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 C_2v 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.

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.

On 15 October 2020, it was announced that small amounts of cyclopropenylidene had been found in the atmosphere of Titan, the largest moon of Saturn using the ALMA telescope. Cyclopropenylidene became the fifth C₃H_n (three carbon) hydrocarbon molecule detected on Titan, after previous detections of CH₃CCH (propyne) and C₃H₈ (propane); C₃H₆ (propene); and CH₂CCH₂ (propadiene).

The formation reaction of c-C₃H₂ has been speculated to be the dissociative recombination of c-C
₃H⁺
₃.

c-C
₃H⁺
₃ is a product of a long chain of carbon chemistry that occurs in the ISM. Carbon insertion reactions are crucial in this chain for forming C
₃H⁺
₃. However, as for most ion-molecule reactions speculated to be important in interstellar environments, this pathway has not been verified by laboratory studies. The protonation of ammonia by c-C
₃H⁺
₃ is another formation reaction. However, under typical dense cloud conditions, this reaction contributes less than 1% of the formation of C₃H₂.

Crossed molecular beam experiments indicate that the reaction of the methylidyne radical (CH) with acetylene (C₂H₂) forms cyclopropenylidene plus atomic hydrogen and also propadienylidene plus atomic hydrogen. The neutral–neutral reaction between atomic carbon and the vinyl radical (C₂H₃) also forms cyclopropenylidene plus atomic hydrogen. Both reactions are rapid at 10 K and have no entrance barrier and provide efficient formation pathways in cold interstellar environments and hydrocarbon-rich atmospheres of planets and their moons.

Matrix isolated cyclopropenylidene has been prepared by flash vacuum thermolysis of a quadricyclane derivative in 1984.

Cyclopropenylidene is generally destroyed by reactions between ions and neutral molecules. Of these, protonation reactions are the most common. Any species of the type HX⁺ can react to convert the c-C₃H₂ back to c-C
₃H⁺
₃. Due to rate constant and concentration considerations, the most important reactants for the destruction of c-C₃H₂ are HCO⁺, H⁺
₃, and H₃O⁺.