cyclopropenylidene

Cyclopropenylidene, or '''c-C3H2''', is a partially aromatic molecule belonging to a highly reactive class of organic molecules known as carbenes. On Earth, cyclopropenylidene is only seen in the laboratory due to its reactivity. However, cyclopropenylidene is found in significant concentrations in the interstellar medium (ISM) and on Saturn's moon Titan. Its C2v symmetric isomer, propadienylidene (CCCH2) is also found in the ISM, but with abundances about an order of magnitude lower. A third C2 symmetric isomer, propargylene (HCCCH), has not yet been detected in the ISM, most likely due to its low dipole moment.

==History== The astronomical detection of c-C3H2 was first confirmed in 1985. Four years earlier, several ambiguous lines had been observed in the radio region of spectra taken of the ISM, but the observed lines were not identified at the time. These lines were later matched with a spectrum of c-C3H2 using an acetylene-helium discharge. Surprisingly, c-C3H2 has been found to be ubiquitous in the ISM. Detections of c-C3H2 in the diffuse medium were particularly surprising because of the low densities. It had been believed that the chemistry of the diffuse medium did not allow for the formation of larger molecules, but this discovery, as well as the discovery of other large molecules, continue to illuminate the complexity of the diffuse medium. More recently, observations of c-C3H2 in dense clouds have also found concentrations that are significantly higher than expected. This has led to the hypothesis that the photodissociation of polycyclic aromatic hydrocarbons (PAHs) enhances the formation of c-C3H2.