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.
Data were taken from
(1) K. M. T. Yamada, M. Winnewisser, G. Winnewisser, L. B. Szalanski, and M. C. L. Gerry, 1980, J. Mol. Spectrosc. 79, 295.
The predictions should be quite good up to about 400 GHz.
The partition function takes into account higher K_a values.
The ¹⁴N hyperfine splitting is reasonably similar to that of HNCS.
The a-dipole moment component was assumed to agree with that of the main species.