Circumstellar Origin of Chrysene (C ₁₈ H ₁₂ ) via Self‐Recombination of Resonantly‐Stabilized 1‐Indenyl Radicals and Implications to the Aromaticity of the Carbonaceous Asteroid Ryugu

Abstract

ABSTRACT Polycyclic aromatic hydrocarbons (PAHs) represent key molecular building blocks of carbonaceous nanoparticles and have been identified in cold molecular clouds (TMC‐1), meteorites (Murchison, Allende), and asteroids (Bennu, Ryugu). However, the understanding of their formation and molecular mass growth processes in these extreme environments has remained elusive, especially with the emergence of resonantly‐stabilized free radical (RSFR)‐mediated pathways. Here, we report a combined experimental and computational study on the self‐recombination of the aromatic and resonantly‐stabilized 1‐indenyl radical (C 9 H 7 • ) isomer‐selectively forming the 18π‐Hückel PAH chrysene (C 18 H 12 ) in an overall endoergic reaction. These features account for the circumstellar origin of chrysene in pristine samples of the carbonaceous asteroid Ryugu and of the Murchison meteorite. At the microscopic level, the ring expansion involving cyclopentadienyl moieties to two six‐membered aromatic rings via pivotal spiroaromatic intermediates affords a versatile, RSFR‐initiated mass growth channel essentially leading to graphene‐type PAHs and eventually two‐dimensional nanostructures in high temperature circumstellar envelopes and in combustion processes.