THE VIBRATIONAL SPECTRUM AND STRUCTURE OF DICYANOACETYLENE

The infrared spectrum of dicyanoacetylene, $N\\equiv C-C\\equiv C-\\equiv N$, has been measured from 400 to $3400 cm^{-1}$ in both vapor and liquid states. The Raman spectrum of the liquid has also been obtained. The following vibrational assignment was made on the basis of a linear symmetrical structure:$$\\begin{array}{ccc} &Infrared=active &Raman=active\\\\ Stretching\\ modes& 2241& 2333\\\\& 1154 &2268\\\\&&(Unassigned)\\\\Bending\\ modes & 472 &504\\\\&107 &260\\\\\\end{array}$$ The only unassigned fundamental is the symmetrical C-C stretching vibration. These fundamentals explain the considerable number of combination tones very satisfactorily. All features of the spectra, including particularly the infrared band contours and the lack of coincidences between infrared and Raman bands, are compatible with $D \\infty$ symmetry. Results of a normal coordinate treatment will be outlined. Some related results from other techniques will be described briefly. The electronic spectrum shows a vibrational level near $2180 cm^{-1}$ in the upper state, but further analysis has not been made thus far. An x-ray determination of the crystal structure has demonstrated that the molecule is linear in the solid, with the nitrogen atoms at the ends. It also provided the interatomic distances which were used to calculate the moment of inertia.