Electronic Spectrum of Cyclobutanone

The electronic spectrum of cyclobutanone is examined to 78 100 cm⁻¹. Nine electronic transitions are found, of which five form an ns Rydberg series leading to an ionization potential of 75 444 cm⁻¹ (9.35₄ eV). The remaining four transitions are assigned in order of increasing energy as $\mathrm{\pi *\hspace{0.17em}\leftarrow \hspace{0.17em}n,\; \pi *\hspace{0.17em}\leftarrow \hspace{0.17em}n\prime ,\; \Sigma }\mathrm{co}\mathrm{*\hspace{0.17em}\leftarrow \hspace{0.17em}n}$ , and $\mathrm{\pi *\hspace{0.17em}\leftarrow \hspace{0.17em}\pi }$ . Earlier vibrational analyses of the first two of these are extended and corrected, and the excitation of the carbonyl stretch mode in the $\mathrm{\pi *\hspace{0.17em}\leftarrow \hspace{0.17em}n\prime }$ transition is demonstrated. Vibrational analyses are also given for the structure observed in the $\mathrm{\pi *\hspace{0.17em}\leftarrow \hspace{0.17em}\pi }$ and Rydberg transitions. An analysis of existing data on the electronic and photoelectron spectra of acetone is used to support the general assignment of the second electronic transition of ketones as $\mathrm{\pi *\hspace{0.17em}\leftarrow \hspace{0.17em}n\prime }$ . Second and third ionization potentials of cyclobutanone are predicted to be about 11.70 and 13.34 eV.