Tetrakis (methylidene)cyclobutane (‘[4]Radialene’): Electronic states of the molecular ion

The photoelectron spectrum of tetrakis (methylidene)cyclobutane ( 1 , ‘[4]radialene’) is reported. The electronic states of 1 + are assigned on the basis of model calculations and with reference to related systems. Jahn ‐ Teller activity in the degenerate states is discussed. A failure of the simple LCBO‐model for the π(e g )‐orbital of 1 is noted and traced to the fact that this orbital, though having a symmetry‐equivalent π*‐counterpart, does not interact with it. This feature is confined to [4n]radialenes; their total π‐energies are therefore higher than those of the other members. It is shown that radialenes, in principle, do not constitute a class analogous to that of the linear polyenes as inferred earlier.