Preferred Label : delocalization;
IUPAC definition : A quantum mechanical concept most usually applied in organic chemistry to describe
the π-bonding in a conjugated system. This bonding is not localized between two atoms:
instead, each link has a 'fractional double bond character' or bond order. 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 aromatic systems and in symmetrical molecular entities in which a lone
pair of electrons or a vacant p-orbital is conjugated with a double bond (e.g. carboxylate
ions, nitro compounds, enamines, the allyl cation). Delocalization in such species
may be represented by partial bonds or as resonance (here symbolized by a two-headed
arrow) between contributing structures. 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 radicals.;
Origin ID : D01583;
See also
A quantum mechanical concept most usually applied in organic chemistry to describe
the π-bonding in a conjugated system. This bonding is not localized between two atoms:
instead, each link has a 'fractional double bond character' or bond order. 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 aromatic systems and in symmetrical molecular entities in which a lone
pair of electrons or a vacant p-orbital is conjugated with a double bond (e.g. carboxylate
ions, nitro compounds, enamines, the allyl cation). Delocalization in such species
may be represented by partial bonds or as resonance (here symbolized by a two-headed
arrow) between contributing structures. 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 radicals.
