Millimeter‐wave spectrum of vibrationally excited cyclopropenylidene, c‐C3H2

The millimeter‐wave rotational spectrum of c‐C3H2 in four vibrationally excited states was measured and the rotational and quartic centrifugal distortion constants determined. Of the vibrational states observed here, only ν3 has been previously detected in the infrared. Assignment of the three new states to ν6, ν5, and ν2 was based on relative intensities, comparison of calculated and measured inertial defects, and symmetry considerations. The spectroscopic constants determined will guide future infrared investigation of c‐C3H2, needed for a complete elucidation of the vibrational structure of this molecule. The experimental values of the vibration‐rotation coupling constants will allow comparison with theoretical calculations that have recently become feasible for molecules of similar size as c‐C3H2. Detection in space of rotational lines from the lowest vibrationally excited states may be possible.