Far-Infrared and Raman Spectra of the Ring-Puckering Vibration of 2,3-Dihydrothiophene. One- and Two-Dimensional Potential Energy Surfaces and the Barrier to Planarity

The vapor-phase far-infrared and Raman spectra of 2,3-dihydrothiophene have been recorded and analyzed. The infrared spectra show more than fifty transition frequencies corresponding to Δ v P = 1, 2, 3, 4, and 5 transitions. The ring-puckering energy levels were determined for both the ring-twisting ground and excited states. Both one- and two-dimensional potential energy functions, which fit the observed data very well, were determined. The barrier to planarity was determined to be 430 cm -1 from the one-dimensional model and 435 cm -1 for the two-dimensional model. The experimental dihedral angle of puckering is 31°, while an ab initio calculation predicts 29°. The magnitude of the interaction constant between the puckering and twisting was found to be 1.67 × 10 5 cm -1 /Å 4, similar to the values determined for related molecules.