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. 2023 Jan 25;8(5):4972–4975. doi: 10.1021/acsomega.2c07374

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

Abhik Ghosh †,*, Jeanet Conradie †,‡,*
PMCID: PMC9910065  PMID: 36777561

Abstract

graphic file with name ao2c07374_0003.jpg

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 σ* 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 C60. In contrast, the PCE is barely discernible for perfluorinated tetrahedrane and bicyclo[1.1.1]pentane.

Introduction

Polyfluorination and perfluorination typically affect organic molecules in a profound manner.1−3 One such influence is the so-called perfluoro effect, observed for planar conjugated molecules, in which the fluorines exert a much stronger stabilizing influence on the σ molecular orbitals than on the π molecular orbitals.4−7 Perfluorinated polyhedral organic molecules have been theoretically examined and a key prediction is a low-energy, totally symmetric LUMO derived from the overlapping inward-pointing lobes of the C–F σ* orbitals.8,9 The molecules thus exhibit a significant electron affinity (EA), accommodating the electron largely within central cavity of the polyhedra. This prediction has now been experimentally realized in the form of the perfluorocubane anion radical with Oh symmetry.10 Herein, we have used density functional theory (DFT) to explore both the generality and limitations of the electron-encapsulating effect across a wide range of organofluorine cages. The effect, hereafter referred to as the perfluoro cage effect (PCE), indeed appears to be general, with only a handful of exceptions. Several new examples of the PCE are predicted.

Results and Discussion

Sixteen perfluorinated polyhedral and/or cage molecules were examined with three well-tested11−15 exchange-correlation functionals, OLYP,16,17 B3LYP,18,19 and B3LYP*,20,21 augmented with D322 dispersion corrections, and large STO-QZ4P basis sets. Table 1 presents their calculated adiabatic EAs, while Figure 1 presents graphical representations of their LUMOs and the spin densities of their anion-radicals. In pretty much every case examined, the LUMO belongs to the totally symmetric irreducible representation of the point group in question. The vast majority of the molecules also exhibit a significant electron-encapsulating ability, as measured by the adiabatic EAs, which, by and large, are mutually consistent across the three functionals.

Table 1. Adiabatic Electron Affinities (eV) of the Compounds Studied.

      adiabatic EA
compound point group LUMO irrep OLYP OLYP (ZPE) B3LYP* B3LYP
Prismanes
Perfluoroprismane D3h a′1 0.97 1.04 0.87 0.80
Octafluorocubane Oh a1g 1.55 1.67 1.43 1.35
Perfluoropentaprismane D5h a′1 1.98 2.12 1.85 1.77
Perfluorohexaprismane D6h a1g 2.15 2.28 2.04 1.96
Asteranes
Perfluoro[3]asterane C3h a′ 1.36 1.54 1.18 1.08
Perfluoro[4]asterane D4h a1g 2.03 2.19 1.91 1.82
Perfluoro[5]asterane D5h a′1 2.43 2.59 2.39 2.33
Perfluoronorbornadiene dimers
D2d-Perfluoro-NBD D2d a1 2.02 2.18 1.90 1.81
D2h-Perfluoro-NBD D2h ag 2.05 2.26 1.96 1.87
Miscellaneous
Perfluorotetrahedrane Td a1 0.02 0.10 0.07 0.07
Perfluorobicyclo[1.1.1]pentane D3h a –0.03   –0.05 –0.13
Perfluorotwistane D2 a 0.73 0.99 0.59 0.48
Perfluoroadamantane Td a1 1.09 1.29 0.93 0.81
Fullerenes
Perfluorododecahedrane Ih a1g 3.44 3.54 3.52 3.46
Perfluoro-C60 Ih a1g 3.87   4.53 4.50
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Figure 1.

Figure 1

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Ball-and-stick structures, neutral LUMOs, anion spin densities of the perfluorinated cage molecules studied.

All the perfluorinated prismanes exhibit sizable electron affinities, with the following order across all three functionals (the values in eV shown within parentheses are for OLYP): hexaprismane23,24 (2.15) > pentaprismane25,26 (1.98) > cubane27−29 (1.55) > prismane30,31 (0.97). The ordering suggests a pronounced size effect on the PCE: larger fluorinated cages result in greater stabilization of the encapsulated electron.

The three asteranes32−36 examined also exhibit relatively large electron affinities, with that of perfluoro[5]asterane (OLYP: 2.43 eV) and perfluoro[4]asterane (2.03 eV) greatly exceeding that of perfluoro[3]asterane (1.36 eV). Again, there appears to be a pronounced size effect.

The two Td systems examined, tetrafluorotetrahedrane37,38 and perfluoroadamantane,39−43 exhibit dramatically different electron affinities. For the former, the value is near zero, suggesting that the relatively tiny tetrahedral cage cannot effectively encapsulate an electron. The same also holds for perfluorinated bicyclo[1.1.1]pentane.44,45 In contrast, perfluorinated twistane46 (OLYP: 0.73) and adamantane (1.09 eV) exhibit moderate, positive EAs.

Among the molecules examined here, the largest adiabatic EAs have been calculated for perfluorinated dodecahedrane, C20F20,47−49 and buckministerfullerene, C60F60,50−52 the OLYP values being 3.44 and 3.87 eV, respectively. In each case, the spin density of the anion radical has the shape of a spheroidal shell within the interior of the polyhedral skeletons.

Concluding Remarks

Given the large number of polyhedral or cage-shaped molecules that have been synthesized and the vastly greater number that are theoretically possible, a clear conclusion from the present study is that electron encapsulation by their perfluorinated counterparts should be widely prevalent – indeed more the rule than the exception – and limited only by the accessibility of the compounds in question. The only exceptions appear to be the smallest cages such as tetrahedrane and bicyclo[1.1.1]pentane.

Acknowledgments

This work was supported by grant no. 324139 of the Research Council of Norway (A.G.) and grant nos. 129270 and 132504 of South African National Research Foundation (J.C.).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c07374.

  • Optimized Cartesian coordinates (10 pages) (PDF)

The authors declare no competing financial interest.

Notes

The data underlying this study are available in the published article and its online Supplementary Material.

Supplementary Material

ao2c07374_si_001.pdf (258.6KB, pdf)

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Supplementary Materials

ao2c07374_si_001.pdf (258.6KB, pdf)

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