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Radio astronomical data of C6H- observed with the Nobeyama 45 m and IRAM 30 m telescopes have been analyzed by the local thermodynamic equilibrium approximation to give the column density of (6.1-8.0) × 1012 cm-2 and the excitation temperature of 32 ± 3 K, with an assumed source size of 30'' ± 3''. The abundance of C6H- was estimated to be 8.6% of C6H. The observed line shapes of C6H - and C6H indicate that the distribution of C 6H- is more present in the inner region than C6H. The production mechanism of C6H- is discussed.

The ortho-to-para nuclear spin ratio of cyclopropenylidene (cyclic C3H2) in TMC-1 cores was investigated using the 45 m radio telescope at Nobeyama Radio Observatory. We have observed four ortho lines (J = 212-101, 312-303, 321-312, and 330-321) and four para lines (J = 202-111, 211-202, 220-211, and 322-313) of cyclic C3H2 in six cores in TMC-1. A statistical equilibrium analysis of the observed intensities revealed apparent correlation between the ortho-to-para ratio of cyclic C3H2 and chemical evolution of the molecular cloud; the ortho-to-para ratio in younger cores is less than 2, while that in older cores is close to the statistical ratio of 3. A simple chemical model calculation indicates that the correlation is ascribed to the conservation of nuclear spin angular momenta in chemical reactions relevant to the production, destruction, and reproduction of cyclic C3H2. Since cyclic C3H2 has been observed in a variety of sources, the ortho-to-para ratio of cyclic C3H2 may be a useful probe of chemical evolution of molecular clouds.

The millimeter and submillimeter spectrum of the FeCO radical in the 3Σ- ground electronic state has been observed in the laboratory. The radical was generated by a DC glow discharge in a mixture of Fe(CO)5 and argon. A total of 33 rotational transitions in the three spin sublevels with Ω = 1, 0 were observed in the frequency region between 223 and 372 GHz. The observed frequencies were analyzed using a Hamiltonian for the 3Σ- state, and the rotational, centrifugal distortion, spin-spin coupling, and spin-rotation coupling constants were determined. Based on these constants, we obtained the rest frequencies in the 2 and 3 mm regions of FeCO and searched for this radical toward the circumstellar envelope of IRC +10216 and the star-forming regions W3(OH), DR 21, and Sgr B2 with the Instituto de Radioastronomia Milimetrica 30 m radio telescope. The FeCO lines were not detected with an rms noise of 3-7 mK in four sources. The upper limit on the FeCO column density was estimated to be 3 × 1013 cm-2 in IRC +10216.

The rotational spectrum of C 5 S in the X 1 Σ + (υ=0) ground state has been observed for the first time using a Fabry-Perot-type Fourier transform microwave spectrometer combined with a pulsed discharge nozzle. C 5 S was generated by a discharge in a mixture of CS 2 and C 2 H 2 diluted in Ar, and subsequently cooled down to a few kelvins in a supersonic jet. Eight rotational transitions of C 5 32 S have been observed in the 5-20 GHz region. Three lines for the less abundant 34 S species have also been detected to confirm the carrier of the observed lines to be C 5 S