Knowledge Bank

The high resolution infrared spectrum of the quasi-linear molecule HCCN in the region of its $\nu 2$ CCN stretching fundamental was recently measured, At least six separate vibrational origins have been positively identified. Using combination differences and the lower state spectroscopic constants for the ground state and excited bending mode states derived by McCarthy and $Gottlieb,1$ the lower state for each vibrational transition has been assigned. Unambiguous assignment of the Upper states is complicated by the appearance of duplicate sets of transitions originating from the same lower states. The most likely cause for this occurrence is a Fermi resonance between $\nu 2$ and $ \nu_{3}+(\nu_{4} +\nu_{5})^{0+}$ (in linear molecule notation) which gives intensity to the transitions to the combination level.