High-Resolution Infrared Determination of the Structure of Carbon Suboxide

High-resolution spectra have been obtained and the vibration—rotation absorption spectrum has been analyzed for a Σu—Σg and a Πg—Πu transition in C3O2 between 3142 and 3195 cm—1. The absorption region studied contains a parallel combination band and a series of associated ``hot bands.'' The successive ``hot bands'' are displaced toward high frequencies in such a manner that the Σ—Σ and Π—Π bands are relatively free of overlapping lines. The analysis shows that the molecule must have D∞h symmetry with B0=0.073206 cm—1. The large number of ``hot bands,'' the variation of intensity of these with temperature, and the large value of the l-doubling constant q all indicate that the lowest frequency bending mode must be of very low energy.