Knowledge Bank

The pure rotational spectrum of ethane is considered to be forbidden, since the point group of the molecule in its equilibrium position, $D3d$, contains a center of symmetry. However, in the non-rigid molecules, belonging to the symmetry group $G36+,$ torsion-vibration-rotation interaction induce an effective dipole moment of the form $\mu \omega -\mu \omega TJ\omega P\gamma (\omega =0,\pm 1)$ where $J\omega$ and $P\gamma$ are components of the rotational and tensional angular momentum, respectively, and $\mu \omega T$ are coupling constants that obey the relation $\mu 1T-\mu -1T+\mu 0T$. This effective dipole moment operator gives rise to pure rotational transitions in doubly degenerate tensional states, with the selection rules $\Delta J=0,\pm 1$ and $\Delta K=0$. The same operator is also responsible for the pure torsionAL torsion-rotation transitions discussed $previously1,2$. Expression for the integrated intensities of pure rotational transitions will be presented. Based on the estimate $\mu 1T+\mu 0T-10-4D$, derived from the observed torsion and fundamental $band1$, the line strengths for low J transitions in the microwave region were calculated to be of the order of $10-11cm-2atm-1$. Such lines can be observed in the laboratory with the state of the art microwave spectrometers. Estimated transition intensities in the infrared region are of the order of $10-8cm-2atm-1$ and are considered too weak to be observed in the laboratory with conventional far infrared instrumentation.