A high-resolution isotopic study of the rotational spectrum of c-C3H2

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−1 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 Lambdips, allow prediction of the most important astronomical transitions (i.e., those with ΔJ = 1 and Ka 3) to 0.05 km s−1 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.