MILLIMETER-WAVE SPECTRUM OF VIBRATIONALLY EXCITED CYCLOPROPENYLIDENE,C-C3H2

The millimeter-wave rotational spectrum of c-C3H2 in four vibrationall y excited states was measured and the rotational and quartic centrifug al distortion constants determined. Of the vibrational states observed here, only nu3 has been previously detected in the infrared. Assignme nt of the three new states to nu6, nu5, and nu2 was based on relative intensities, comparison of calculated and measured inertial defects, a nd symmetry considerations. The spectroscopic constants determined wil l guide future infrared investigation of c-C3H2, needed for a complete elucidation of the vibrational structure of this molecule. The experi mental values of the vibration-rotation coupling constants will allow comparison with theoretical calculations that have recently become fea sible for molecules of similar size as c-C3H2. Detection in space of r otational lines from the lowest vibrationally excited states may be po ssible.