Ab Initio Study of the Oxidation Reaction of CO by ClO Radicals Florent Louis,* ,² Carlos A. Gonzalez,* ,‡ and Jean-Pierre Sawerysyn ² Physico-Chimie des Processus de Combustion et de l’Atmosphe ` re (PC2A) UMR CNRS 8522, FR CNRS 2416 Centre d’Etudes et de Recherche Lasers et Applications (CERLA), UniVersite ´ des Sciences et Technologies de Lille, 59655 VilleneuVe d’Ascq Cedex, France, and Computational Chemistry Group, Physical and Chemical Properties DiVision, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 ReceiVed: March 31, 2003; In Final Form: August 14, 2003 The oxidation of carbon monoxide by ClO radicals was studied by ab initio molecular orbital theory calculations. Geometry optimizations and vibrational frequencies were computed using two methods: Møller-Plesset second-order perturbation theory (MP2), and quadratic configuration interaction in the space of single and double excitations (QCISD). Single-point energy calculations were performed at the QCISD level with triple excitations treated perturbatively (QCISD(T)) and the aug-cc-pVTZ basis set. Canonical transition state theory was used to predict the rate constants as a function of temperature (550-2500 K), and three-parameter Arrhenius expressions were obtained by fitting to the computed rate constants. The possible impact of the title reaction in combustion chemistry is also discussed. Introduction To model the high-temperature oxidation processes occurring in the incineration of chlorinated hazardous wastes, kinetic data concerning the reactivity of chlorooxy radicals with major species such as carbon monoxide are needed. A recent study 1 of the ignition and combustion of ammonium perchlorate in a hydrogen atmosphere showed that ClO radicals are generated at 1400 K near the surface of the burner at the same level of concentration as the OH radicals. To the best of our knowledge, this is the first time that ClO radicals have been detected in flames including chlorinated compounds. Studies 2 of the equi- librium product distributions associated with the combustion of CH 3 Cl, CH 2 Cl 2 , and CHCl 3 in air show that the mole fractions of ClO radicals under fuel lean conditions (equivalence ratio  ) 0.5) are at least 100 as high as the ones of OH radicals at temperatures lower than 900 K and remain predominant until about 1100, 1500, and 1800 K for CH 3 Cl, CH 2 Cl 2 , and CHCl 3 , respectively. These estimates suggest that ClO might contribute to the oxidation of CO to CO 2 in the combustion of chlorinated hydrocarbons under air excess conditions. This contribution is expected to be largely predominant in oxidation processes of chlorinated organic species without hydrogen atoms. Very few determinations of the rate constant of the reaction ClO + CO f Cl + CO 2 have been published in the literature. Clyne and Watson 3 reported an upper limit to the rate constant at 587 K (k < 2.0 × 10 -15 cm 3 ‚molecule -1 ‚s -1 ) for the reaction between ClO and CO. On the basis of these data, DeMore et al. 4 have estimated the following rate expression over the atmospheric temperature range (200-300 K): where the activation energy is assumed to be a lower limit. To assess the relevance of the title reaction in the incineration of chlorinated hazardous waste, it is important to understand the mechanisms and kinetics of this reaction at combustion temperatures. High-level ab initio molecular orbital theory can be very helpful in the prediction of the kinetic parameters over a wide temperature range and in the understanding of the details of the mechanism governing the reaction ClO + CO f Cl + CO 2 . In this work, we apply this methodology combined with canonical transition state theory, to study the temperature dependence of the kinetics of this reaction. To our knowledge, this is the first theoretical study of the reaction between ClO and CO. Computational Methods 5 All calculations described were carried out with the Gaussian 98 6 suite of programs on a 4-processor Compaq, 32-processor NEC SX-5, and 32-processor Silicon Graphics Origin 2000 parallel computers. Fully optimized geometries, harmonic vibrational frequencies, and zero-point energy corrections (ZPE) of reactants, transition states, molecular complexes and products, were calculated with the unrestricted second-order Møller- Plesset perturbation theory (UMP2) 7 using the 6-311G(d) basis set. 8 Electron correlation was computed with second-order Møller-Plesset perturbation theory with full annihilation of spin contamination 9 as implemented in the Gaussian 98 package (noted PMP2 in our results). To confirm the connection of the transition states with reactants and products, the reaction path was followed using IRC 10 (intrinsic reaction coordinate) calcula- tions at the UMP2/6-311G(d) level. The geometrical parameters were reoptimized with the unrestricted quadratic configuration interaction theory using single and double excitations method 11 (UQCISD) and the 6-311G(d) basis set. Harmonic vibrational frequencies were also computed numerically at the UQCISD/ 6-311G(d) level. The geometries previously optimized at the QCISD/6-311G(d) level were used in order to perform single- point energy calculations for all species at the quadratic configuration interaction level of theory using single, double, and triple excitations, QCISD(T), 11 and basis sets ranging from * To whom correspondence should be addressed. F.L.: fax, (33)-3- 20436977; e-mail, florent.louis@univ-lille1.fr. C.G.: fax, (301)869-4020; e-mail, carlos.gonzalez@nist.gov. ² Universite ´ des Sciences et Technologies de Lille. ‡ National Institute of Standards and Technology. k ) 1.0 × 10 -12 (cm 3 ‚molecule -1 ‚s -1 ) × exp(-30764 (J‚mol -1 )/RT) (1) 9931 J. Phys. Chem. A 2003, 107, 9931-9936 10.1021/jp0348272 CCC: $25.00 © 2003 American Chemical Society Published on Web 10/25/2003