A Combined Experimental and Theoretical Study of the Reaction of Dicarbon (c2) with D1-acetylene (hccd): Possible Mechanisms for Deuterium Enrichment in the Interstellar D1-butadiynyl Radical, Ccccd(x

The reaction of the dicarbon molecule (C2) with D1-acetylene (HCCD) was carried out under single-collision conditions in a crossed molecular beams machine to investigate deuterium enrichment processes in cold molecular clouds and in circumstellar envelopes of carbon stars. The experiments were merged with electronic structure calculations. Our combined experimental and theoretical studies suggest that this barrierless neutral-neutral reaction can induce a deuterium enrichment in the D1-1,3-butadiynyl radical (CCCCD) over a broad range of collision energies formally corresponding to temperatures from 10 to a few thousand kelvin as present in cold molecular clouds and circumstellar envelopes close to the photosphere, respectively. Generalized, each rapid, neutral-neutral reaction, which proceeds through indirect scattering dynamics via a reaction intermediate residing in a deep potential energy well, can induce a deuterium enrichment in cold molecular clouds (10 K) and even at elevated temperatures of a few thousand K as present, for instance, in the envelope of the dying carbon star IRC +10216. These findings should trigger an astronomical survey of the D1-1,3-butadiynyl molecule in the circumstellar shells of IRC +10216. Subject headinggs: ISM: molecules — molecular processes