Far‐infrared spectra and two‐dimensional potential energy surface for the ring‐bending and ring‐twisting vibrations of 5,6‐dihydro‐4H‐thiopyran
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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 × 10 4 x 4 -4.13 × 10 4 x 2 + 1.37 × 10 4 τ 4 -1.82 × 10 4 τ 2 + 1.10 × 10 5 x 2 τ 2 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.
author list (cited authors)
Choo, J., Meinander, N. T., Villarreal, J. R., & Laane, J.