Kinetic and thermodynamic investigations on the HF elimination reactions from neutral and ionized CF3CH2F

Abstract

This work discusses the possible HF formation routes via recombination reactions from CF₃CH₂F (R-134a) and its cation. The molecular properties of the main reagents were first evaluated at the M06-2X/cc-pVTZ level. Then, changes in energy (ΔE) for all reactions comprising a possible HF formation from the studied systems were evaluated at the CCSD(T)/CBS//M06-2X/cc-pVTZ level. With the aid of Intrinsic Reaction Coordinate (IRC) calculations for each path, it is found that the HF formation reaction takes place majorly through the "1,2" elimination, resulting in an olefin as the secondary product. In turn, the IRC associated with "2,2" reactions allowed to find a post-barrier complex between the carbene :CHCF₃ and HF in its exit channel, with dissociation energy of ∼4 kcal mol^-1. Similarly, the cationic system exhibits favoritism towards the "1,2" elimination, and an ion-dipole post-barrier complex is found. The ΔE of such a complex production is negative in both directions, indicating this complex (25.5 kcal/mol more stable than CF₃CH₂F⁺) should be a minimum on the R-134a cation surface. However, unlike the neutral "2,2" path, there is no F atom migration transition state for the 2,2-HF elimination from CF₃CH₂F⁺. Hence, the F migration is expected to occur simultaneously with the rest of the structural changes towards the ion-dipole complex. The rate coefficients computed at the current level of theory, including corrections for anharmonicity and hindered rotations, showed a reasonable agreement with the available experimental data, inspiring confidence in our predictions for the cationic system.

Keywords: Electronic structure; HF elimination; HFC-134a; Kinetics; Photofragmentation.