Spectroscopic Determination of the Ring-Twisting Potential Energy Function of 1,3-Cyclohexadiene and Comparison with Ab Initio Calculations

Abstract

The high-temperature vapor-phase Raman spectrum of 1,3-cyclohexadiene shows nine transitions resulting from the ν19 (A2) twisting mode (labeled according to C2v symmetry). Ab initio calculations predict barriers in the 1197−1593 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 infrared combination bands also verify the assignments and provide information on the vibrational coupling. The twisting angles were determined to be 9.1° and 30.1°. Vibrational frequencies calculated by ab initio methods generally give good agreement with all of the experimental values.