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 tran sition frequencies corresponding to Delta v(P) = 1, 2, 3, 4, and 5 transiti ons. The ring-puckering energy levels were determined for both the ring-twi sting ground and excited states. Both one- and two-dimensional potential en ergy functions, which fit the observed data very well, were determined. The barrier to planarity was determined to be 430 cm(-1) from the one-dimensio nal model and 435 cm(-1) for the two-dimensional model. The experimental di hedral angle of puckering is 31 degrees, while an ab initio calculation pre dicts 290. The magnitude of the interaction constant between the puckering and twisting was found to be 1.67 x 10(5) cm(-1)/Angstrom(4), similar to th e values determined for related molecules.