Microwave Spectrum and Ring-Bending Vibration of 3-Methyleneoxetane

The microwave spectrum of 3‐methyleneoxetane, $$\begin{array}{ccccccc}\mathrm{O}& –& {\mathrm{CH}}_2& –& \mathrm{C}& =& {\mathrm{CH}}_2,\\ |& & & & |& & \\ |& –& {\mathrm{CH}}_2& –& |& & \end{array}$$ in the ground vibrational state and first four excited states of the ring‐bending vibrational mode has been observed and assigned. The features of the microwave spectrum and four previously observed far‐infrared vibrational transitions have been used to determine the ring‐bending potential function, $\mathrm{V(Z)\hspace{0.17em}=\hspace{0.17em}}\frac{1}{4}{\mathrm{;\nu }}_0{\mathrm{(Z}}^2{\text{\hspace{0.17em}+\hspace{0.17em}0.011Z}}^4)$ , where $Z$ is a dimensionless reduced coordinate describing the ring bending and ${\nu }_0\text{\hspace{0.17em}=\hspace{0.17em}108.60}{\mathrm{cm}}^{\text{−1}}$ . This potential function is nearly harmonic with a single minimum, implying a planar configuration for the equilibrium ring structure. Assuming curvilinear motion of the atoms, an approximate model for the vibration has been developed which reproduces reasonably well the observed rotational constant variations. Using this model, the potential function has been interpreted in terms of the energy contributions from the torsional interactions and ring‐angle deformations. The rotational constant data has been used to improve an electron diffraction structural determination using a least‐squares technique. The dipole moment for the ground state of 3‐methyleneoxetane has been measured as 1.63 ± 0.03 D.