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remove from the proper or usual locality

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/05/11 09:01:48」(JST)

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Benzene, with the delocalization of the electrons indicated by the circle.

In chemistry, delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or one covalent bond. ^[1] Delocalized electrons are contained within an orbital that extends over several adjacent atoms. Classically, delocalized electrons can be found in conjugated systems and mesoionic compounds. It is increasingly appreciated that electrons in sigma bonding levels are also delocalized. For example, in methane, the bonding electrons are shared by all five atoms equally. Pervasive existence of delocalization is implicit in molecular orbital theory.

Examples [edit]

In the simple aromatic ring of benzene the delocalization of six π 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 of this delocalization. In valence bond theory, delocalization in benzene is represented by resonance structures.

Delocalized electrons also exist in the structure of solid metals, where the d-subshell interferes with the above s-subshell. Metallic structure consists of aligned positive ions (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 as conductivity.

In diamond all four outer electrons of each carbon atom are 'localized' between the atoms in covalent bonding. The movement of electrons is restricted and diamond does not conduct an electric current. In graphite, 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 at right angles to the plane.

In the case of ions it is common to speak about delocalized charge (charge delocalization) when meaning delocalized electrons. An example of delocalized electrons (delocalized charge) in ions can be found in the carboxylate group, wherein the negative charge is centered equally on the two oxygen atoms. Charge delocalization in anions is an important factor determining their reactivity (generally: the higher the extent of delocalization the lower the reactivity) and, specifically, the acid strength of their conjugate acids. As a general rule, the better delocalized is the charge in an anion the stronger is its conjugate acid. For example, the negative charge in perchlorate anion (ClO₄^-) is evenly distributed among the symmetrically oriented oxygen atoms (and a part of it is also kept by the central chlorine atom). This excellent charge delocalization combined with the high number of oxygen atoms (four) and high electronegativity of the central chlorine atom leads to perchloric acid being one of the strongest known acids (with pK_a values cited in the range of -7 .. -10). The extent of charge delocalization in an anion can be quantitatively expressed via the WAPS parameter.^[2]

Delocalization in reactions [edit]

Delocalized electrons are important for several reasons. One, an expected chemical reaction may not occur because the electrons delocalize to a more stable configuration, resulting in a reaction that happens at a different location. An example attempting the Friedel–Crafts alkylation of benzene with 1-chloro-2-methylpropane; the carbocation rearranges to a tert-butyl group stabilized by hyperconjugation, a particular form of delocalization. Delocalization leads to lengthening of wavelength of electron therefore decreases the energy.

References [edit]

^ IUPAC Gold Book delocalization
^ Kaupmees K., Kaljurand I., Leito I. (2010). "Influence of Water Content on the Acidities in Acetonitrile. Quantifying Charge Delocalization in Anions". J. Phys. Chem. A 114: 11788–11793. doi:10.1021/jp105670t.

English Journal

Molecular structure, vibrational spectroscopy, NBO and HOMO, LUMO studies of o-methoxybenzonitrile.

Elanthiraiyan M1, Jayasudha B2, Arivazhagan M3.
Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy.Spectrochim Acta A Mol Biomol Spectrosc.2015 Jan 5;134:543-52. doi: 10.1016/j.saa.2014.04.103. Epub 2014 Jun 16.
In the present study, the FT-IR and FT-Raman spectra of o-methoxybenzonitrile (O-MBN) have been recorded in the region 4000-400cm(-1) and 3500-50cm(-1), respectively. The fundamental modes of vibrational frequencies of O-MBN are assigned. Theoretical information on the optimized geometry, harmonic v
PMID 25058575

Vibrational spectroscopic (FTIR and FT-Raman), first-order hyperpolarizablity, HOMO, LUMO, NBO, Mulliken charge analyses of 2-ethylimidazole based on Hartree-Fock and DFT calculations.

Arivazhagan M1, Manivel S2, Jeyavijayan S3, Meenakshi R4.
Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy.Spectrochim Acta A Mol Biomol Spectrosc.2015 Jan 5;134:493-501. doi: 10.1016/j.saa.2014.06.108. Epub 2014 Jun 25.
The FTIR and FT-Raman spectra of 2-ethylimidazole (2EIDZ) have been recorded in the region 4000-400cm(-1) and 3500-50cm(-1), respectively. Utilizing the observed FTIR and FT-Raman data, a complete vibrational assignment and analysis of the fundamental modes of the compound were carried out. The opti
PMID 25048284

Vibrational spectra, molecular structure, natural bond orbital, first order hyperpolarizability, thermodynamic analysis and normal coordinate analysis of Salicylaldehyde p-methylphenylthiosemicarbazone by density functional method.

Porchelvi EE1, Muthu S2.
Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy.Spectrochim Acta A Mol Biomol Spectrosc.2015 Jan 5;134:453-64. doi: 10.1016/j.saa.2014.06.018. Epub 2014 Jun 20.
The thiosemicarbazone compound, Salicylaldehyde p-methylphenylthiosemicarbazone (abbreviated as SMPTSC) was synthesized and characterized by FTIR, FT-Raman and UV. Density functional (DFT) calculations have been carried out for the title compound by performing DFT level of theory using B3LYP/6-31++G
PMID 25033238

Japanese Journal

Magnetic and Electronic Properties of URu2Si2Revealed by Comparison with Nonmagnetic References ThRu2Si2and LaRu2Si2

Emi Naoya,Hamabata Ryosuke,Nakayama Daisuke,Miki Toshihiro,Koyama Takehide,Ueda Koichi,Mito Takeshi,Kohori Yoh,Matsumoto Yuji,Haga Yoshinori,Yamamoto Etsuji,Fisk Zachary,Tsujii Naohito
Journal of the Physical Society of Japan 84(6), 2015-04-28
NAID 160000000724

Photoelectron Spectroscopic Study on Decay Processes of Core-Excited States of NaNO2

Kamada Masao,Azuma Junpei,Ueda Yuki,Yamamoto Isamu,Imamura Masaki,Takahashi Kazutoshi
Journal of the Physical Society of Japan 84(5), 2015-04-22
NAID 160000000714