Knowledge Bank

The infrared absorption spectrum of gaseous diacetylene has been measured from $500cm-1$ to $4000cm-1$ and the contours of the bands resolved. Several new bands have been found including a very strong perpendicular-type band at $629cm-1$. In the photographic infrared region three parallel-type infrared bands have been resolved with a 21-ft grating. The lines of these bands show an intensity alternation similar to that found for acetylene. The Raman spectrum of liquid diacetylene at $-20\circ C$ to $-30\circ C$ has been photographed and the polarization of the lines measured. Several new Raman lines were obtained. These results lead to a new assignment of the vibrational spectrum. The symmetrical C-C stretching mode, $\nu 3$ (in Herzberg's notation), is assigned to a polarized Raman line at $878cm-1$, While the depolarized line at $642cm-1$ is ascribed to the $\pi g$ hydrogen bending mode. The other $\pi g$ vibrational mode, $\nu \ast$, has been identified with the Raman line at $483cm-1$, while the $\pi \ast$ mode, $\nu 8$, has been correlated with the strong infrared band at $629cm-1$. This assignment leads to a force constant for the central C-C bond which is higher than that in ethane, in agreement with the anomolously small C-C single bond distance. The difficulty experienced by previous authors with regard to this point is thus removed. A provisional analysis of the photographic infrared bands indicates that the moment of inertia of the molecule is consistent with the internuclear distances obtained from electron diffraction experiments by Pauling, Springall, and Palmer.