Quantum chemical study of infrared and Raman spectra of dicyanodiacetylene

Structural and vibrational spectral features of dicyanodiacetylene, a molecule of considerable interstellar significance, using ab-initio (MP2) and density functional theory (DFT) based quantum chemical methods have been studied. Double and triple-zeta basis sets with polarization and diffuse functions (6-31G**, 6-311+G**) as well as Dunning's correlation consistent basis sets (cc-pvDZ, cc-pvTZ and aug cc-pvTZ) have been used in the study. Optimized geometry, infrared and Raman spectra have been calculated and a complete vibrational assignment based on potential energy distribution along the internal coordinates and depolarization ratios of the Raman bands has been provided. Dicyanodiacetylene is found to be a symmetric linear chain molecule of D(infinity h) symmetry having significant conjugation in the chain. This affects the length of both the single and triple bonds. Conjugation effect is most pronounced for the central C-C bond which has a length of about .1.35 angstrom. While some of the earlier assignments to the infrared and Raman bands have been confirmed, several new assignments have been suggested using symmetry considerations and position, intensity and polarization of the vibrational bands.