Microwave Spectrum, Ring-Puckering Potential Function, Ring Structure, and Dipole Moment of Cyclobutanone

The microwave spectrum of cyclobutanone in the ground state and the first 10 excited states of the ring‐puckering vibration has been assigned. Most features of the observed variation of rotational constants with ring‐puckering state and of the far‐infrared spectrum are accounted for with the ring‐puckering potential function $${\mathrm{V(Z)\hspace{0.17em}=\hspace{0.17em}\nu }}_0[\frac{1}{2}{Z}^2{\text{\hspace{0.17em}+\hspace{0.17em}0.4918Z}}^4\text{\hspace{0.17em}+\hspace{0.17em}1.598}\exp {\text{(−1.0Z}}^2\mathrm{)],}$$ with ${\nu }_0\text{\hspace{0.17em}=\hspace{0.17em}29.85}{\mathrm{cm}}^{\text{−1}}$ . $Z$ is a dimensionless coordinate for the ring‐puckering motion. A barrier of 7.6 ± 2 cm⁻ exists at the planar ring conformation; with the $\text{υ\hspace{0.17em}=\hspace{0.17em}0}$ level lying 9.2 cm⁻¹ above the top of the barrier. A perturbation of the $\text{υ\hspace{0.17em}=\hspace{0.17em}8}$ state is discussed. The ground‐state rotational spectra of the three ¹³C monosubstituted species have also been assigned and the structure of the ring determined. With the carbonyl carbon labeled C₁ and other carbons numbered sequentially around the ring, the ring structural parameters are $$\mathrm{r(}{\mathrm{C}}_1{\mathrm{C}}_2\text{)\hspace{0.17em}=\hspace{0.17em}1.527\hspace{0.17em}+\hspace{0.17em}0.003 Å, r(}{\mathrm{C}}_2{\mathrm{C}}_3\text{)\hspace{0.17em}=\hspace{0.17em}1.556\hspace{0.17em}±\hspace{0.17em}0.001 Å,}$$ $$\angle {\mathrm{C}}_2{\mathrm{C}}_1{\mathrm{C}}_{\text{2′}}\text{\hspace{0.17em}=\hspace{0.17em}93.1\hspace{0.17em}±\hspace{0.17em}0.3°, ∠}{\mathrm{C}}_1{\mathrm{C}}_2{\mathrm{C}}_3\text{\hspace{0.17em}=\hspace{0.17em}88.0\hspace{0.17em}±\hspace{0.17em}0.3°,}$$ $$\angle {\mathrm{C}}_2{\mathrm{C}}_3{\mathrm{C}}_{\text{2′}}\text{\hspace{0.17em}=\hspace{0.17em}90.9\hspace{0.17em}±\hspace{0.17em}0.2°.}$$ Stark‐effect measurements have yielded a value of 2.89 ± 0.03 D for the dipole moment of the common isotopic species in the ground vibrational state.