Far-infrared spectra and two-dimensional potential energy surface for the ring-bending and ring-twisting vibrations of 5,6-dihydro-4H-thiopyran

Abstract

5,6-Dihydro-4H-thiopyran has been synthesized and its far-infrared spectrum has been recorded. Eleven ring-bending bands originating at 120.7 cm−1 and four ring-twisting bands originating at 274.5 cm−1 were observed. Twelve sum and difference bands in the 383–397 and 148–166 cm−1 regions were also observed and these facilitated the construction of a detailed energy map including numerous excited vibrational states of the two coupled vibrations. The two-dimensional potential energy surface, which satisfactorily fits the observed data, was determined to be V=9.48 ×104x4−4.13×104x2+1.37×104τ4−1.82×104τ2+1.10 ×105x2τ2, where x and τ are the bending and twisting coordinates, respectively. The minima on the potential energy surface correspond to twisting angles of ±48° (half-chair conformation). The lowest energy bent (boat) conformation corresponds to a saddle point 1500 cm−1 above the twisted conformation on the potential energy surfaces, and the barrier to planarity was estimated to be 6000 cm−1. Both of these values have large uncertainties since the vibrational data only extend to 800 cm−1 above the potential surface minimum. The relatively low bending energy and high barrier to planarity can both be explained by the low force constant for the C–S–C angle bending.