Example Ar···SO2

The experimental lines were taken from the article "The rotational-tunneling spectrum of the Ar···SO2 van der Waals complex" by L. H. Coudert, K. Matsumura, and F. J. Lovas, published in J. Mol. Spectrosc. 147 (1991) 46–60.
The spectrum was fit with an internal axis method (IAM) ansatz. However, it is also possible to obtain a fit of identical quality using a reduced axis system (RAS) ansatz. The present fits follow the article "Weakly bound complexes: Structure and internal motion data obtained from rotational spectra" by Martin Schäfer, published in J. Mol. Struct. 599 (2001) 57–67.
Ar···SO2 has a double minimum potential, allowing tunneling. There is a symmetric (v = 0) and an antisymmetric tunneling substate (v = 1). Transitions of a-type symmetry occur within the tunneling substates, and c-type transitions occur between the substates.
Ar···SO2 has two identical 16O nuclei, which have a spin of 0. Thus half of the states are missing: only even and odd Ka states exist for v = 0 and v = 1, respectively.