Hyperfine interaction induced K-type doubling in the mmw spectrum of (HSSH)-S-33

The rotational spectrum of the disulfane isotopomer (HSSH)-S-33 has been recorded and assigned between 70 and 300 GHz. In the analysis, rotational constants and S-33 hyperfine parameters are derived, the latter accounting for nuclear electric quadrupole as well as nuclear spin - rotation hyperfine interactions. The quadrupole constants are in qualitative agreement with a published ab-initio calculation. The electric field gradient principal axes are considerably rotated against the principal inertial axes, giving rise to off-diagonal elements in the electric field gradient tenser. In combination with the very small inertial asymmetry of the molecule, this causes the K-doubling in the lowest rotational levels to be dominated by the hyperfine structure rather than the inertial asymmetry contributions. By diagonalization of the quadrupole tensor, an estimate for the dihedral angle (eta approximate to 94 degrees) can be obtained, in good agreement with a previous value.