Theoretical study of the low-lying excited states of cyclopropane and annelated derivatives Original Research Article

Extensive ab initio symmetry-adapted-cluster configuration-interaction (SAC-CI) calculations have been performed for the electronic spectra of cyclopropane, bicyclobutane, [1.1.1]propellane, spiropentane, dispiroheptane, and trispirononane. The theoretical results for the singlet and triplet Rydberg and valence transitions are in reasonable agreement with spectroscopic data for cyclopropane, bicyclobutane and [1.1.1]propellane. The spectroscopic assignments provided by the SAC-CI approach, however, differ in many respects from those obtained by previous ab initio calculations. The present results support the empirical speculations by Robin and Schafer et al. as to the location of the lowest valence transitions of cyclopropane, bicyclobutane and [1.1.1]propellane. Also, the assignment of the feature at 4.7 eV of [1.1.1]propellane with the lowest valence state 3A″2 is confirmed. The theoretical predictions for the spiro compounds are, instead, in sharp disagreement with the scarce experimental data. According to theory, spiropentane and its congeners do not absorb below 6.5 eV.