Spectroscopic Evidence for Pseudorotation. II. The Far-Infrared Spectra of Tetrahydrofuran and 1,3-Dioxolane

The absorption spectra of tetrahydrofuran and of 1,3-dioxolane at about 0.3-cm−1 resolution are reported in the region of 20–370 cm−1. It is found that dioxolane pseudorotates, and the earlier evidence for the pseudorotation of tetrahydrofuran is confirmed. Both compounds have barriers to pseudorotation of about 50 cm−1. Pseudorotation constants in both the ground and in the first excited radial states are obtained. The spectra show the effects of a considerable number of complex rotation–vibration interactions, which can be understood qualitatively by comparison with the spectra of four-membered-ring compounds. The band shapes are explained semiquantitatively using a number of Coriolis terms in the effective Hamiltonian.