Inchemistry,delocalized electrons areelectrons in amolecule,ion or solidmetal that are not associated with a singleatom or acovalent bond.[1]
The termdelocalization is general and can have slightly different meanings in different fields:
In thesimple aromatic ring ofbenzene, thedelocalization of sixπ electrons over the C6 ring is often graphically indicated by a circle. The fact that the six C-C bonds are equidistant is one indication that the electrons are delocalized; if the structure were to have isolated double bonds alternating with discrete single bonds, the bond would likewise have alternating longer and shorter lengths. Invalence bond theory, delocalization in benzene is represented byresonance structures.
Delocalized electrons also exist in the structure of solid metals. Metallic structure consists of aligned positiveions (cations) in a "sea" of delocalized electrons. This means that the electrons are free to move throughout the structure, and gives rise to properties such asconductivity.
Indiamond all four outer electrons of eachcarbon atom are 'localized' between the atoms in covalent bonding. The movement of electrons is restricted and diamond does not conduct an electric current. Ingraphite, each carbon atom uses only 3 of its 4 outer energy level electrons in covalently bonding to three other carbon atoms in a plane. Each carbon atom contributes one electron to a delocalized system of electrons that is also a part of the chemical bonding. The delocalized electrons are free to move throughout the plane. For this reason, graphite conducts electricity along the planes of carbon atoms, but does not conduct in a direction atright angles to the plane.
Standardab initio quantum chemistry methods lead todelocalized orbitals that, in general, extend over an entire molecule and have the symmetry of the molecule.Localized orbitals may then be found aslinear combinations of the delocalized orbitals, given by an appropriateunitary transformation.
In the methane molecule, ab initio calculations show bonding character in four molecular orbitals, sharing the electrons uniformly among all five atoms. There are two orbital levels, a bonding molecular orbital formed from the 2s orbital on carbon and triply degenerate bonding molecular orbitals from each of the 2p orbitals on carbon. The localized sp3 orbitals corresponding to each individual bond invalence bond theory can be obtained from a linear combination of the four molecular orbitals.
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