Laboratory and astronomical spectroscopy of C3H2, the first interstellar organic ring

Abstract

The first laboratory and astronomical observations of the carbene ring molecule C3H2 briefly reported in a recent Letter are described in detail. In the laboratory, 22 millimeter-wave lines have been measured in a He and C 2 H 2 discharge. From these the three rotational constants and the five fourth-order centrifugal distortion constants of C 3 H 2 have been determined to high precision, allowing, in turn, calculation to 0.1 km s 1 of the most important lines in the radio and far-IR spectrum; 123 lines in the rotational spectrum with E/k < 100 K are tabulated. Eleven astronomical transitions have been identified, and in Sgr B2 and Orion KL, rotational temperatures and column densities were derived and compared with those of other hydrocarbons with a similar number of C atoms. Owing to the ubiquity in space of its strongest low-lying transitions, C 3 H 2 promises to be a generally useful molecular probe; further, it may be the first of a new class of ring molecules in space. Detection of a carbene ring in an He and C 2 H 2 discharge suggests searches for other hydrocarbon rings and cyclopropenylidene derivatives.