Analysis of the ν7+ ν9 and 2 ν7− ν9 band complexes of dicyanoacetylene (NC–CC–CN)

Abstract

The very weak ν 7+ ν 9 band complex near 368 cm −1 and the 2 ν 7 0 − ν 9 system near 404 cm −1 in the infrared gas spectrum of dicyanoacetylene (NC–CC–CN) have been analysed. In the ν 7+ ν 9 band system, we have assigned 68 subbands, reaching upper vibrational states with ν 7 up to 3 and ν 9 up to 8. The 11 subbands of the 2 ν 7 0 − ν 9 system with ν 9 up to 7 could be observed because of intensity transfer from the ν 3− ν 9 band, a result of the strong anharmonic interaction between the ν 3 and 2 ν 7 0 states. The combined data from the previously studied ν 9 band together with the ν 7+ ν 9 and 2 ν 7− ν 9 bands were used for a global rovibrational analysis from which a full set of molecular parameters for the ν 7 and ν 9 modes has been deduced. A recently developed theory of intensities in such band systems has been applied to the ν 7+ ν 9 band. Examples of intensity calculations illustrate some spectacular effects of the rotational and vibrational ℓ-type resonances on intensities. The observed intensities are in good agreement with the predicted values within the expected J-ranges.