The Vibrational Energy Levels and Specific Heat of Ethylene

In a recent review of the experimental infra-red and Raman spectra of ethylene and tetradeuteroethylene 1, Conn and Sutherland have succeeded in giving very reasonable estimates of the positions of the four fundamental lelvels associated with the "rocking frequencies" of the methylene groups in C2H4. They have at the same time revised the previous estimate of the position of the "torisional" lelvel 2 to 700 cm^-1, for reasons which are in our opinion inadequate, and with results which prove to be unsatisfactory. In particular, as they themselves remark, the calculated heat capacities then become too large. We wish to point out that a return to the Bonner assignment of this level not only yields a more reasonable explanation of the weak combination and harmonic lines in the Raman spectra, but also leads to a theoretical heat capacity which is in good agreement with the existing measured values 3 and with new experimental values recently obtained in these Laboratories.