Spectroscopic determination of the ring-twisting potential energy functionof 1,3-cyclohexadiene and comparison with ab initio calculations

The high-temperature vapor-phase Raman spectrum of 1,3-cyclohexadiene shows nine transitions resulting from the v(19) (A(2)) twisting mode (labeled ac cording to C-2v symmetry). Ab initio calculations predict barriers in the 1 197-1593 cm(-1) range. Far-infrared absorption bands confirm five of these transitions. A one-dimensional potential energy function with a barrier of 1132 cm(-1) does an excellent job of fitting the data. Other Raman and infr ared combination bands also verify the assignments and provide information on the vibrational coupling. The twisting angles were determined to be 9.1 degrees and 30.1 degrees. Vibrational frequencies calculated by ab initio m ethods generally give good agreement with all of the experimental values.