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The ‘Inverted Bond’ revisited. Analysis of ‘in silico’ models and of [1.1.1]Propellane using Orbital Forces

preprint
revised on 28.10.2019, 11:03 and posted on 29.10.2019, 20:54 by Rubén Laplaza, Julia Contreras-García, Franck Fuster, François Volatron, Patrick Chaquin
This article dwells on the nature of “inverted bonds”, which make reference to the σ interaction between two s-p hybrids by their smaller lobes, and their presence in [1.1.1]propellane 1. Firstly we study H 3 C-C models of C-C bonds with frozen HCC angles reproducing the constraints of various degrees of “inversion”. Secondly, the molecular orbital (MO) properties of [1.1.1]propellane 1 and [1.1.1]bicyclopentane 2 are analyzed with the help of orbital forces as a criterion of bonding/antibonding character and as a basis to evaluate bond energies. Triplet and cationic state of 1 species are also considered to confirm the bonding/antibonding character of MOs in the parent molecule. These approaches show an essentially non-bonding character of the σ central CC interaction in propellane. Within MO theory, this bonding is thus only due to π-type MOs (also called ‘banana’ MOs or ‘bridge’ MOs) and its total energy is evaluated to ca. 50 kcal/mol. In bicyclopentane 2, despite a strong σ-type repulsion, a weak bonding (15-20 kcal/mol) exists between both central CC, also due to π-type interactions, though no bond is present in the Lewis structure. Overall, the so-called ‘inverted’ bond, as resulting from a σ overlap of the two s-p hybrids by their smaller lobes, appears highly questionable.

History

Email Address of Submitting Author

rlaplaza@lct.jussieu.fr

Institution

Sorbonne Université

Country

France

ORCID For Submitting Author

0000-0001-6315-4398

Declaration of Conflict of Interest

No conflicts to declare.

Version Notes

Version 3.0.

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