The electronic spectra of the unsaturated threehyphen;membered rings cyclopropene, 3,3hyphen;dimethylcyclopropene, and difluorodiazirine were recorded down to 85 000 cmminus;1and are compared with calculations of their optical spectra performed using Gaussianhyphen;type orbitals, with limited configuration interaction. Thepgr;thinsp;rarr;thinsp;pgr;*lpar;1A1thinsp;rarr;thinsp;1B1rpar;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 lowernthinsp;rarr;thinsp;pgr;*lpar;1A1thinsp;rarr;thinsp;1B2rpar;excitation energies and, in addition, predict a second, forbiddennthinsp;rarr;thinsp;pgr;*lpar;1A1thinsp;rarr;thinsp;1A2rpar;band about 3 eV higher than the lower, allowed one. This too has been observed experimentally. In the cyclopropenes, the analog of the lowernthinsp;rarr;thinsp;pgr;*bands of the diazirines is a valence shellsgr;thinsp;rarr;thinsp;pgr;*lpar;1A1thinsp;rarr;thinsp;1B2rpar;excitation, which is computed to lie well below eitherpgr;thinsp;rarr;thinsp;pgr;*orpgr;thinsp;rarr;thinsp;sgr;*excitations. A lowhyphen;lying valence shell excitation which is observed in cyclopropene and 3,3hyphen;dimethylcyclopropene is tentatively assigned as a Berryhyphen;typesgr;thinsp;rarr;thinsp;pgr;*promotion. Just as in the spectra of the saturated rings, the unsaturated rings also display intense externalsgr;thinsp;rarr;thinsp;externalsgr;*bands in the deep ultraviolet. Various groundhyphen;state properties were also calculated for the threehyphen;membered rings and compare favorably with experimental values where such are available. Adequate agreement was obtained with groundhyphen;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.
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