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Two-Electron Multicenter Bonding (‘Pancake Bonding’) in Dimers of 5,6-Dichloro-2,3-dicyanosemiquinone (DDQ) Radical Anions

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journal contribution
posted on 23.07.2020, 17:43 by Valentina Milašinović, Anna Krawczuk, Krešimir Molčanov, Biserka Kojić-Prodić
An X-ray charge density study of 4-cyano-N-methylpyridinium salt of 5,6-dichloro-2,3-dicyanosemiquinone radical anion ([4-CN-N-MePy]+[DDQ]) revealed fine details of two-electron multicenter (2e/mc) covalent bonding within stacked dimers of DDQ anion radicals with a short separation distance of 2.87 Å. Since the electron pair is not localized but rather spread over a large area between two semiquinoid rings (28 atoms), the maximum electron density is 0.085 e Å–3; the electron density value is consistent with the existence of a nonlocalized electron pair. However, the presence of multiple bond (3,–1) critical points and a local electron density minimum (which is usually found in cage-like moieties, such as adamantyl) between the rings point to a 3D shape of frontier orbitals, consistent with the presence of a 2e/mc bond.