Vinyl isocyanate exists in the form of two conformers: trans and cis. The latter is higher in energy by approximately 301 cm^–1 (or 433 K). The experimental transition frequencies were summarized in
(1) K. Vávra, L. Kolesniková, A. Belloche, R. T. Garrod, J. Koucký, T. Uhlíková, K. Luková, J.-C. Guillemin, P. Kania, H. S. P. Müller, K. M. Menten, and Š. Urban, 2022, Astron. Astrophys. 666, Art. No. A50.
The analysis takes into account lower frequency data from
(2) C. Kirby and H. Kroto, 1978, J. Mol. Spectrosc. 70, 216.
The ground state rotational spectrum reveals strong perturbations due to interactions with low-lying excited vibrational states which allowed only transitions involving levels up to K_a = 6 to be analyzed in (1). Calculations of a-type transitions should be reliable up to J = 68 for K_a = 0 to 5 and up to J = 44 for K_a = 6. This corresponds basically to the data sets encompassed in (1) with interpolations. b-Type transitions were limited to K_a = 3 and J = 80 in (1). Given the large value of A, these calculations should be sufficient to identify the molecule in cold as well as in fairly warm interstellar sources. Calculations with K_a > 6 should only be viewed with great caution; some caution is also advised for transition frequencies beyond 330 GHz.
¹⁴N hyperfine splitting may be resolvable in cold sources at low quantum numbers, but were not considered here. Thus, the partition function does not take into account the spin multiplicity of the ¹⁴N nucleus !
The partition function was calculated as a sum over the rotational levels in the ground vibrational states of both conformers assuming they are in local thermodynamical equilibrium. Individual contributions as well as vibrational correction factors for a posteriori corrections are available.
The experimental dipole moment components are from (2).