Ring Puckering in Five-Membered Rings. II. The Microwave Spectrum, Dipole Moment, and Barrier to Pseudorotation in Tetrahydrofuran

Abstract

The microwave spectrum of tetrahydrofuran has been studied. Nine complete rotational spectra have been observed. These arise from the ground and eight excited states. All of these states are less than 200 cm⁻¹ from the ground state. The rotational constants and dipole moments exhibit a strong nonlinear dependence on the quantum number of the excited state. Vibration–rotation interaction is strong and the spectra of the first four states deviate from that of rigid rotor spectra. These deviations permit the determination of two energy separations: ${\Delta }_{01}\text{\hspace{0.17em}=\hspace{0.17em}0.67}{\mathrm{cm}}^{\text{−1}}$ and ${\Delta }_{23}\text{\hspace{0.17em}=\hspace{0.17em}1.5}{\mathrm{cm}}^{\text{−1}}$ . All of the results are interpreted in terms of a model of restricted pseudorotation with a potential function of $\text{[30(1‐}\cos \text{2φ)\hspace{0.17em}/\hspace{0.17em}2]\hspace{0.17em}+\hspace{0.17em}[40(1‐}\cos \text{4φ)\hspace{0.17em}/\hspace{0.17em}2]}{\mathrm{cm}}^{\text{−1}}$ , where $\phi$ is the angle of pseudorotation. The dipole moment varies from 1.52 to 1.76 D depending upon the pseudorotation state. The details of this variation indicate that the twisted configuration is at lower energy than the bent configuration.