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https://doi.org/10.1351/goldbook.D01583
A quantum mechanical concept most usually applied in organic chemistry to describe the π-bonding in a @[email protected] This bonding is not localized between two atoms: instead, each link has a 'fractional double bond character' or @[email protected] There is a corresponding 'delocalization energy', identifiable with the stabilization of the system compared with a hypothetical alternative in which formal (localized) single and double bonds are present. Some degree of delocalization is always present and can be estimated by quantum mechanical calculations. The effects are particularly evident in @[email protected] systems and in symmetrical @[email protected] in which a lone pair of electrons or a vacant p-orbital is conjugated with a double bond (e.g. carboxylate ions, @[email protected], @[email protected], the allyl @[email protected]). Delocalization in such species may be represented by partial bonds or as @[email protected] (here symbolized by a two-headed arrow) between @[email protected]
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These examples also illustrate the concomitant delocalization of charge in ionic conjugated systems. Analogously, delocalization of the spin of an unpaired electron occurs in conjugated @[email protected]