Knowledge Bank

Redetermination of the depolarization factors of dimethyldiacetylene indicates that the $475cm-1$ line is depolarized. Using force constants taken from previous work on methylacetylene, methylhaloacetylenes, and dimethylacetylene, it is found that the $475cm-1$ line should be a bending frequency instead of the central C-C stretching frequency. This stretching frequency falls in the region $450-550cm-1$ when the central C-C stretching force constant varies from 4-6.5 millidynes/{\AA}. the end C-C stretching frequency lies in the region $950-1150cm-1$, its value being greatly affected by the value chosen for the central C-C stretching force constant. Additional Raman spectra of liquid dimethyldiacetylene are being taken in order to determine more accurately the location of these two frequencies, since for the spectra obtained from solutions of dimethyldiacetylene only one very weak line was found in each region. Assuming the $644cm-1$ band of diacetylene is a bending frequency, a reasonable set of force constants for the doubly degenerate vibrations results. The determination of the force constants for the mondegenerate vibrations and the calculated value of the C0-C stretching frequency is in progress for this molecule.