Laboratory and Astronomical Detection of the Negative Molecular Ion C3N−

The negative molecular ion C 3 N − has been detected at millimeter wavelengths in a low-pressure laboratory discharge, and then with frequencies derived from the laboratory data in the molecular envelope of IRC+10216. Spectroscopic constants derived from laboratory measurements of 12 transitions between 97 and 378 GHz allow the rotational spectrum to be calculated well into the submillimeter-wave band to 0.03 km s −1 or better in equivalent radial velocity. Four transitions of C 3 N − were detected in IRC+10216 with the IRAM 30 m telescope at precisely the frequencies calculated from the laboratory measurements. The column density of C 3 N − is 0.5% that of C 3 N, or approximately 20 times greater than that of C 4 H − relative to C 4 H. The C 3 N − abundance in IRC+10216 is compared with a chemical model calculation by Petrie & Herbst. An upper limit in TMC-1 for C 3 N − relative to C 3 N (<0.8%) and a limit for C 4 H − relative to C 4 H (<0.004%) that is 5 times lower than that found in IRC+10216, were obtained from observations with the NRAO 100 m Green Bank Telescope (GBT). The fairly high concentration of C 3 N − achieved in the laboratory implies that other molecular anions containing the CN group may be within reach.