FAR-INFRARED SPECTRA AND 2-DIMENSIONAL POTENTIAL-ENERGY SURFACE FOR THE RING-BENDING AND RING-TWISTING VIBRATIONS OF 5,6-DIHYDRO-4H-THIOPYRAN

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 cm1 were observed. Twelve sum and difference bands in the 383-397 and 148-166 cm1 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 × 104τ 2 + 1.10 × 105 x2τ2 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. © 1995 American Institute of Physics.