Knowledge Bank

Examination of the infrared and Raman spectra of dimethyl acetylene and perfluorodimethyl acetylene (hexafluoro-2-butyne) suggested a re-examination of the selection rule arguments for freely rotating ethane-type molecules. Teller and $Topley1$ originally assumed $D3$ selection rules for freely rotating ethane. $Howard2$ later showed that the degenerate species is split by the higher symmetry of the potential field. Because he used a plane of symmetry ($\sigma h$) as the additional symmetry element, he called the new selection rules $D3h$. Howard had also pointed out, however, that a center of symmetry (i) could have been used as the additional element. This would lead to $D3d$ selection rules rather than $D3h$. Although these groups were equivalent for Howard’s work, they produce significant differences in the Raman spectrum and in infrared combination tones. The selection rules followed by a freely rotating ethane-type molecule are shown to be those of the point group $D3d$.