Vibrational spectra and potential energy surface for the ring bending and ring twisting of 5,6-dihydro-2H-thiopyran

Abstract

5,6-Dihydro-2H-thiopyran, CH2CH2CH=CHCH2S, has been synthesized and its far-infrared and Raman spectra recorded. Two series of sharp bands were observed originating from 139 and 235 cm−1 in the infrared spectrum for the out-of-plane ring-bending and the ring-twisting vibrations, respectively. A detailed energy level diagram including numerous excited states was determined for the two coupled vibrations. The two-dimensional potential energy surface, which satisfactorily fits more than two dozen observed transitions, was calculated to be V=2.431×104 x41 −0.383×104x21 +2.258×104x42 −1.966×104 x22 +1.026×105x21 x22 , where x1 is the ring-bending coordinate and x2 is the ring-twisting coordinate. The minimum energy on the potential surface corresponds to a twisting angle of 37.8° (the half-chair conformation). The lowest energy bent (boat) conformation corresponds to a saddle point 4130 cm−1 above the twisted conformation on the potential energy surface. The results are compared to analogous molecules and to molecular mechanics calculations.