QUASI-LINEARITY IN HCCN - EFFECTS OF THE NU(2) FUNDAMENTAL

The high resolution infrared spectrum of the quasi-linear molecule HCCN in the region of its nu(2) CCN antisymmetric stretching fundamental has been observed. Six separate vibrational bands have been positively identified and rotationally analyzed. The lower states of these transitions belong to the ground state and the first and second excited states of the HCC bending mode (nu(5) in linear molecule notation), nu(5)(+/-1) and 2 nu(5)(+/-2). Multiple occurrences of transitions originating from each of these lower levels combined with the presence of partially resolved fine structure splitting in some bands and its absence in others suggest a strong coupling of the nu(2) mode with otherwise ''dark'' vibrational states. Fermi resonance between nu(2) and a combination level, such as nu(3) + (nu(4) + nu(5))(0+), provides such an intensity borrowing mechanism.