Raman Spectra and the Potential Energy Function for the Internal Rotation of 1,3-Butadiene and its Isotopomers

The gas-phase Raman spectra of 1,3-butadiene-d$_{0}$, 2,3-d$_{2}$, 1,1,4,4-d$_{4}$, and --d$_{6}$ have been collected with CCD detection with numerous scans of ten hours or more. For each isotopomer eight Raman transitions in the 240-330 cm$^{-1}$ region corresponding to double quantum jumps of the A$_{u}$ internal rotation ($\nu _{13})$ were observed for the \textit{trans} conformer. Weaker bands in the 170-260 cm$^{-1}$ region were assigned to the \textit{gauche} conformation, which lies at higher electronic energy. A periodic potential function for the internal rotation, which fits the data for all the isotopomers, was determined. This function shows the \textit{gauche} form to be 966 cm$^{-1}$ higher in energy and the barrier between the \textit{trans} and \textit{gauche} structures to be 2055 cm$^{-1}$. The \textit{cis} structure has an energy 408 cm$^{-1}$ higher than the \textit{gauche}. Fourteen combination band or hot band series involving $\nu _{13}$ for the \textit{trans} conformer were also observed, and these allow the internal rotation levels in various excited vibrational states to be determined.