20 July 2001

Chemical Physics Letters 342 (2001) 659±666

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Theoretical study of the H-abstraction reaction of the CH3O radical with formaldehyde E. Henon *, F. Bohr Laboratoire de Chimie Physique±GSMA, Equipe associ ee au CNRS UMR 6089, Universit e de Reims Champagne Ardenne, Moulin de la Housse, Naturelles, BP 1039-51687, Reims Cedex 2, France Received 10 July 2000; in ®nal form 13 February 2001

Abstract The purpose of the present Letter is to investigate the mechanism of the bimolecular abstraction reaction of the methoxy radical …CH3 O† with formaldehyde and to compute gas-phase thermal rate constants using conventional transition state theory including tunneling corrections. The potential information needed to calculate the rate constants is obtained from ab initio electronic structure calculations. We report the ab initio UHF, MP2, MP4, CASSCF, CASPT2 and MRCI results obtained by using ANO basis sets as well as HF-DFT (B3LYP) results obtained by using the 6-311++G basis set. The theoretical rate constants obtained from the MRSDCI potential are in good agreement with the experimental values. Ó 2001 Elsevier Science B.V. All rights reserved.

1. Introduction Alkoxy radicals are important intermediate species during the oxidation of volatile organic compounds (VOC) in atmospheric chemistry. Isomerization of alkoxy radicals by intramolecular H-atom transfer plays an important role in the degradation of heavier alkoxy radicals (number of carbon atoms C P 4). The estimation of the corresponding rate constants by direct experimental measurements is dicult as explained by Fittschen et al. [1]. These authors suggest that measurement of the rate parameters for bimolecular H-atom abstraction reactions of alkoxy radicals might be a basis for the estimation of isomerization rates. This suggestion is based on the recent results of *

Corresponding author. Fax: +33-326-91-33-98. E-mail address: [email protected] (E. Henon).

Viskolcz et al. [2] who have shown by systematic ab initio calculations that the barrier heights for isomerization reactions of di€erent alkyl radical (C3 to C6 ) are the sum of two independent terms: the ring strain energy of the corresponding cyclic transition state (depending mainly on the size of the ring) and the activation energy of the relevant bimolecular abstraction by the CH3 radical. Atkinson [3] suggested that this additive rule could be valid for alkoxy radicals as well. As mentioned above, the rate constant for the reaction CH3 O ‡ H2 CO ! CH3 OH ‡ HCO has been measured by Fittschen et al. [1]. Their results, obtained by two di€erent direct experimental techniques, show that the rate constant does not depend on the pressure and that the Arrhenius activation energy is positive. The given Arrhenius expression is

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