High-Resolution Fourier Transform Infrared Absorption Spectroscopy of the ν6 Band of c-C3H2

The gas-phase high-resolution absorption spectrum of the ν6 band of cyclopropenylidene (c-C3H2) has been observed using a Fourier transform infrared spectrometer for the first time. The molecule has been produced by microwave discharge in an allene (3.3 Pa) and Ar (4.0 Pa) mixture inside a side arm glass tube. The observed spectrum shows a pattern of c-type ro-vibrational transitions in which the Q-branch lines strongly and distinctly stand out in the spectrum. A combined least-squares analysis of the observed 216 ro-vibrational transitions together with 28 millimeter-wave rotational transitions from the previous study has resulted in an accurate determination of the molecular constants in the ν6 state. The band center is found to be at 776.11622(13) cm–1 with one standard deviation in parentheses, which is 2.3% lower than the matrix isolation value. The intensity ratio I33)/I66) obtained from the observed ν3 and ν6 bands, 1.90(9), is somewhat lower than the ratio estimated from ab initio (2.4–2.6) and DFT (2.8) calculations.