Knowledge Bank

Spectra of gaseous $CH3CH3$ and $CD3CD3$ have been recorded using a modified Bomem spectrometer. For $CD3CD3$, the region of the torsional fundamental (208 $cm-1$) was measured at a resolution of $0.016cm-1$ and an absorption path length of 20 m. The gas temperature and pressure were 168 K and 80 Torr, respectively. In addition, the $\nu 9$ fundamental (650 $cm-1$) was studied at very high resolution ($0.0014cm-1$) with a path length of 2 m at a temperature of 133 K and a pressure of 0.6 Torr. The ground state torsion-rotation parameters were obtained from a combination of (I) a frequency analysis of 442 lower state combination differences in $\nu 9$ band and its associated hot band $\nu 9+\nu 4-\nu 4$ with (II) a frequency/intensity analysis of the P, Q, and R branches of the torsional fundamental and the Q branch of the first torsional overtone. The torsional dipole moments $\mu 1$ and $\mu |-\mu \perp$ were determined to be 7.41(74) and -19.9(2.0) $\mu $D, respectively. Similar measurements and analyses were carried out for $CH3CH3$. The torsional dipole moments are: $\mu \perp =9.27(93)$ and $\mu |-\mu \perp =-30.81(3)$ $\mu $D. The experimental values of $\mu \perp$ and $\mu |-\mu \perp$ are shown to be in excellent agreement with values calculated from theoretical expressions. The shape of the torsional bands can be reproduced in detail with the model used.