Reaction of CF 3 CH 2 OCF 3 by Hydroxyl Radical Bull. Korean Chem. Soc. 2010, Vol. 31, No. 12 1 Hydrogen-Atom Abstraction Reaction of CF 3 CH 2 OCF 3 by Hydroxyl Radical Hari Ji Singh, * Bhupesh Kumar Mishra, and Pradeep Kumar Rao Department of Chemistry, DDU Gorakhpur University, Gorakhpur - 273009, India. * E-mail: hari_singh81@hotmail.com Received August 26, 2010, Accepted October 19, 2010 Theoretical investigations are carried out on the title reaction by means of ab-initio and DFT methods. The optimized geometries, frequencies and minimum energy path are obtained at UB3LYP/6-311G(d,p) level. Single point energy calculations are performed at MP2 and MP4 levels of theory. Energetics are further refined by calculating the energy of the species with a modified Gaussian-2 method, G2M(CC,MP2). The rate constant of the reaction is calculated using Canonical Transition State Theory (CTST) utilizing the ab-initio data obtained during the present study and is found to be 5.47 × 10 ‒12 cm 3 molecule ‒1 s ‒1 at 298 K and 1 atm. Key Words: Theoretical chemistry, Hydrofluoroethers, Potential energy surface, Canonical transition state theory Introduction It is now a well recognized fact that atomic chlorine transport- ed to the stratosphere on account of release of a variety of chlo- rine containing compounds particularly chlorofluorocarbons (CFCs) into the atmosphere are responsible for the catalytic destruction of ozone in the atmosphere. 1-4 Chlorine atoms gene- rated by the decomposition of CFCs in the stratospheric region through a series of catalytic reactions lead to a net decrease in the total ozone concentration in the upper atmosphere, with a net increase in ultraviolet radiation resulting in adverse effects on plants and animals. The Montreal Protocol and its conti- nuous updates on substances that deplete the ozone layer led to a global consensus to restrict the use of chlorinated com- pounds on account of its deleterious effect on the environment and agreed to phase out of their production. Serious attempts have been made to find out alternatives of CFCs, and hydro- chlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) come out as viable alternatives to chlorofluorocarbons (CFCs). 5-6 Studies performed with many of the HFCs and HCFCs have shown that these are not a reliable solution to protect the ozone layer. Recently hydrofluoroethers (HFEs) have been the focus of intense attention as replacement materials for CFCs and HFCs. 7 Hydrofluoroethers (HFEs) are being used as third gene- ration replacements to CFCs, HCFCs and HFCs. 8-9 The absence of chlorine atoms in HFEs shows that such compounds would have little impact on stratospheric ozone and that they would possess a negligible ozone depleting potential (ODP). 10-11 How- ever, the presence of C-F and C-O bonds in HFEs may enhance the absorption features in the atmospheric infrared region (800 - 1400 cm ‒1 ) and could play a significant role as green house gases. 12 Therefore, considerable attention has been paid in re- cent years to perform experimental and theoretical studies on the decomposition kinetics of HFEs. 13-16 Tropospheric degradation is expected to be initiated mainly by the attack of OH radicals in the gas phase 17 and CF 3 CH 2 OCF 3 (HFE-246cb2) may undergo H atom abstraction as follows. CF 3 CH 2 OCF 3 + OH → CF 3 CHOCF 3 + H 2 O (1) The reactant, CF3CH2OCF3 would possess Cs symmetry. Thus the two hydrogen atoms of the -CH 2 position are equivalent and therefore, only one channel is identified for the title reaction as given by reaction (1). Literature survey reveals that no experimental study is per- formed for this reaction yet. Thus, there is desirable need to perform theoretical studies utilizing DFT methods. In the pre- sent study we have theoretically investigated the kinetics of hy- drogen atom abstraction reaction of CF 3 CH 2 OCF 3 with OH radical. The aim of the present paper is to have a more accurate thermochemical data using modified composite method, G2M (CC, MP2). Canonical Transition State Theory (CTST) is also utilized to predict the rate constant of the title reaction on the basis of ab-initio data obtained during the present investigation. Computational Methods All calculations performed during the course of the present investigation were done using the GAUSSIAN 03 suits of pro- grams. 18 Amongst a series of available options B3LYP/6-311G (d,p) method was found to be sufficiently accurate for predict- ing reliable geometries of the stationary points. At the same time it is not computationally expensive to scan the potential energy surfaces. Therefore, geometries of reactants, products and transition state for reaction (1) were optimized at the UB3LYP/ 6-311G(d,p) level. 19-20 Vibrational frequency calculations em- ployed for the characterization of stationary points on the po- tential energy surface of the title reaction have been identified to correspond to local minima with all positive values of vib- rational frequencies (NIMAG = 0). Zero point energy (ZPE) corrections, and rate constant calculation were made too. Transi- tion state is characterized by the occurrence of only one ima- ginary frequency (NIMAG = 1) on the potential energy surface. To confirm that there is smooth transition from reactants to pro- ducts through the observed transition state structure, intrinsic reaction coordinate (IRC) calculations 21 were performed with the same level of theory at which optimization and frequency calculation had been performed. In order to obtain a more refine energy values for various