Breakdown of the reduction of the rovibrational Hamiltonian: The case of S18O2F2

The ground state rotational spectrum of the near-spherical top molecule S18O2F2 (sulfuryl fluoride) has been measured from 50 to 700 GHz. As for the parent isotopologue, S16O2F2 [K. Sarka, J. Demaison, L. Margulès, I. Merke, N. Heineking, H. Bürger, H. Ruland, J. Mol. Spectrosc. 200 (2000) 55-64], it was necessary to use a non-reduced Hamiltonian in order to obtain a satisfactory fit. It was possible to determine six quartic centrifugal distortion constants (instead of five for a standard asymmetric top) and five sextic constants (one of them not existing in the reduced Hamiltonian) could also be determined. This ground state level has also been analysed thanks to a tensorial formalism developed in Dijon. Only two tensorial sextic constants are fixed to zero, all others have been adjusted. Although S18O2F2 is less spherical than S16O2F2, the analysis was more difficult. It is partly due to the fact that S18O2F2 is oblate whereas S16O2F2 is prolate. The experimental quartic centrifugal distortion constants were found in good agreement with those calculated from the force field, confirming the correctness of the analysis.