How (not) to Cage an Electron: The Perfluoro Cage Effect as an Extrinsic Molecular Property

Abstract

The notion of a (molecular) electron cage was coined for the unique hosting capacity of perfluorocubane, suggesting the “encapsulation of an electron” within the highly symmetric molecular framework. Extensions to this occurrence of a perfluoro cage effect (PCE) have been found in a variety of different cage structures, showing a remarkable transferability within the chemical subspace of perfluorinated prismanes and fullerens. Classifications of this cage effect based on Sigma Stellation fail for extended carbon frameworks, whereas Baders Atoms-in-Molecules analysis of non-nuclear attractors is inconclusive for smaller systems. Our investigation of the electronic structure within the molecular cage of perfluorocubane does not support the hypothesis of a caged electron as the common cause of the unique electron affinity characteristic for the PCE. Instead, we observe a central confining nodal surface within the spin density for perfluorocubane, which also occurs for any other radical anion of the aforementioned chemical subspace considered here. The association of the spin density to the singly occupied molecular orbital of a corresponding restricted-open-shell Kohn–Sham reference system enables us to construct a quantitative single-particle model that captures the main tendencies of the vertical electron affinity on the symmetry and size of the system. The derivation is based on the so-called nodal variational principle and thereby clarifies the effectiveness of a confined particle model for the PCE without any reference to the encapsulation of an electron.