Knowledge Bank

The Raman and infrared spectra of liquid 1,1,2,2-tetrachloro- and 1,1,2,2-tetrabromoethane have been investigated. Using a grating spectrograph of 3.8 \AA/mm dispersion, we have observed the chlorine isotope effect for the $\Delta \nu =353$ and $366cm-1$ line pair, and have verified the previous results of Rank, Sheppard, and Szasz concerning the anomolous intensity contour exhibited by the $353cm-1$ vibration. From temperature intensity measurements in the Raman effect and Raman-infrared correspondences, we conclude that the two forms posses $C2h$ and $C2$ symmetry, with the $C2$ form being the more stable one. On the basis of this conclusion we are able to explain the intensity contours observed in the chlorine isotope effect. The value of $\Delta H$ for the reaction $C2\to C2h$ in s-tetrabromoethane was found to be 910 cal/mole.