Microwave vibration rotation spectrum of diacetylene

Abstract

Microwave spectrum of diacetylene (HCCCCH) arising from vibration-rotation transitions between closely lying excited vibrational states was observed. The ν 8- ν 6 band which has been reported in our previous paper was reinvestigated in a frequency region extended to 175 GHz, where ν 6 and ν 8 respectively represent the symmetric ( π g ) and antisymmetric ( π u ) CCH bending vibrations. The ν 8 + ν 9 − ν 6 − ν 9 band, where ν 9 is the lowest frequency (220 cm −1) bending mode with π u symmetry, was newly identified. Precise molecular constants associated with the ν 8 - ν 6 band and the three subbands Σ g + - Σ u +, Σ g − - Σ u −, and Δ g - Δ u of the ν 8 + ν 9 − ν 6 − ν 9 band were determined by least-squares analyses. l-Type resonance interactions in the ν 6 + ν 9 and ν 8 + ν 9 vibrational states were analyzed. Also, a Fermi resonance which perturbs the Σ g + substate of the ν 8 + ν 9 state was discussed. The transition dipole moments were determined from the Stark effect for the Σ g + - Σ u + and Σ g − - Σ u − subbands of ν 8 + ν 9 − ν 6 − ν 9.