Optical and photoelectron spectra of small rings. III. The saturated three-membered rings

The electronic states of cyclopropane, ethylene oxide, ethylenimine, and diaziridine were investigated using gas-phase and condensed-phase vacuum ultraviolet spectroscopy, photoelectron spectroscopy, and Gaussian orbital self-consistent field calculations. Correlation of the gas-phase and condensed-phase optical spectra of cyclopropane with the first band in its photoelectron spectrum confirms the presence of Rydberg transitions in the optical spectrum involving excitation of an electron from the 3e' sigma level. The valence shell spectrum of cyclopropane is complex but is dominated by two very strong ¹A₁'→¹E', σ→σ ^* excitations. The Rydberg spectrum of ethylene oxide is reassigned to include the two absorptions in the 55 000-65 000-cm^-1 region, all bands originating from the 2b₂π orbital, while its valence shell spectrum is closely related to that of cyclopropane. Similar but less conclusive results were obtained for ethylenimine. Virtual orbital calculations of σ→σ^* excitation energies gave results uniformly too high by about 3 eV, but calculated dipole velocity oscillator strengths seem to agree closely with experiment. Comparison of photoelectron spectra and the Koopmans' theorem ionization potentials of the first six levels of the three-membered rings showed that the calculated values are too high by 8% and that reorganization and correlation energy corrections improve the agreement markedly.