Molecular Acoustic and Spectroscopic Studies on the Internal Rotation in 1-Fluoro-1,1,2,2-tetrachloroethane

An ultrasonic relaxation observed in liquid 1‐fluoro‐1,1,2,2‐tetrachloroethane has been attributed to a perturbation of the equilibrium between the trans and gauche isomers. From the temperature dependence of the relaxation parameters, the rates of isomerization have been derived. The temperature dependence of the rate constants were found to obey the Arrhenius and Eyring rate equations giving the following parameters: $$\begin{array}{cc}{\text{A\hspace{0.17em}=\hspace{0.17em}2.2\hspace{0.17em}±\hspace{0.17em}1.5\hspace{0.17em}×\hspace{0.17em}10}}^{12}{\sec }^{\text{−1}},& \text{E\hspace{0.17em}=\hspace{0.17em}9.0\hspace{0.17em}±\hspace{0.17em}0.4}\mathrm{kcal}/\mathrm{mole},\\ & {\mathrm{\Delta H}}_{\mathrm{gt}}^{+}\text{\hspace{0.17em}=\hspace{0.17em}8.3\hspace{0.17em}±\hspace{0.17em}0.3}\mathrm{kcal}/\mathrm{mole}.\end{array}$$ The vibrational frequency of the torsional oscillations of the trans isomer is 86 cm⁻¹, which is equivalent to a torsional barrier ${\mathrm{(\Delta H}}_{\mathrm{gt}}^{+})$ of 8.89 kcal/mole. The acoustic and spectroscopic barrier are compared to the potential barrier found from a recent NMR study, ${\mathrm{(\Delta H}}_{\mathrm{gt}}^{+}\text{\hspace{0.17em}=\hspace{0.17em}7.25}\mathrm{kcal}/\mathrm{mole})$ .