The one-electron carbon-carbon bond: Hexafluoroethane and ethane radical cations.

In the previous post, I looked[1] at the recently reported[2] hexa-arylethane containing a carbon-carbon one-electron bond, its structure having been determined by x-ray diffraction (XRD). The measured C-C bond length was ~2.9aÅ and my conclusion was that the C…C region represented more of a weak “interaction” than of a bond as such. How about a much simpler system, hexafluoroethane? Here, the two-electron C-F bonds are much lower in energy than the C-C bond, so when the molecule is ionised, it escapes from the C-C bond rather than any of the C-F bonds. The below is the structure computed at the ωB97XD/Def2-TZVPP level, revealing a much shorter C-C bond of 2.149Å. The computed C-C stretching vibrational frequency is 179 cm-1.

An electron density difference map, obtained by subtracting the computed density of the dication from that of the radical cation at the geometry of the former is shown below, confirming that the electron has been removed from the C-C region, with a smaller removal from the C-F bonds.

The Laplacian of the electron density is shown below contoured for negative values of this function. Unlike the previous molecule, this now has a (small) negative value along the C-C region (contour -0.001).

A calculation of the NCI surface gave a null result! The parameters for computing a non-covalent analysis are thus: [0.5 1 0.0005 0.05 0.95 1.00], being the ones used in the previous analysis. The value of 0.05 is the density cutoff used to remove covalent density and using this value, no non-covalent features are detected. Or, put another way, only covalent features are present, as supported by the -ve Laplacian noted above.

Whilst C2F6+. cannot be claimed to be typical of a molecule with a hypothetical “pure” one-electron C-C bond, it is certainly very different from the previous example.[1],[2] Time to go all the way and try ethane itself, C2H6+.. Again the same behavour is seen, whilst the calculated C-C length reduces to 1.933Å. The C-C stretching vibrational frequency is elevated to 477 cm-1. We might take these last values as the natural ones for a one-electron C-C bond?

This alternative subtraction involves the density difference between neutral ethane and its radical cation. The result is essentially the same.

So these two ethane derivatives add some further context to the properties of a one-electron C-C bond. We have seen them range from a low of ~1.9Å to a high of ~2.9Å This variation of around 1Å as a function of the substituents on the two carbons must be the largest ever seen for any kind of bond!

References

  1. H. Rzepa, "A carbon-carbon one-electron bond! Or a weak carbon-carbon interaction?", 2024. http://dx.doi.org/10.59350/xp5a3-zsa24
  2. G.N. Lewis, "THE ATOM AND THE MOLECULE.", Journal of the American Chemical Society, vol. 38, pp. 762-785, 1916. http://dx.doi.org/10.1021/ja02261a002

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