2012ApJS..200....1S

The rotational spectra of the normal and seven isotopic species of cyclopropenylidene c-C₃H₂ have been measured at high spectral resolution by Fourier transform microwave spectroscopy of a supersonic molecular beam between 10 and 43 GHz. Deuterium quadrupole coupling and carbon-13 spin-rotation hyperfine constants were determined in addition to the rotational constants. Quartic and sextic centrifugal distortion constants derived from 28 lines between 150 and 316 GHz of the doubly deuterated species c-C₃D₂ allow the rotational spectrum to be calculated to 0.5 km/s or better in equivalent radial velocity up to 500 GHz. Spectroscopic constants determined from four centimeter-wave and 19 millimeter-wave lines of the normal species c-C₃H₂, including 15 with sharp Lamb-dips, allow prediction of the most important astronomical transitions (i.e., those with ΔJ = 1 and K_a ≤ 3) to 0.05 km/s or better at 500 GHz. The doubly deuterated species is a good candidate for detection in cold dark clouds, because deuterium fractionation is high in c-C₃H₂ and lines of C₃HD are fairly intense in these sources. An accurate empirical equilibrium structure of c-C₃H₂, derived from the experimental rotational constants of normal and isotopic c-C₃H₂, corrected for zero-point vibrational effects, is compared with previously reported structures.