The cyanobutadiynylide ion has been detected very recently in the circumstellar shell of IRC +10216. Even though no laboratory data are available at present, very accurate ab initio calculations, see below, provide very reliable spectroscopic parameter. Combined with the chemistry in that source, the assignment of the C₅N^– appears to be rather secure; see also the Molecules in Space section. Rest frequencies were taken from
(1) J. Cernicharo, M. Guélin, M. Agúndez, M. C. McCarthy, and P. Thaddeus, 2008, Astrophys. J., accepted.
U33332.3, reported by
(2) K. Kawaguchi, Y. Kasai, S.-I. Ishikawa, and N. Kaifu, 1995, Publ. Astron. Soc. Japan 47, 853;
is, at least in part, due to C₅N^–. Therefore, this line was also used in the fit.
The ¹⁴N hyperfine splitting may be be resolvable for low values of J. Therefore, a separate hyperfine calculation is provided up to J" = 4. The eQq value was taken from C₃N^–; this is probably a good, but not perfect assumption. The spin multiplicity g_I of 3 for the ¹⁴N nucleus is NOT included in the partition function or in the upper state degeneracy g_up of the general entries. The partition function values have to be multiplied by 3 if hyperfine splitting is taken into account.
The reported line uncertainties seem to be somewhat conservative; based on the fit results, they seem to be 2 σ values. The predictions are probably reliable up to 200, maybe even 300 GHz.
The dipole moment is from an ab initio calculation by
(3) P. Botschwina and R. Oswald, 2008, J. Chem. Phys. 129, Art. No. 044305.