Low-temperature gas-phase formation of indene in the interstellar medium

Polycyclic aromatic hydrocarbons (PAHs) are fundamental molecular building blocks of fullerenes and carbonaceous nanostructures in the interstellar medium and in combustion systems. However, an understanding of the formation of aromatic molecules carrying five-membered rings-the essential building block of nonplanar PAHs-is still in its infancy. Exploiting crossed molecular beam experiments augmented by electronic structure calculations and astrochemical modeling, we reveal an unusual pathway leading to the formation of indene (C9H8)-the prototype aromatic molecule with a five-membered ring-via a barrierless bimolecular reaction involving the simplest organic radical-methylidyne (CH)-and styrene (C6H5C2H3) through the hitherto elusive methylidyne addition-cyclization-aromatization (MACA) mechanism. Through extensive structural reorganization of the carbon backbone, the incorporation of a five-membered ring may eventually lead to three-dimensional PAHs such as corannulene (C20H10) along with fullerenes (C-60, C-70), thus offering a new concept on the low-temperature chemistry of carbon in our galaxy.

Automated summary

This study reveals a new mechanism for the formation of aromatic molecules with five-membered rings such as indene, shedding light on the low-temperature chemistry of carbon in interstellar and combustion systems. The incorporation of a five-membered ring through the MACA mechanism may eventually lead to the formation of three-dimensional PAHs and fullerenes, offering a new concept for carbon chemistry in our galaxy.