The experimental laboratory frequencies from the millimeter region were reported in
(1) S. Saito and S. Yamamoto, 1997, J. Chem. Phys. 107, 1732.
The parameters used in the present fit differ slightly from those in (1). Moreover, the lines omitted from the fit because of overlap or asymmetric shape due to broad, but unresolved hyperfine splitting differ from those in (1).
With respect to the Oct. 2003 entry, additional accurate transition frequencies from the microwave and lower millimeter wave regions were included, which were published in
(2) H. Ozeki, T. Hirao, S. Saito and S. Yamamoto, 2004, Astrophys. J. 617, 680.
Since the rather complex hyperfine splitting will be unresolvable at higher frequencies or hard to be resolved at lower frequencies in warmer inter- or circumstellar sources the main entry does NOT take into account hyperfine splitting. NOTE HOWEVER: both the ¹H and ¹⁴N hyperfine splitting may be resolvable at lower frequencies and in cooler sources. Therefore, a calculation with hyperfine structure is available with N" up to 10.
Transitions both without and with hyperfine splitting have been included in the fit. A separate state number was used. The main entry has been simplified to include only 4 quantum numbers: N, K_a, K_c, and J + 1/2; the coding of the quantum numbers has been corrected with respect to the first entry. The partition function and the upper state degeneracies of the hyperfine free entry take into account the hyperfine splittin ! The present predictions with hyperfine splitting also has been simplified. The state number has been omitted to avoid confusion. Now the fourth quantum number is an aggregate spin number. It can be decoded with part of the hfs.out file. The fifth quantum number, F, designates the total spin.
The dipole moment is from an ab initio calculation by
(3) H. S. P. Müller, unpublished.
At low temperatures, it may be necessary to discern between ortho-H₂CCN and para-H₂CCN. The ortho states are described by K_a even, the para states by K_a odd. There are three times as many levels for ortho-H₂CCN than there are for para-H₂CCN. Thus, for transitions with unresolved ¹H hyperfine splitting the nuclear spin-weight ratio is 9 : 3 between ortho-H₂CCN and para-H₂CCN. However, for transitions with resolved ¹H hyperfine splitting no non-trivial spin-statistics have to be considered. The N_KaKc = 1₁₁; J + 1/2 = 2 is the lowest para state. It is 9.8339 cm^–1 above ground.