Cyclopropenylidene

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.

Formation

The major formation reaction of c-C₃H₂ is the dissociative recombination of c-C₃H₃⁺.

c-C₃H₃⁺ 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 C₃H₃⁺. The protonation of NH₃ by c-C₃H₃⁺ is the second most important formation reaction. However, under typical dense cloud conditions, this reaction contributes less than 1% of the formation of C₃H₂.

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

Destruction

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-C₃H₃⁺. Due to rate constant and concentration considerations, the most important reactants for the destruction of c-C₃H₂ are HCO⁺, H₃⁺, and H₃O⁺.

Notice that c-C₃H₂ is mostly destroyed by converting it back to C₃H₃⁺. Since the major destruction pathways only regenerate the major parent molecule, C₃H₂ is essentially a dead end in terms of interstellar carbon chemistry. However, in diffuse clouds or in the photodissociation region (PDR) of dense clouds, the reaction with C⁺ becomes much more significant and C₃H₂ can begin to contribute to the formation of larger organic molecules.

Spectroscopy

Detections of c-C₃H₂ in the ISM rely on observations of molecular transitions using rotational spectroscopy. Since c-C₃H₂ is an asymmetric top, the rotational energy levels are split and the spectrum becomes complicated. Also, it should be noticed that C₃H₂ has spin isomers much like the spin isomers of hydrogen. These ortho and para forms exist in a 3:1 ratio and should be thought of as distinct molecules. Although the ortho and para forms look identical chemically, the energy levels are different, meaning that the molecules have different spectroscopic transitions.

When observing c-C₃H₂ in the interstellar medium, there are only certain transitions that can be seen. In general, only a few lines are available for use in astronomical detection. Many lines are unobservable because they are absorbed by the Earth's atmosphere. The only lines that can be observed are those that fall in the radio window. The more commonly observed lines are the 1₁₀ to 1₀₁ transition at 18343 MHz and the 2₁₂ to 1₀₁ transition at 85338 MHz of ortho c-C₃H₂.