Cyclopropanes Rydberg transitions

Bicyclo[1.1.0]butane has been studied in some detail. " Again, all of the states appear to involve Rydberg transitions. The transition energies are at lower energies than those for cyclopropane, which may be a result of the strain in the bicyclobutane ring that further increases the ground-state energy. The spectrum of [l.l.ljpropellane has not as yet been subjected to a detailed analysis. 

The spectrum of cyclopropane is rather similar in that it has only Rydberg transitions. The spectrum of cyclobutane is also dominated by Rydberg transitions, and its spectrum resembles that of the larger cycloalkanes, and is quite different to that for cyclopropane.104 Cyclopropene and cyclobutene have also been examined, and here a n -> n transition is seen along with the Rydberg transitions.101... 

In the expansion 22 the expressions in parentheses represent zeroth-order excitations localized at the carbonyl or cyclopropane moieties, respectively. The term CT includes charge-transfer components and depends upon the extent of mesomeric interactions between the subunits. The sum in equation 22 is over all the HOMO - LUMO (3e, 4e ) transitions of cyclopropane (cf. Subsection III.A). Actually, of course, more excitations have to be included into expansion 22. In particular, expansion 22 does not include Rydberg excitations. 

The UV spectrum of cyclopropane is quite different from those of the alkanes we have examined so far. Whereas acyclic alkanes and cyclic alkanes other than cyclopropane and cyclobutane have only Rydberg bands in the lower frequency (and photo chemically important) part of the spectrum, for cyclopropane the lowest Rydberg bands intermingle with bands due to valence-shell transitions. Two structured bands at 63000 and 78000 cm (159 and 128 nm) have been assigned to the 3e a,3p) and (3e o,4p) transitions on the grounds of the similarity of their vibrational fine structure with that of the respective photoelectron bands and their term values. Two other bands, near 70000 cm (143 nm) and 83000 cm (120 nm)... 

As to the identity of the state in which most cyclopropane molecules photolyse the most likely candidate seems to be the 3p Rydberg state (or rather states since under Dsh symmetry 3p splits to 32" + e and (32" + e ) x e = E" + A/ + A2 + E transitions to E and A2 states being allowed). However, it is important to note that the 3s bands have not been so far identified. They ould be near 56 or 57000 cm with normal term values. They must be photochemically important even though they are very weak. 

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