Reaction mechanisms and kinetics of the β-elimination processes of compounds CHF₂CH₂SiF_nMe_{3– n} (n = 0–3): DFT and CBS-QB3 methods using Rice-Ramsperger-Kassel-Marcus and transition state theories

The gas-phase β-elimination kinetics of 2,2-difluoroethyltrifluorosilane (1), 2,2-difluoroethylmethyldifluorosilane (2), 2,2-difluoroethyldimethylfluorosilane (3), and 2,2-difluoroethyltrimethylsilane (4) have been investigated computationally using M06-2x exchange-correlation functional as well as the benchmark CBS-QB3 quantum chemical approach. The obtained energy profile has been enhanced with kinetic calculations using statistical Rice-Ramsperger-Kassel-Marcus (RRKM) theory and transition state theory (TST). The calculated results are in good agreement with the available experimental data which obtained by the CBS-QB3 approach. The comparison between all our calculations and experiments indicates that a thermodynamically-controlled reaction that gives more stable products derived from the compound 2 species will be the vinyl fluoride and methyltrifluorosilane species, whereas the elimination of compound 1 into the vinyl fluoride and silicon tetrafluoride species is favorable process from kinetic point of view. In proportion to rather larger barrier heights, pressures where P > 10^―4 bar are insufficient to ensure a saturation of the calculated rate constant compared with the RRKM unimolecular rate kinetics (in high-pressure limit). Natural bond orbital analysis revealed that in accordance with an increase of barrier height from compounds 1 to 4, the HOMO-LUMO energy-gaps decreases. Furthermore, the obtained order of barrier heights could be explained by the number of electron-withdrawing fluorine atoms attached to the silicon atom. The occupancies of σ_C1―F3 bonding orbital for the studied compounds are as follows: 1>2>3>4 and those of σ^* _C1―F3 antibonding orbital increase in the opposite order (4>3>2>1) by NBO analysis. This fact explains a comparatively easier elimination of the σ_C1―F3 bond in compound 1 compared to the other compounds. The calculated data reveal that the polarization of the C₁―F₃ bond in the sense C₁ ^{δ +}–F₃ ^{δ −} is the determining factor in the elimination reaction of the studied compounds.