The experimental lines are from (1) S. Saito and S. Yamamoto, 1999, J. Chem. Phys. 111, 7916.
Several experimental frequencies omitted in (1) were used in the present fit leading to slightly different parameters. The fourth quantum number is an aggregate spin number.
The dipole moment is from an ab initio calculation by
(2) H. S. P. Müller, unpublished.
At low temperatures, it may be necessary to discern between ortho-H₂CP and para-H₂CP. 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-H₂CP than there are for para-H₂CP. Thus, for transitions with unresolved ¹H hyperfine splitting the nuclear spin-weight ratio is 3 : 1 between ortho-H₂CP and para-H₂CP. However, for transitions with resolved ¹H hyperfine splitting no non-trivial spin-statistics have to be considered. The N_KaKc = 1₁₁; n, F = 11, 0 (or J, F₁ = 0.5, 0.0) is the lowest para state. It is 10.3442 cm^–1 above ground.