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 × 10⁴x⁴₁ - 0.383 × 10⁴x ₁² + 2.258 × 10⁴x₂⁴ - 1.966 × 10⁴x₂² + 1.026 × 10 ⁵x₁²x₂², where x ₁ 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.