Unraveling the Formation of Interstellar Benzene. Ralf I. Kaiser, University of Hawaii at Manoa, Honolulu, HI, 96822

Polycyclic aromatic hydrocarbons (PAHs) and related species have been suggested to play a key role in the astrochemical evolution of the interstellar medium, but the formation mechanism of even their simplest building block - the aromatic ben zene molecule – has remained elusive for decades. Here we demonstrate in cros sed molecular beam experi ments combined with electronic structure and statistical calcu la ti ons that benzene (C6H6) can be syn the si zed via the barrier-less, exoergic reaction of the ethynyl radical (C2H) and 1,3-butadiene (H2CCHCHCH2) under single collision con di tions. Novel gas–grain astrochemical models imply that this low-tem perature route controls the synthesis of the very first aromatic ring from acyclic precursors in cold mole cu lar clouds such as in TMC-1.

B. M. Jones, F. Zhang, R. I. Kaiser, A. M. Mebel, M. A. Cordiner, S.B. Charnley, Unraveling the Formation of Interstellar Benzene. PNAS, 108, 452-457 (2011).