Application of the reaction class transition state theory to the kinetics of hydrogen abstraction reactions of alkanes by atomic chlorine Tammarat Piansawan a , Nawee Kungwan a,⇑ , Siriporn Jungsuttiwong b a Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand b Center for Organic Electronics and Alternative Energy, Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Ubon Ratchathani University, Ubon Ratchathani 34190, Thailand article info Article history: Received 16 August 2012 Received in revised form 14 February 2013 Accepted 14 February 2013 Available online 24 February 2013 Keywords: Transition state theory Reaction-class transition state theory (RC-TST) Atomic chlorine Hydrogen abstraction reaction Linear energy relationship (LER) BH&HLYP abstract Kinetics of the hydrogen abstraction reaction of the class alkane + Cl ? alkyl + HCl was studied using reaction class transition state theory (RC-TST) combined with linear energy relationships (LERs). The thermal rate coefficients for the reference reaction of ethane + Cl ? ethyl + HCl, calculated by the micro- canonical variational transition state theory (lVT) incorporating small curvature tunneling (SCT), were taken from the literature. All necessary parameters were derived from density functional theory (DFT) calculations for a representative set of 29 reactions involving a range of hydrocarbons. Direct comparison to available experimental data reveals that the RC-TST/LER can predict rate coefficients for any reaction in the reaction class with acceptable accuracy. For the two test reactions outside of the representative set, our derived rate coefficients were in reasonable agreement with available data. Furthermore, our analy- ses indicate that RC-TST/LER gave systematic errors of less than 25% when compared to TST with one- dimensional Eckart tunneling approximation rate coefficients. Ó 2013 Elsevier B.V. All rights reserved. 1. Introduction Hydrogen abstraction reactions involving alkanes and atomic chlorine (Cl) are a significant class of reactions in atmospheric chemistry and combustion chemistry. More than 60 theoretical and experimental studies for the reaction CH 4 + Cl ? HCl + CH 3 have been published and about 40 theoretical and experimental studies have appeared for the reaction C 2 H 6 + Cl ? HCl + C 2 H 5 [1]. The fate of Cl is not well-characterized since the rate coefficients for its reactions with many hydrocarbons, including alkanes, are not completely understood. These reactions are the major removal process for alkanes and other hydrocarbons in the global tropo- sphere and marine boundary layer [2]. Despite their importance, there are only 13 such elementary reactions of alkane + Cl for which some direct experimental measurements of kinetic data are available, and these are available only over limited temperature ranges. Of these 13 reactions, only three reactions have been stud- ied more than twice, and for four reactions, rate coefficients only at room temperature are reported. Experimental difficulties include secondary reactions of the intermediate alkyl radicals and instru- mental limitations for wide temperature ranges. To construct the global detailed kinetic mechanisms for all possible reactions, rate coefficients of all of the reactions in the class are needed. Recently, first-principles based theoretical calculations are providing an alternative route to the necessary kinetic information for hydrogen abstractions by Cl [3–5]. Rate coefficients for a large number of reactions in a given class can be estimated using the reaction class transition state theory with linear energy relationship (RC-TST/LER) method. The RC-TST/LER theory provides a cost-effective approach to estimating the thermal rate coefficients of all arbitrary reactions in the given class under the transition state theory framework. The central principle of RC-TST is that all reactions in the same class have the same reactive moiety in- volved in bond changes during the reaction, and thus are expected to have similar features on their potential surfaces along the specific reaction pathway. Rate coefficients of any reaction in the given class can be determined by the extrapolation from that of a reference reaction with a relative rate scaling factor expression that applies to the whole class. Moreover, within a given reaction class there is a linear energy relationship (LER) between the classical barrier heights and reaction energies [6–8]. By combining the basics of RC-TST with LER, thermal rate coefficients for any reaction in the class can be predicted from just its reaction energy once all reaction class parameters are determined. The definitions of these parame- ters are given in Section 2.1. Within the RC-TST/LER method the reaction energy can be simply obtained at a relatively low level of theory, e.g., AM1. Although AM1 is a low level of theory, it has proven to be very practical when combined with RC-TST/LER. Thus the use 2210-271X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.comptc.2013.02.010 ⇑ Corresponding author. Tel.: +66 53 943341x101; fax: +66 53 892277. E-mail address: naweekung@gmail.com (N. Kungwan). Computational and Theoretical Chemistry 1011 (2013) 65–74 Contents lists available at SciVerse ScienceDirect Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc