Elsevier

Chemical Physics

Volume 322, Issue 3, 20 March 2006, Pages 279-288
Chemical Physics

Mechanism and kinetics properties for the reaction: Chloroethane with atomic O (3P)

https://doi.org/10.1016/j.chemphys.2005.08.029Get rights and content

Abstract

In this paper, we present direct dynamics calculations for the multiple-channel reaction of CH3CH2Cl with atomic O (3P) in a wide temperature range (200–3000 K), based on canonical variational transition state theory including small curvature corrections. Four distinct saddle points, one for α-abstraction and three for β-abstraction, have been located for this reaction. The potential energy surface information has been calculated at the MP2/6-311G(d,p) level. The energies along the minimum energy path have been further improved by single-point energy calculations at the G3MP2 level. In the β-abstraction channel, Jahn–Teller effect has been found. Changes of geometries, generalized normal-mode vibrational frequencies, and potential energies along the reaction paths for all channels have been discussed and compared. The calculated total rate constants match the available experimental values reasonable well over the measured temperature range. The results show the variational effect can be negligible and the small curvature tunneling contribution plays an important role for the calculation of the rate constant. At low temperature α-abstraction may be the major reaction channel, while β-abstraction will have more contribution to the whole reaction rate as the temperature increase.

Introduction

Chlorinated hydrocarbons (CHCs) are the most common contaminant found at hazardous waste sites and are the most prevalent contaminants on (Department of Energy) DOE weapons production sites. Many of chlorinated hydrocarbons (CHCs) are highly persistent environmental pollutants, which have been found to be carcinogenic and adversely affect the central nervous system [1]. Use of incineration as a method of disposal of hazardous industrial wastes, including chlorinated hydrocarbons, has stimulated the research directed at mechanistic and kinetic modeling of chlorinated hydrocarbon combustion. Hydrogen abstraction reactions of simple alkanes and their halogenated derivatives are of particular interest because of the natural abundant of such species in the atmosphere. Understanding the reactivity of these species with various radicals, such as OH, Cl and O(1D, 3P), is essential to estimating their atmosphere lifetime, which determines the efficiency of transport of haloalkanes to the stratosphere. Estimating CHC lifetimes requires an understanding of the reactions mechanisms for all of the major pathways and their associated rate constants. Experimentally and theoretically, the mechanism and kinetics of such reactions of haloalkanes with various free radicals have been extensively studied and continue to receive considerable attention [2], [3], [4], [5], [6], [7], [8], [9], [10]. In this paper, we present an exhaustive and theoretical investigation for the multiple-channel H abstraction reaction of CH3CH2Cl with atomic O (3P).

Experimentally, two studies are on record [11], [12]. However, only the thermal rate constants are available for this reaction. The detailed reaction mechanism has not been known well. No experimental information has been reported on the branching fraction. Furthermore, in chloroethane, the H atom can be abstracted from α-position (Cl-bearing carbon atom) and β-position (unsubstituted carbon atom). For the β-abstraction, several channels may be existed due to three unequivalent β-hydrogen atoms. However, it is difficult to know which hydrogen is abstracted in experiment. Therefore, it is necessary to further investigate theoretically the reaction. To our best knowledge, little theoretical attention has been paid to the reaction of CH3CH2Cl with atomic O (3P). In the current study, ab initio electronic theory was basis for the information of the energy profile surface. Kinetic simulations are becoming more and more widely used in modeling of chemical processes of practical interest. The rate constants for the reaction of CH3CH2Cl with atomic O (3P) have been deduced using interpolated canonical variational transition state (CVT) theory [13], [14], [15] and the centrifugal-dominant, small-curvature tunneling approximation (SCT) [16], including the information at the reactants, products, saddle point, and extra points along the minimum energy path.

Section snippets

Computational methods

All the electronic structure calculations were carried out with Gaussian 98 [17] for the reaction of CH3CH2Cl with atomic O (3P). Full geometry optimizations were performed for all species at the unrestricted second-order perturbation Moller–Plesset level of theory (MP2) using the standard 6-311G(d,p) basis set. Force constant matrixes and related normal-mode harmonic vibrational frequencies were calculated at the same level for each stationary point in order to determine the nature of the

Result and discussion

The optimized geometries of the reactant, saddle points, and products along with the experimental values are shown in Fig. 1. The vibrational frequencies of the reactant, products, and saddle points are listed in Table 1, Table 2. The potential barriers ΔE and the reaction enthalpies ΔH calculated at the G3MP2//MP2/6-311G(d,p) level are summarized in Table 3. Figs. 2(a) and (b) show the classical potential energy (VMEP) and vibrationally adiabatic potential energy (VaG) curves as functions of

Conclusion

In this paper, we present an exhaustive and theoretical study on the multiple-channel reaction of CH3CH2Cl with atomic O (3P) using ab initio direct dynamics method. The H atom in CH3CH2Cl can be abstracted by O (3P) from α-position (Cl-bearing carbon atom) and β-position (unsubstituted carbon atom). Four saddle points, in particular one for α-H abstraction and three for H abstraction from β-position, have been identified. The comprehensive dynamics study on all the channels involved in the

Acknowledgments

The authors thank Professor Donald G. Truhlar for providing the Polyrate 9.3 program. This work is supported by Program for New Century Excellent Talents in University.

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