Knowledge Bank

Cyclopropenylidene, c-C$3$H$2$, is the most widely distributed ring in our galaxy; it has been detected in more than 50 astronomical sources, and its isotopic species c-C$3$HD has been observed towards several dense cores in cold dark clouds. Because of the high observed abundance and large deuterium fractionation for this small hydrocarbon ring, other isotopic species of c-C$3$H$2$ may be good candidates for astronomical detection. For these reasons, an exhaustive isotopic study of c-C$3$H$2$ has now been undertaken in which rotational spectra of c-C$3$D$2$, c-C$3$HD, and the carbon--13 isotopic species of c-C$3$HD and c-C$3$H$2$ have been detected in the centimeter-wave band by Fourier transform microwave (FTM) spectroscopy between 10 and 40~GHz. For c-C$3$D$2$, millimeter- and submillimeter-wave spectra were subsequently measured between 140 and 400~GHz. Rotational and centrifugal distortion constants derived either from previous measurements or those predicted from theory are compared with the precise constants determined here. % #######################################################################