February 2007

C6H- anion

In late 2006, C6H- became the first molecular anion to be identified in the gas phase interstellar and circumstellar media. Michael McCarthy and his co-workers observed the rotational spectra in the lab and found matches to 17 lines in the circumstellar envelope of IRC+10216 and in the TMC-1 complex. The lab detection was deduced on the basis of the similarity of the new species' rotational constants to those of C6H, a well-known and abundant astromolecule, Other clues included the relative simplicity of its spectra, indicating that the species posesses a singlet electronic state, and the large electron affinity of C6H. The group at the Harvard-Smithsonian Center for Astrophysics and their colleages have subsequently reported lab detections of C4H- and C8H-, as well as C2H-.

While many anions have been formed in the laboratory, often to determine electron affinities, producing enough to perform rotational spectroscopy is one of the technical achievements of the new experiments.

In addition to the possibility that more gas-phase anions will be identified in interstellar or circumstellar sources, the existence of interstellar anions raises questions about how they are formed and what chemistry they may participate in.

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