Spectra and Structure of Small Ring Compounds. XXIV. Microwave Spectrum of 2,3-Dihydrofuran

The microwave spectrum of 2,3‐dihydrofuran has been recorded from 12.4 to 40.0 GHz. Only A‐type transitions were measured. Both Q‐branch and R‐branch assignments have been made for the ground and first five vibrationally excited states of the ring‐puckering mode. From a consideration of the rotational constants for the ground and excited states and the comparative intensities of lines in the vibrationally excited states of the ring puckering mode with similarly assigned lines in the ground state, it is concluded that the potential function for this out‐of‐plane skeletal bending mode has a double minimum with a low barrier (around 83 cm⁻¹). In the ground states the expectation value for the dihedral angle is consistent with the 19° previously reported. The total dipole moment was determined to be $\text{1.32± 0.02\hspace{0.17em}}\mathrm{D}$ for the ground state and the following components were obtained: ${\mu }_a{\text{=1.30± 0.02,\hspace{0.17em}μ}}_b{\text{=0.25± 0.02,\hspace{0.17em}μ}}_c\text{= 0.00± 0.01\hspace{0.17em}}\mathrm{D}$ . The dipole moments for the first three excited states of the ring‐puckering mode was also determined and only the μ_b component was outside the experimental error of that found for the ground state.