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

The high resolution infrared spectrum of the quasi-linear molecule HCC N 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 trans itions belong to the ground state and the first and second excited sta tes of the HCC bending mode (nu(5) in linear molecule notation), nu(5) (+/-1) and 2 nu(5)(+/-2). Multiple occurrences of transitions originat ing from each of these lower levels combined with the presence of part ially 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 combin ation level, such as nu(3) + (nu(4) + nu(5))(0+), provides such an int ensity borrowing mechanism.