Spectroscopic Studies of Rotational Isomerism in Fluorinated Ethanes. III. CF2Cl-CF2Cl and CF2Br-CF2Cl
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 (C2h) 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 (Cs 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 (Cs 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.