Cryogenic Bottom-up Formation of the Benzene Cation from Acetylene in Helium Nanodroplets.

Bottom-up ion-molecule reaction networks are thought to drive molecular complexity in space. Nevertheless, even the formation of the first aromatic ring (benzene) remains poorly understood. In this publication, we report the formation of the benzene radical cation from an acetylene cation and two acetylene molecules via the sequential two-step reaction C2H2•+ + C2H2 → C4H4•+ followed by C4H4•+ + C2H2 → C6H6•+ inside cryogenic helium nanodroplets. The superfluid helium-environment, used as a model system to study chemical reactivity in an environment with efficient energy dissipation such as interstellar dust and ice, stabilizes reaction intermediates and products and enables direct spectroscopic investigation. Helium-tagging spectroscopy in the visible wavelength range unambiguously identifies the C6H6•+ product as the benzene radical cation. This is consistent with quantum-chemical calculations that predict a reaction pathway that is barrierless in both steps. These results establish a low-temperature route to the formation of the first aromatic ring in very cold condensed phases, which may impact our understanding of polycyclic aromatic hydrocarbon formation in the interstellar medium.