Computational Studies on the Mechanisms and Dynamics of OH Reactions with CHF2CHFOCF3 and CHF2CH2OCF3

The dual-level direct dynamics method has been employed to investigate the multichannel hydrogen abstraction reactions of CHF2CHFOCF3 + OH (R1) and CHF2CH2OCF3 + OH (R2) theoretically. The optimized geometries and frequencies of the stationary points and the minimum energy path are calculated at the B3LYP/6-311G(d,p) level, and the energetic information is further refined by the MC-QCISD method. There are two hydrogen abstraction channels for reaction R1 and three hydrogen abstraction channels for reaction R2. And four products, CF2CHFOCF3, CHF2CFOCF3, CF2CH2OCF3, and CHF2CHOCF3, are produced during these two reactions. The enthalpies of formation for the species involved in the two reactions (CHF2CHFOCF3, CF2CHFOCF3, CHF2CFOCF3, CHF2CH2OCF3, CF2CH2OCF3, and CHF2CHOCF3) are calculated via isodesmic reaction. Furthermore, the reaction mechanisms of subsequent reactions of product radicals (CF2CHFOCF3, CHF2CFOCF3, CF2CH2OCF3, and CHF2CHOCF3) with the OH radical are studied at the same level. The rate constants of reactions R1 and R2 are evaluated by means of canonical variational transition-state theory including the small-curvature tunneling correction over a wide range of temperatures, from 250 to 1200 K. Our results show that the tunneling correction plays an important role in the rate constant calculation in the low-temperature range. Agreement between the calculated and experimental data available at 298 K is good. It is found that the fluorine substitution decreases the reactivity of the C-H bond, and as a result, reaction R2 may proceed much easier than reaction R1. The Arrhenius expressions over a wide temperature range are obtained, and the kinetic isotope effects for reactions CHF2CHFOCF3/CDF2CDFOCF3 + OH and CHF2CH2OCF3/CDF2CD2OCF3 + OH are estimated so as to provide theoretical estimation for future laboratory investigation.