SELECTION RULES FOR FREELY ROTATING ETHANE-TYPE MOLECULES
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Date
1954
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Ohio State University
Abstract
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 $Topley^{1}$ originally assumed $D_{3}$ selection rules for freely rotating ethane. $Howard^{2}$ 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 $D_{3{h}}$. Howard had also pointed out, however, that a center of symmetry (i) could have been used as the additional element. This would lead to $D_{3{d}}$ selection rules rather than $D_{3{h}}$. 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 $D_{3{d}}$.
Description
$^{1}$E. Teller and B. Topley, J. Chem. Soc. 1935, 885 $^{2}$ J. B. Howard, J. Chem. Phys. 5:442 (1937)
Author Institution: Mellon Institute
Author Institution: Mellon Institute