Direct ab Initio Dynamics Studies of the Hydrogen Abstraction Reactions of Hydrogen Atom with Fluoromethanes Dilip K. Maity, Wendell T. Duncan, and Thanh N. Truong* Henry Eyring Center for Theoretical Chemistry, Department of Chemistry, UniVersity of Utah, Salt Lake City, Utah 84112 ReceiVed: October 30, 1998; In Final Form: January 19, 1999 A direct ab initio dynamics study on the gas-phase reactions of atomic hydrogen with different fluoromethanes has been carried out. The thermal rate constants were calculated using canonical variational transition state (CVT) theory augmented by multidimensional semiclassical zero and small curvature tunneling approximations. The potential energy surfaces for the reactions were calculated using hybrid density functional theory, namely, Becke’s half-and-half (BH) nonlocal exchange and the Lee-Yang-Parr (LYP) nonlocal correlation functionals using the cc-pVDZ basis set. The reaction energies and barrier heights were improved by single-point energy calculations along the minimum energy path (MEP) at the spin-projected fourth order Moller-Plesset perturbation theory (PMP4) using the cc-pVTZ basis set. The calculated forward and reverse thermal rate constants are in the good agreement with the experimental data. The electronic effects of fluorine substitution on the rate of this class of reactions are examined. Introduction Halons (chlorofluorobromocarbons) have been widely used as a chemical fire extinguisher in ships, aircraft, and computer rooms; however, their production and use are banned worldwide because of their destructive effects on stratospheric ozone. It is understood that the ozone depletion potential of halons is caused by bromine. As an alternative to halons, hydrofluorocarbons (HFC) have been proposed as fire suppressants. These com- pounds contain no chlorine or bromine, and fluorine is believed to be relatively innocuous in its effect on the ozone layer. 1,2 Among the hydrocarbons, fluorinated methanes are considered to be safe alternative potential candidates as flame suppres- sants. 3,4 Accurate kinetic data for the reactions of hydrogen atoms with fluorinated methanes are needed to assess the mechanism and effectiveness for their use as alternative flame suppressants and to model the combustion chemistry of fluori- nated hydrocarbons. The reliability of computational kinetic modeling is critically dependent on the accuracy of estimated rate constants for the different channels of the reactions involving these species under combustion temperatures. Un- fortunately, the required kinetic data are often unavailable or else have been obtained at only a few temperatures. Recent work has indicated that the abstraction of hydrogen atoms from fluoromethanes by H atoms is one of the major destruction pathways in a flame, and therefore, a basic under- standing of this process may help in designing more effective fire suppression agents. Three different pathways for the reaction of H atoms with CH 3 F, CH 2 F 2 , and CHF 3 have been discussed in the literature because of their exothermicities: abstraction of H (CH 4-x F x + H T CH 3-x F x + H 2 ), abstraction of F (CH 4-x F x + H T CH 4-x F x-1 + HF), and substitution (CH 4-x F x + H T CH 5-x F x-1 + F). It has also been shown in the literature, based on ab initio molecular electronic structure calculations at various levels of theory, that the reactions of F abstraction or substitution have much higher barriers compared to the H abstraction reaction channel, thus making those two channels minor. 5 For this reason, in our present study, we focus only on the H abstraction reaction channel (CH 4-x F x +H T CH 3-x F x + H 2 ; x ) 1, 2, or 3), at a fairly accurate level of calculation. To examine the electronic effect of fluorine substitution and for the sake of comparison, we have also presented results for the H abstraction reaction of methane (x ) 0) at the same level of theory. To date, unlike the H + CH 4 reaction, only a limited number of experimental measurements of the thermal rate constants are available for H abstraction reactions of fluoromethanes. How- ever, for purpose of comparison in this study we use several sets of experimental data available in the literature at different temperatures for these four reactions. 6-19 Several previous groups 20-22 have looked at the equilibrium structures, but only Berry et al. 5 have studied the kinetics of the H + CH 4-x F x (x ) 0-3) reactions using conventional transition state theory (TST). Recently, Hranisavljevic and Michael 19 have also studied the reaction CHF 3 + H T CF 3 + H 2 , theoretically also employing TST with one-dimensional Eckart tunneling corrections. It is known that tunneling contributions in hydrogen abstraction reactions are significant. Thus, one-dimensional tunneling models associated with the TST framework may not be sufficient. Variational transition state theory 23 provides a framework for more accurate tunneling treatments. Since an analytical potential energy function for these reactions is not available, the conventional approach of reactive dynamic calculations is not viable. 23 Direct dynamics approach provides a viable alternative for using the more extensive variational transition state theory and tunneling methods to obtain accurate thermal rate constants. 24,25 In this approach, all potential information required for evaluating dynamic properties are obtained directly from ab initio electronic structure calculations rather than from empirical analytical force fields. In the present study, we employ the direct ab initio dynamics 26-30 method developed in our laboratory to obtain thermal rate constants for H abstraction reactions and look into the substitution effects of fluorine on the rate of H atom transfer reaction. 2152 J. Phys. Chem. A 1999, 103, 2152-2159 10.1021/jp984281x CCC: $18.00 © 1999 American Chemical Society Published on Web 03/13/1999