Spectroscopic Determination of the Pseudorotation Barrier in Selenacyclopentane

The infrared and Raman spectra of selenacyclopentane have been studied. A series of far-infrared absorption peaks is interpreted to indicate that the five-membered ring molecule has a large barrier to pseudorotation. The derived potential function is V(φ) in cm−1 = − (1795.4 / 2)[1 − cos(2φ)] + (13.45 / 2) × [1 − cos(4φ)] − (86.19 / 2)[1 − cos(6φ)] with the pseudorotation constant B = 1.55 cm−1. Evidence that the molecule has a twisted (c2) configuration in its lowest few vibrational states has been obtained.