The ground state fundamental rotational transition with ¹⁴N and ¹³C hyperfine splitting was published in
(1) M. Krüger, W. Stahl, and H. Dreizler, 1993, J. Mol. Spectrosc. 158, 298.
Two additional lower J transition frequencies were taken from astronomical observations by
(2) J. Cernicharo, B. Tercero, C. Cabezas, M. Agúndez, E. Roueff, R. Fuentetaja, N. Marcelino, and P. de Vicente, 2024, Astron. Astrophys. 682, L13.
The D value was kept fixed to the value of the main isopolog scaled by the square of the ratio of the B values. The ¹⁴N-¹³C nuclear spin-nuclear spin coupling parameter was kept fixed to the value in (1). The nuclear spin-rotation parameters were kept fixed to the values from a quantum-chemical calculation by
(3) H. S. P. Müller, 2020, unpublished.
The calculation may be accurate enough throughout, but some caution is advised.
¹⁴N and possibly ¹³C hyperfine splitting matter in some astronomical observations. Therefore, a separate hyperfine calculation is provided for J' ≤ 5 below 50 GHz. NOTE: The partition function does take into account the spin multiplicity of the ¹⁴N and ¹³C nuclei !
The experimental dipole moment of HCCNC was published by
(4) M. Krüger, H. Dreizler, D. Preugschat, and D. Lentz, 1991, Angew. Chem. 103, 1674.