Isothiocyanic acid is a pseudolinear molecule with a large A rotational parameter – analogously to isocyanic acid, HNCO. The Hamiltonian is very slowly converging, in particular for the purely K-dependent terms. Moreover, at higher values of K_a, there is extensive vibration rotation interaction bewteen the ground vibrational state of HNCS and its three lowest excited vibrational states. Because of this and because only transitions up to K_a = 1 have been observed in the interstellar medium, only levels up to K_a = 2 were considered in the present calculation.
The measurements were taken from
(1) K. M. T. Yamada, M. Winnewisser, G. Winnewisser, L. B. Szalanski, and M. C. L. Gerry, 1979, J. Mol. Spectrosc. 78, 189; from
(2) M. Rodler, S. Jans-Büli, M. Oldani, and A. Bauder, 1987, Chem. Phys. Lett. 142, 10; and from
(3) M. Niedenhoff, G. Winnewisser, K. M. T. Yamada, and S. P. Belov, 1995, J. Mol. Spectrosc. 169, 224.
The partition function takes into account higher K_a values.
The far-infrared transitions from (3) with uncertainties of about 2 MHz have not been merged.
The ¹⁴N hyperfine splitting is unlikely to be be resolved for b-type transitions in the ISM. The spin multiplicity g_I of 3 for the ¹⁴N nucleus is neither included in the partition function nor in the upper state degeneracy g_up !
The a-dipole moment component was mentioned in
(4) L. B. Szalanski, M. C. L. Gerry, G. Winnewisser, K. M. T. Yamada, and M. Winnewisser, 1978, Can. J. Phys. 56, 1297.
The determination of the b-dipole moment component of DNCS was reported in that article as 1.08 (15) D. The respective value of HNCS should be slightly larger on geometrical grounds. However, because of the large uncertainty of the DNCS value 1.0 D was used as the b-component of HNCS for simplicity reasons. Moreover, because of the pseudoliearity of the molecule, the HNCS value may be somewhat different from what would be calculated from the geometry of the molecule. In addition, the dipole moment components may change non-negligibly with K_a.
Since the origin of the K_a = 1 – 0 branch is near 1.3 THz (43 cm^–1), it may well be that the excitation of these transitions differs from that of the ΔK_a = 0 transitions. For this reason and because of the uncertainties in the b-component of the dipole moment, separate entries are given for the a-type transitions (ΔK_a = 0) and the b-type transitions (ΔK_a ≡ ΔK_a ≡ 1 mod 2); see also e059503.cat.