TORSIONAL FREQUENCIES IN THE FAR INFRARED THE FORM OF THE POTENTIAL CURVE FOR HINDERED INTERNAL ROTATION OF A METHYL GROUP$^{\ast}$

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1962

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Ohio State University

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Transitions between excited torsional levels $(0\rightarrow 1, 1\rightarrow 2, 2\rightarrow 3, \ldots)$ has been measured in the infrared for several compounds. The observed frequencies provide the best test of the potential function for hindered internal rotation which has yet been made. The constants of the usual expression. $$V (\alpha) = V_{3} (1-\cos 3\alpha)/2+V_{6} (1-\cos 6\alpha)/2$$ are: $$\begin{array}{ccc}Compound & V_{3} & V_{6}\\ CH_{3}CH_{2}Cl& 1310\pm10 cm^{-1} &10\pm0.5 cm^{-1}\\ CH_{3}CH = CH_{2} & 706 \pm 10 & -21 \pm 1\\ CH_{3}CF = CH_{2} & 806\pm 4 & 0\\ O\\ CH_{3} -CH-CH &895\pm 8 & -9\pm 0.05\\\end{array}$$ Thus $V_{6} < 3\%$ of $V_{3}$, and may be either positive or negative. Frequencies are also given for propylene sulfide, but lack of structural information prevents further interpretation. Pitzer and Hollenberg's earlier results for $CH_{3}CCl_{3}$ could not be confirmed. The spectra for all six compounds were examined from 100 to $435 cm^{-1}$, allowing some bending frequencies to be observed.

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$^{\ast}$ This work was supported by grant 16833 from the National Science Foundation.
Author Institution: Mellon Institute

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