The Perfluoro Cage Effect: A Search for Electron-Encapsulating Molecules

Quantum chemical calculations have for some time predicted that perfluorinated polyhedral organic molecules should exhibit a low-energy LUMO consisting of the overlapping inward-pointing lobes of the C-F sigma* orbitals. Accordingly, these molecules should be able to encapsulate an electron within the interior of their cavities. Inspired by the recent confirmation of this prediction for perfluorocubane, we have sought to identify additional perfluorinated cage molecules capable of this remarkable behavior, which we refer to as the perfluoro cage effect (PCE). Using DFT calculations with multiple well-tested exchange-correlation functionals and large STO-QZ4P basis sets, we have identified several systems including [n]prismanes (n = 3-6), [n]asteranes (n = 3-5), twistane, and two norbornadiene dimer cages that clearly exhibit the PCE. In other words, they exhibit a low-energy LUMO belonging to the total symmetric irreducible representation of the point group in question and adiabatic electron affinities ranging from somewhat under 1 eV to over 2 eV. A pronounced size effect appears to hold, with larger cages exhibiting higher electron affinities (EAs). The largest adiabatic EAs, well over 3 eV, are predicted for perfluorinated dodecahedrane and C-60. In contrast, the PCE is barely discernible for perfluorinated tetrahedrane and bicyclo[1.1.1]pentane.

Automated summary

Quantum chemical calculations have predicted the existence of a low-energy LUMO in perfluorinated polyhedral organic molecules, allowing them to encapsulate an electron. Inspired by the confirmation of this prediction for perfluorocubane, the authors identified other perfluorinated cage molecules that exhibit the same behavior, known as the perfluoro cage effect (PCE). The size of the cage molecule affects its electron affinity, with larger cages exhibiting higher values. The PCE is more prominent in larger cages like perfluorinated dodecahedrane and C-60, while it is barely discernible in smaller cages like perfluorinated tetrahedrane and bicyclo[1.1.1]pentane.