Deciphering the irradiation induced fragmentation-rearrangement mechanisms in valence ionized CF3CH2F.

The increasing presence of 1,1,1,2-tetrafluoroethane (CF3CH2F) in the atmosphere has prompted detailed studies into its complex photodissociation behavior. Experiments focusing on CF3CH2F irradiation have unveiled an array of ions, with the persistent observation of the rearrangement product CHF2+ not yet fully understood. In this work, we combine density functional theory, coupled-cluster calculations with a complete basis set formalism, and atom-centered density matrix propagation molecular dynamics to investigate the energetics and dynamics of different potential pathways leading to CHF2+. We found that the two-body dissociation pathway involving an HF rearrangement, which was previously considered complex for CHF2+ formation, is actually straightforward but not likely due to the facile loss of HF. In contrast, our calculations reveal that the H elimination pathway, once thought of as a potential route to CHF2+, is not only comparably disadvantageous from both thermodynamic and kinetic points of view but also does not align with experimental data, particularly the lack of a rebound peak at m/z 101-102. We establish that the formation of CHF2+ is predominantly via the HF elimination channel, a conclusion experimentally corroborated by studies involving the trifluoroethylene cation CF2CHF+, a key intermediate in this process.

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

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.