474161.molcanov.doc - Strong π···π interactions of...

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Strong π···π interactions of dianionic quinoid rings: alkali salts of 2,5-dihydroxybenzoquinone Krešimir Molčanov and Biserka Kojić-Prodić Rudjer Bošković Institute, P.O.B. 180, HR-10002, Zagreb, Croatia, e-mail: [email protected] A first study of π···π interactions between quinoid rings revealed that they stack unlike aromatic rings: they form face-to-face stacks without offset [1,2]. Centroid···centroid distances of about 3.30 Å [1,2] are much shorter than in aromatic stacks, 0.3 Å shorter than sum of van der Waals radii and are actually the shortest ever observed. The novel type of interaction may play an important role in supramolecular synthesis and crystal engineering. Due to a specific distribituon of electron density (quinoid rings are not aromatic and their π-electrons are not delocalised, Fig. 1a), repulsion of electron-rich π-systems is minimal, and is easily outperformed by σ-π attractions. All previously described quinoid
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