A plethora of vibrational states of the HCN molecule are of relevance for astronomical observations. The states are designated by (v₁v₂v₃). Note: The (100) mode is the CH stretching mode at 3311.5 cm^–1; the (010) mode is the doubly degenerate bending mode at 712.0 cm^–1; and the (001) mode is the CN stretching mode at 2096.8 cm^–1. The designation of the (100) and (001) modes has been interchanged on occasion – sometimes without recognizing this.
The Feb. 2003 entry was based on data for (000) and (010). The May 2007 entry employs in addition data for (020) and (030) as well as (001) and (100). Below (100), the Coriolis interacting (040) and (011) states have to be included in the fit.
The observed v = 0 transitions were summarized largely in
(1) V. Ahrens, F. Lewen, S. Takano, G. Winnewisser, S. Urban, A. A. Negirev, and A. N. Koroliev, 2002, Z. Naturforsch., 57a, 669.
One additional high J transition was reported in
(2) S. Thorwirth, H. S. P. Müller, F. Lewen, S. Brünken, V. Ahrens, and G. Winnewisser, 2003, Astrophys. J. 585, L163.
For the May 2007 entry the J = 2 – 1 transition frequencies were taken from
(3) G. Cazzoli and C. Puzzarini, unpublished; cited in A. V. Lapinov, 2006, Proc. SPIE 6580, Art. No. 658001.
Sources for the v₂ = 1 data are given in e027503.cat, those for higher vibrational states in one of the documentations e027507.cat to e027510.cat.
Extensive infrared data was taken from
(4) A. G. Maki, G. C. Mellau, S. Klee, M. Winnewisser, and W. Quapp, 2000, J. Mol. Spectrosc. 202, 67.
The dipole moment was reported by
(5) W. L. Ebenstein and J. S. Muenter, 1984, J. Chem. Phys. 80, 3989.
The ¹⁴N hyperfine splitting can be resolved for low values of J. Therefore, a separate hyperfine calculation is provided up to J" = 10 up to 1 THz. While the general entries do NOT include hyperfine effects, the spin multiplicity g_I of 3 for the ¹⁴N nucleus is included in the partition function as well as in the upper state degeneracy g_up.
The partition function takes into account all vibrational states used in the fit. Contributions of individual vibrational states to the partition function are available. The vibrational contributions are essentially complete at 300 K.