Theoretical study for conformational analysis and kinetics on the internal halogen exchange thermally induced of trichloro-(1,1-difluoroethyl)silane in the gas phase

Abstract Consecutive first-order, homogeneous intramolecular fluorine-chlorine exchange reactions of trichloro-(1,1-difluoroethyl)silane have been theoretically studied using ab initio post-Hartree-Fock G4MP2 and DFT (B3LYP, M06, M06-2X) levels of theory. The reactions were studied at 127 °C and 0.1 Torr of temperature and pressure respectively, yielding (1-choro-1-fluoroethyl)-dichlorofluorosilane, and chloro-difluoro(1,1-dichloroethyl)silane. A very good agreement has been found between B3LYP/6-311+G(2df,p) calculations and previously reported experimental results. Conformational mobility in the structure of the intermediate and final product was studied in order to assign the highest and lowest in energy conformers. Analysis of activation parameters, Natural Bond Order (NBO) and Atoms In Molecules (AIM) suggests that in the first exchange reaction, the fluorine atom is almost completely transferred to silicon prior to chlorine migration, producing a penta-coordinated Si center and a positive-charged carbon in the transition state (TS). The second fluorine-chlorine switch proceeds through a more synchronized three-membered cyclic transition state, as opposed to the results obtained in a previous work, which proposes that the cyclic TS take place in the first halogen interchange. The differences in synchronicity (Sy) parameters between the two reactions steps are attributed to the difference in the uniformity of the electron distribution. According to IRC calculations, transition states are early and both exchanges are exothermic.