Microwave Spectrum and Ring-Bending Vibration of 3-Oxetanone

The microwave rotational spectrum of 3‐oxetanone, $$\begin{array}{ccccccc}\mathrm{O}& –& {\mathrm{CH}}_2& –& \mathrm{C}& =& \mathrm{O}\\ |& & & & |& & & ,\\ & –& {\mathrm{CH}}_2& –& & & \end{array}$$ has been observed and assigned for the ground vibrational state, the first five excited states of the ring‐bending vibrational mode, and the two carbon‐13 isotopically substituted species. The features of the microwave spectrum indicate that the molecule has a single‐minimum ring‐bending vibrational potential and consequently a planar equilibrium configuration. Approximate models for the ring‐bending vibrational mode were set up and the vibrational potential function interpreted in terms of contributions from ring‐angle deformations and torsional interactions. The ¹³C data were used to derive a partial substitution structure. With the carbonyl carbon being labeled C_β, the structural parameters determined in this way were $$\begin{array}{c}\mathrm{r(}{\mathrm{C}}_{\alpha }{\mathrm{C}}_{\beta }\text{)=1.522±0.002\hspace{0.17em}Å,}\\ \angle {\mathrm{C}}_{\alpha }{\mathrm{C}}_{\beta }{\mathrm{C}}_{\mathrm{\alpha \prime }}\text{=88.06°±0.15°.}\end{array}$$ The moment equations for the hypothetically planar molecule were solved to give some indication of the remaining parameters. Stark effect measurements yielded the dipole moment as $\text{0.887± 0.005\hspace{0.17em}}\mathrm{D}$ for the common isotopic species in the ground vibrational state.