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

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 m...