Laboratory and astronomical identification of cyclopropenylidene, C3H2

Twenty-seven rotational lines of C3H2 have been identified in the laboratory or in astronomical sources, and the rotational and centrifugal distortion constants of this previously unobserved carbene ring determined to high accuracy. The assigned astronomical transitions include the strong, ubiquitous interstellar lines at 85,338 MHz and 18,343 MHz, which are the lowest lying transitions of ortho C3H2:2(12) to 1(01) and 1(10) to 1(01), respectively. Interstellar C3H2 can be rapidly formed by dissociative recombination of the very stable ion C3H3(+), which in turn can be produced from acetylene in only two stars. In standard molecular sources such as Ori A and Sgr B2, C3H2 is only moderately abundant, but in diffuse molecular clouds it may be one of the most abundant molecules. There is some radio spectroscopic evidence for two related molecules in Sgr B2 or TMC-1: ethynylmethylene HCCCH, a hypothetical carbon chain isomer, and cyclopropene, C3H4, a known, stable three-membered ring.