Far Infrared Spectra of Ring Compounds. X. Hindered Pseudorotation in Six-Membered Rings: Estimation of the Barrier Height for Half-Chair Inversion in Dioxene and 2,3-Dihydropyran

High‐resolution far‐infrared spectra of 1,4‐dioxene (I) and 2,3‐dihydropyran (II) have been determined. The spectra are divided into three principal regions associated with the twisting mode, the bending mode, and difference tones between the two. Analysis of the data was made in terms of the two‐dimensional potential function, V(β, τ), of Ikeda and Lord [J. Chem. Phys. 56, 4450 (1972)]. The results for dioxene are: $\mathrm{V\hspace{0.17em}(\beta ,\hspace{0.17em}\tau )\hspace{0.17em}}{\mathrm{in\; cm}}^{\text{−1}}{\text{=−4580β}}^2{\text{−19059}}_{\tau }^2{\text{+835.5β}}^4{\text{+13280}}_{\tau }^4{\text{+15422β}}^2{\tau }^2$ , and for dihydropyran are: $\mathrm{V\hspace{0.17em}(\beta ,\hspace{0.17em}\tau )\hspace{0.17em}}{\mathrm{in\; cm}}^{\text{−1}}{\text{=−4447β}}^2{\text{−15526}}_{\tau }^2{\text{+851.4β}}^4{\text{+9783}}_{\tau }^4{\text{+12738β}}^2{\tau }^2$ . To the extent that these potential functions can be extrapolated reliably beyond the immediate neighborhood of the potential minimum, they imply that the most stable conformation is a twisted (half‐chair) form with the respective twist angles 48.54° (I) and 51.04° (I), and barriers to planarity of 6838 cm⁻¹ (I) and 6160 cm⁻¹ (II). A metastable bent form appears to exist at 561 (I) and 352 cm⁻¹ (II), respectively, above the energy minimum in the twisted conformation, with barriers to conversion from the twisted form of 2890 (I) and 2940 cm⁻¹ (II).