The experimental lines are from (1) S. Yamamoto and S. Saito, 1992, J. Chem. Phys. 96, 4157.
The parameters used in the present fit differ slightly from those in (1). The fourth quantum number is an aggregate spin number; see general section. Translation to the more commonly used quantum numbers is possible with the h2cn.out file. Note: Here the spin angular momentum I due to the two equivalent H nuclei is the last one coupled to the rotational angular momentum because of restrictions in the fitting program. In contrast, in (1) I(N) is the spin angular momentum coupled to the rotational angular momentum last as would be expected from the magnitudes of the hyperfine structure effects.
The dipole moment is from an ab initio calculation by
(2) D. C. Cowles, M. J. Travers, J. L. Frueh, and G. B. Ellison, 1991, J. Chem. Phys. 95, 4702. At low temperatures, it may be necessary to discern between ortho-H₂CN and para-H₂CN. 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-H₂CN than there are for para-H₂CN. Thus, for transitions with unresolved ¹H hyperfine splitting the nuclear spin-weight ratio is 3 : 1 between ortho-H₂CN and para-H₂CN. However, for transitions with resolved ¹H hyperfine splitting no non-trivial spin-statistics have to be considered. The N_KaKc = 1₁₁; n, F + 1/2 = 13, 1 (or J, F₁ = 0.5, 0.5) is the lowest para state. It is 10.6256 cm^–1 above ground.