Optical Spectra of Small Rings. II. The Unsaturated Three-Membered Rings

The electronic spectra of the unsaturated three‐membered rings cyclopropene, 3,3‐dimethylcyclopropene, and difluorodiazirine were recorded down to 85 000 cm⁻¹ and are compared with calculations of their optical spectra performed using Gaussian‐type orbitals, with limited configuration interaction. The ${\mathrm{\pi \hspace{0.17em}\to \hspace{0.17em}\pi *(}}^1{A}_1{\mathrm{\hspace{0.17em}\to \hspace{0.17em}}}^1{B}_1)$ excitations are computed to fall in the region expected for olefins and azoalkanes, but with oscillator strengths only half their normal value, as observed. Calculations on diazirine and difluorodiazirine correctly predict the lower ${\mathrm{n\hspace{0.17em}\to \hspace{0.17em}\pi *(}}^1{A}_1{\mathrm{\hspace{0.17em}\to \hspace{0.17em}}}^1{B}_2)$ excitation energies and, in addition, predict a second, forbidden ${\mathrm{n\hspace{0.17em}\to \hspace{0.17em}\pi *(}}^1{A}_1{\mathrm{\hspace{0.17em}\to \hspace{0.17em}}}^1{A}_2)$ band about 3 eV higher than the lower, allowed one. This too has been observed experimentally. In the cyclopropenes, the analog of the lower $\mathrm{n\hspace{0.17em}\to \hspace{0.17em}\pi *}$ bands of the diazirines is a valence shell ${\mathrm{\sigma \hspace{0.17em}\to \hspace{0.17em}\pi *(}}^1{A}_1{\mathrm{\hspace{0.17em}\to \hspace{0.17em}}}^1{B}_2)$ excitation, which is computed to lie well below either $\mathrm{\pi \hspace{0.17em}\to \hspace{0.17em}\pi *}$ or $\mathrm{\pi \hspace{0.17em}\to \hspace{0.17em}\sigma *}$ excitations. A low‐lying valence shell excitation which is observed in cyclopropene and 3,3‐dimethylcyclopropene is tentatively assigned as a Berry‐type $\mathrm{\sigma \hspace{0.17em}\to \hspace{0.17em}\pi *}$ promotion. Just as in the spectra of the saturated rings, the unsaturated rings also display intense external $\mathrm{\sigma \hspace{0.17em}\to \hspace{0.17em}}\mathrm{external}\mathrm{\sigma *}$ bands in the deep ultraviolet. Various ground‐state properties were also calculated for the three‐membered rings and compare favorably with experimental values where such are available. Adequate agreement was obtained with ground‐state dipole, quadrupole, and second moments, but the agreement was poor for the components of the electric field gradient tensor at the N nuclei of diazirine. Ionization potentials and diamagnetic susceptibilities were also calculated.