Spectroscopic Studies of Rotational Isomerism in Fluorinated Ethanes. III. CF2Cl–CF2Cl and CF2Br–CF2Cl

Abstract

The phenomenon of rotational isomerism has been studied in CF₂Cl–CF₂Cl and CF₂Br–CF₂Cl by observing the temperature and state dependence of their infrared absorption spectra in the 2–18 μ region. In CF₂Cl–CF₂Cl, the trans‐isomer persists in the crystalline solid, and is also the more stable form in the liquid and vapor states. The energy difference between the trans (C_2h) and gauche (C₂) forms in the vapor was observed to be 500±200 calories/mole. In CF₂Br–CF₂Cl, the selection rules for the two probable isomeric species (C_s and C₁) are not sufficiently different to facilitate a direct determination of the symmetry of the form which persists in the solid state. However, a comparison of the infrared spectrum of CF₂Br–CF₂Cl with those of CF₂Cl–CF₂Cl and CF₂Br–CF₂Br indicates that the persistent isomer is the pseudo‐trans‐form (C_s symmetry). This trans‐form is also the more stable isomer in both the liquid and vapor phases, the energy difference being 520±200 and 600±200 calories/mole, respectively.