INFRARED STUDIES OF ROTATIONAL ISOMERISM IN $CF_{2}CL-CF_{2}CL$ AND $CF_{2}BR-CF_{2}CL$

Abstract

The phenomenon of rotational isomerism has been studied in $CF_{2}CL-CF_{2}CL$ and $CF_{2}BR-CF_{2}CL$ by observing the temperature and state dependence of their infrared absorption spectra in the $2-18\mu$ region. In $CF_{2}CL-CF_{2}CL$, the trans isomer persists in the crystalline solid, and is also the more stable form in the liquip and vapor states. The energy difference between the trans ($C_{2}H$) and gauche ($C_{2}$) forms in the vapor was observed to be 500 $\pm$ 200 calories/mole. In $CF_{2}BR-CF_{2}CL$, the selection rules for the two probable isomeric species ($C_{s}$ and $C_{1}$) 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 $CF_{2}BR-CF_{2}CL$ with those of $CF_{2}CL-CF_{2}CL$ and $CF_{2}BR-CF_{2}BR$ 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 $\pm$ 200 and 600 $\pm$ 200 calories/mole, respectively.