Excited Electronic States of Cyclopropane

The cyclopropane molecule is treated as a system of delocalized σ electrons with respect to the carbon skeleton by a method similar to the Pariser—Parr treatment of π-electronic states. The results predict an A2′ state as the lowest singlet excited state and two 1E′ states above it. Experimental findings are seemingly in agreement with these calculations, corroborating the application of the delocalization model at least qualitatively. A triplet state of A2′ orbital symmetry is predicted to lie about 0.5 eV below the lowest 1A2′ state. If the configuration interaction is much larger than these calculations indicate, the lowest excited singlet state would be of E′ symmetry and the absorption now characterized as 1A1′→1A2′ could be due to the lowest singlet→triplet transition.