Millimeter Wave Rotational Spectrum of HSSH and DSSD. II. Anomalous K Doubling Caused by Centrifugal Distortion in DSSD

Abstract

The microwave spectrum of DSSD in the ground vibrational state has been measured and assigned. Although the DSSD molecule in the ground state is extremely close to a symmetric top ${\mathrm{(b}}_p{\text{=−1.69× 10}}^{\text{−7}}\text{; κ=−0.99999934)}$ the observed spectrum could not be explained by the symmetric‐rotor centrifugal‐distortion formula, even if higher distortion terms such as P⁶ or P⁸ are included. The most remarkable anomaly in the observed spectra is the splitting of the K=2 energy levels, which is several orders of magnitude larger than that expected from the small asymmetry splitting. For $\text{J>10}$ the splitting of the K=2 levels is larger than that of the K=1 levels. These anomalies are shown to be due to the additional K‐type doubling caused by the centrifugal‐distortion operator ${\mathrm{(Ĵ}}_{+}^4{\mathrm{+Ĵ}}_{-}^4)$ in the rotational Hamiltonian. This effect, which is predicted to exist in both symmetric‐top as well as asymmetric‐top molecules, is normally masked in the latter ones by the much larger asymmetry splitting.