Knowledge Bank

The pure rotational spectra of $CH3CD3$ in the three lowest torsional states has been observed using a mm-wave spectrometer. A total 87 rotational frequencies were measured between 230 to 363 GHz for $J=7\leftarrow 6$ to $11\leftarrow 10$ in $\nu 6=0,1$, and 2, where $\nu 5$ is the torsional quantum number. For the lowest two torsional states, the spectra have the classic form expected for a symmetric top in the ground vibrational state. For $\nu 6-2$ and for a given $(J+1)\leftarrow J$, a markedly different splitting pattern is observed as a result of the $(K,\sigma )$ dependence of the effective rotational constant B, where $\sigma$ labels the torsional sublevels. In order to identify the individual features in the $(\nu 6=2)$ spectrum, an assignment procedure was developed which is based on the fact that the ratio of moment of inertia of the top about the molecular symmetry axis to that of the whole molecule about the same axis is to a very good approximation 1/3. The torsion-rotation Hamiltonian discussed earlier in connection with $CH3SiH3$ (N. Moazzen.Ahmadi et al. J. Mol. Spectrosc., 119, 299 (1986)) was used to analyse the rotational frequencies along with the molecular beam anticrossing data and the origin of the torsional fundamental. Several constants which characterize the J-dependence of the energy levels were determined. Effective values for the barrier height $V~3$ and the shape parameter $V~6$ associated with the first order correction in the Fourier expansion of the potential function were obtained. The effect of redundancies on the interpretation of the measurements will be discussed.