 1.093 1.093 1.197 0.963 1.421

In the transition state, the unpaired electron on the hydroxyl attacks one of the C-C sigma bonds of acetone. The two electrons in the C2-C3 bond become unpaired. One of these pairs of electrons remains in the acyl radical and the other electron forms a sigma C3-O2 bond in methanol. In this structure, the computed bond lengths describing the main part of the reaction coordinate are C2-C3 = 1.833 Å and C3-O2 = 1.785 Å. The relative energies (including the zero-point correction energy) of the reactants, transition state and products are collected in Table 2, and reported in Fig. 2 jointly with the H-abstraction and OH-addition channels. Due to a high-spin contamination of the UHF wavefunction (=0.88) for the transition state, we give rather PMP2 and PMP4 results. The PMP4 and CCSD(T) relative energies are consistent. This reaction is found to be exothermic (-5 to -8 kcal mol-1), which is consistent with the heat of reaction given by Talukdar et al. (-8 kcal mol-1). As shown in Table 2, the energy barrier is very large: 171.9 kJ mol-1. Thus, our theoretical results show that such a mechanism (1c) has a substantial activation barrier and cannot be envisaged as another possible channel of reaction (1). This confirms the previous conclusion of Talukdar et al. [13].

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Fig. 2. Potential energy profile for the OH + acetone reaction using the CCSD(T)/6-311G(d,p)//MP2/6-31G(d,p) level of theory (energies given in kJ mol-1). The nomenclature for the designation of stationary points (H-abstraction and OH addition channel) is the same as that used in our previous work [11]

Table 2 Relative energies (in kJ mol-1) for reactants, transition state and products for the direct channel towards the formation of methanol. Computational results are presented for PMP2, PMP4 and CCSD(T) 6-311G(d,p) levels of theory. The numbers in brackets do not include ZPE corrections

Reactants Transition state Products

PMP2

PMP4

CCSD(T)

0.0 (0.0) 162.1 (154.1) -34.0 (-40.8)

0.0 (0.0) 164.4 (156.4) -26.6 (-33.5)

0.0 (0.0) 171.9 (163.9) -21.6 (-28.5)

Acknowledgement. I.D.R.I.S., C.I.N.E.S. and C.R.I.H.A.N. Computing centers and the computational center of the Université de Reims Champagne-Ardenne are acknowledged for the CPU time donated. The authors thank Prof. Sándor Dóbé for the helpful discussions. We are particularly grateful to the Region Champagne-Ardenne for financial support.

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