Far-Infrared Spectra and Potential Energy Surface for 5,6-Dihydro-4H-thiopyran

Abstract

The far infrared spectrum of 5,6-dihydro-4H-thiopyran shows eleven ring bending bands (near 120 cm−1), four ring-twisting bands (near 275 cm−1), and twelve sum and difference bands in the 383–397 cm−1 and 148–166 cm−1 regions. From these frequency data a detailed energy map was constructed for many of the excited vibrational states of the two coupled out-of-plane ring vibrations. A two-dimensional potential energy surface, which satisfactorily fits the observed data, was determined in terms of 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 correspnds 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.