Millimeter Wave Rotational Spectrum of HSSH and DSSD. IV. P, Q, and R Branches of HSSH

The earlier microwave measurements on the ground‐state rotational spectrum of HSSH have been extended into the submillimeter wave region to a wavelength of 0.7 mm, or 420 GHz, for the observation of the Q branch of the $\text{K=2← 1}$ rotational subband. In the lower‐frequency range 50–200 GHz, various Q‐, R‐, and P‐branch transitions have also been assigned. From these measurements the ground‐state rotational constants are (in megahertz): $\text{A=146\hspace{0.17em}858.170± 0.032, B=6\hspace{0.17em}970.430± 0.003, C=6\hspace{0.17em}967.689± 0.003}$ , which yield an asymmetry parameter $b_p{\text{=−0.98× 10}}^{\text{−5}}$ or $\text{κ =−0.999960}$ . The most remarkable feature of the perpendicular‐type spectrum is the anomalous splitting of the K=2 energy levels. It is found that the splitting of the K=2 levels is not only one order of magnitude larger than that expected from the small asymmetry of the molecule, but also that the sequence of these levels is reversed as compared to their ``normal'' order, determined by the asymmetry splitting. These anomalies are explained as in paper II of this series by the additional K‐type doubling (centrifugal‐distortion splitting) caused by the centrifugal‐distortion operator ${\mathrm{(Ĵ}}_{+}^4{\mathrm{+Ĵ}}_{-}^4)$ in the rotational Hamiltonian. Unlike DSSD, the centrifugal‐distortion splitting in HSSH is nearly masked by the larger asymmetry splitting.