This H₂C₃H isomer is better described as a combination of the 2-propynyl and the propadienyl resonance forms. These resonance forms are also known as propargyl and allenyl, respectively.
The experimental lines were taken from
(1) K. Tanaka, Y. Sumiyoshi, Y. Ohshima, Y. Endo, and K. Kawaguchi, 1997, J. Chem. Phys. 107, 2728.
The parameters used in the present fit differ slightly from those in (1).
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: the ¹H hyperfine splitting may be resolvable at lower frequencies and in cooler sources. Therefore, a calculation with hyperfine structure is available with N and K_a up to 10 and 2, respectively.
The main entry has been simplified to include only 4 quantum numbers: N, K_a, K_c, and J + 1/2. Since only transition with N up to 2 have been observed the predictions have been limited to N and K_a = 30 and 6, respectively. But even for these the predicted uncertainties should be viewed with great caution !
The predictions with hyperfine splitting have six quantum numbers. In this case, the fourth quantum number is 1 for transitions with hyperfine splitting. The fifth one is an aggregate spin number. It can be decoded with part of the hfs.out file. The sixth quantum number, F, designates the total spin.
The dipole moment is from an ab initio calculation by
(2) P. Botschwina, R. Oswald, J. Flügge, and M. Horn, 1995, Z. Phys. Chem. 188, 29.
At low temperatures, it may be necessary to discern between ortho-H₂C₃H and para-H₂C₃H. 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-H₂C₃H than there are for para-H₂C₃H. Thus, for transitions with unresolved ¹H hyperfine splitting the nuclear spin-weight ratio is 6 : 2 between ortho-H₂C₃H and para-H₂C₃H. 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 9.9103 cm^–1 above ground.