Far-infrared spectra and hindered pseudorotation of 1,3-oxathiolane

The far-infrared spectra arising from the two out-of-plane ring vibrations of 1,3-oxathiolane have been recorded and analyzed. Fifteen pseudorotational (ring-bending) transition frequencies were observed in the 25–125 cm−1 region and were fit with the one-dimensional potential function V(cm−1)=−541(1−cos 2φ)+139(1−cos 4φ). The molecule is twisted with a barrier to pseudorotation of 541±20 cm−1 (1.54±0.06 kcal/mol). The spectra recorded include transition frequencies occurring above the barrier, and this is the first molecule (other than a nearly free pseudorotor) for which such bands have been seen. Seven radial transitions arising from three different pseudorotational states have also been observed in the 297–317 cm−1 region. Those in the pseudorotational ground state can be fit with a one-dimensional double-minimum function with a barrier to planarity of 2720 cm−1. However, this value for the one-dimensional approximation is believed to be too high. Difference bands (180–240 cm−1), sum bands (398–426 cm−1), and double quantum transitions (144–220 cm−1) were also detected and used to confirm the assignments.