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[[File:Non-Kekule-Structures.svg|frame|right|Examples of non-Kekulé (a) polyenes, (b) quinodimethanes, and (c) polynuclear aromatics]]

A '''non-Kekulé molecule''' is a [[conjugated system|conjugated]] [[hydrocarbon]] that cannot be assigned a classical [[Kekulé structure]]. 

Since non-Kekulé molecules have two or more formal [[radical (chemistry)|radical]] centers, their [[spin (physics)|spin-spin]] interactions can cause [[electrical conductivity]] or [[ferromagnetism]] ([[molecule-based magnets]]), and applications to functional materials are expected. However, as these molecules are quite reactive and most of them are easily decomposed or [[Polymerization|polymerized]] at room temperature, strategies for stabilization are needed for their practical use. Synthesis and observation of these reactive molecules are generally accomplished by [[matrix-isolation]] methods.


==Biradicals==
The simplest non-Kekulé molecules are [[biradical]]s. A biradical is an even-electron [[chemical compound]] with two [[free radical]] centres which act ''independently'' of each other. They should not be confused with the more general class of [[diradical]]s.<ref name=IUPACdef/> 

One of the first biradicals was synthesized by [[Wilhelm Schlenk]] in 1915 following the same methodology as [[Moses Gomberg]]'s [[triphenylmethyl radical]].  The so-called '''Schlenk-Brauns hydrocarbons''' are:<ref name=Moss/>

[[Image:Schlenk-brauns biradicals.png|center|400px|Schlenk-brauns hydrocarbons]]

Eugene Müller, with the aid of a [[Gouy balance]], established for the first time that these compounds are [[paramagnetic]] with a [[triplet state|triplet ground state]].

Another classic biradical was synthesised by Tschitschibabin in 1907.<ref name=Tschit/><ref name=Mont/>  Other classical examples are the biradicals described by Yang in 1960<ref name=Yang/> and by Coppinger in 1962.<ref name=Coppi62/><ref name=Coppi64/><ref name=Baumg/>

{|align="center"  class="wikitable"
|[[File:Tschitschibabin biradical.svg|200px|Tschitschibabin biradical (1907)]]
||||[[File:Yang biradical.svg|180px|Yang biradical (1960)]] ||||  [[File:Coppinger biradical.svg|200px|Coppinger biradical (1962)]] 
|-
|'''Tschitschibabin biradical (1907)'''||||'''Yang biradical (1960)'''||||'''Coppinger biradical 1962'''
|-
|}

===Trimethylenemethane===
A well studied biradical is [[trimethylenemethane]] (TMM), {{chem|C|4|H|6}}. In 1966 Paul Dowd determined with [[electron spin resonance]] that this compound also has a [[diradical|triplet state]]. In a crystalline host the 6 hydrogen atoms in TMM are identical.

===Quinodimethanes and PAHs ===
Other examples of non-Kekulé molecules are the biradicaloid quinodimethanes, that have a six-membered ring with methylene substituents. 

Non-Kekulé [[polynuclear aromatic hydrocarbons]] are composed of several fused six-membered rings. The simples member of this class is [[triangulene]]. After unsuccessful attempts by Eric Clar in 1953, [[trioxytriangulene]] was sinthesized by Richard J. Bushby in 1995, and kinetically stabilized triangulene by Kazuhiro Nakasuji in 2001.  A related class of biradicals are [[para-benzyne]]s.

Other studied biradicals are those based on [[pleiadene]],<ref name=Kolc/> [[viologen|extended viologens]],<ref name=Porter/><ref name=Casado/> [[corannulene]]s,<ref name=Ueda/> [[nitronyl-nitroxide]],<ref name=Ziessel/> [[bis(phenalenyl)]]s <ref name=Kubo/> and [[teranthene]]s.<ref name=Konishi/><ref name=Lambert/>

{|align="center"  class="wikitable"
|[[File:Teranthene.svg|400px|Teranthene biradical]]
||||[[File:Bisphenalenyl biradical.png|370px|Bisphenalenyl biradical]] 
|-
|'''Teranthene biradical''' ''Singlet. max. 3 stabilizing [[Clar's rule|Clar sextets]], stable rt, air. 50% biradical, molecular section of [[graphene]]''||||'''Bisphenalenyl biradical''' ''Singlet. max. 6 stabilizing Clar sextets, stable rt, air. 42% biradical''
|-
|}

'''Pleiadene''' has been synthesised from [[acenaphthylene]] and [[anthranilic acid]] / [[amyl nitrite]]:

{|align="center"  class="wikitable"
|[[File:Pleiadene.svg|600px|Pleiadene generation]]
|-
|'''Pleiadene generation and dimerization''' 
|-
|}

===Oxyallyl===
The [[oxyallyl radical|oxallyl diradical]] (OXA) is a timethylenemethan molecule with one [[methylene group]] replaced by [[oxygen]]. This [[reactive intermediate]] is postulated to occur in ring opening of [[cyclopropanone]]s, [[allene oxide]]s and in the [[Favorskii rearrangement]]. The intermediate has been produced by reaction of [[oxygen radical anion]]s with [[acetone]] and studied by [[photoelectron spectroscopy]].<ref name=Ichino/> The experimental [[electron affinity]] of OXA is 1.94 eV.

==Classification==
[[File:Disjoint-nondisjoint--nonkekule-molecules.svg|frame|right|NBMOs of ''non-disjoint'' (top) and ''disjoint'' (bottom) Non-Kekulé molecules]]
Non-Kekulé molecules with two formal radical centers (non-Kekulé diradicals) can be classified into ''non-disjoint'' and ''disjoint'' by the shape of their two non-bonding [[molecular orbital]]s (NBMOs). 

Both NBMOs of molecules with ''non-disjoint'' characteristics such as trimethylenemethane have [[electron density]] at the same [[atom]]. According to [[Hund's rule]], each orbital is filled with one electron with parallel spin, avoiding the [[Coulomb's law|Coulomb repulsion]] by filling one orbital with two electrons. Therefore, such molecules with ''non-disjoint'' NBMOs are expected to prefer a [[triplet state|triplet]] [[ground state]]. 

In contrast, the NBMOs of the molecules with ''disjoint'' characteristics such as [[tetramethyleneethane]] can be described without having electron density at the same atom. With such MOs, the destabilization factor by the Coulomb repulsion becomes much smaller than with ''non-disjoint'' type molecules, and therefore the relative stability of the [[diradical|singlet]] ground state to the triplet ground state will be nearly equal, or even reversed because of [[exchange interaction]].

==References==
<references>

<ref name=IUPACdef>
  [[IUPAC Gold Book]] definitions of [http://goldbook.iupac.org/B00671.html ''biradical''] and [http://goldbook.iupac.org/D01765.html ''diradicals'']
</ref>

<ref name=Moss>
  Robert A. Moss ed. (2004), "Reactive Intermediate Chemistry" (Book) Wiley-Interscience. ISBN 0-471-23324-2
</ref> 

<ref name=Tschit>
  A. E. Tschitschibabin (1907), "Über einige phenylierte Derivate des p, p-Ditolyls." Berichte der Deutschen Chemischen Gesellschaft, volume 40, pages 1810–1819. {{doi|10.1002/cber.19070400282}}
</ref>

<ref name=Mont>
  Lawrence K. Montgomery, John C. Huffman, Edward A. Jurczak, Martin P. Grendze Jr. (1986), "The molecular structures of Thiele's and Chichibabin's hydrocarbons."  Journal of the American Chemical Society, volume 108, issue 19, pages 6004–6011 {{doi|10.1021/ja00279a056}}
</ref>

<ref name=Yang>
  N. C. Yang and A. J. Castro (1960), "Synthesis of a stable biradical"  n P. Grendze Jr. (1986), "The molecular structures of Thiele's and Chichibabin's hydrocarbons."  Journal of the American Chemical Society, volume 82, issue 23, page 6208 {{doi|10.1021/ja01508a067}}
</ref>

<ref name=Coppi62>
  G. M. Coppinger (1962), "A stable phenoxy radical inert to oxygen" Tetrahedron, volume 18, issue 1, pages 61-65. {{doi|10.1016/0040-4020(62)80024-6}}
</ref>

<ref name=Coppi64>
  G. M. Coppinger (1964), "Inhibition Reactions of Hindered Phenols"  Journal of the American Chemical Society, volume 86, issue 20, pages 4385–4388 {{doi|10.1021/ja01074a032}}
</ref>

<ref name=Baumg>
  M. Baumgarten (2003/2004), "High spin molecules directed towards molecular magnets", chapter 12 in "EPR of free radicals in solids, Trends in methods and application", A. Lund, M. Shiotani (eds), Kluwer, pages 491-528
</ref>

<ref name=Kolc>
  Jaroslav Kolc and Josef Michl (1973), "π,π-Biradicaloid hydrocarbons. Pleiadene family. I. Photochemical preparation from cyclobutene precursors."  Journal of the American Chemical Society, volume 95, issue 22, pages 7391–7401 {{doi|10.1021/ja00803a030}}
</ref>

<ref name=Porter>
  William W. Porter III, Thomas P. Vaid, and Arnold L. Rheingold (2005), "Synthesis and characterization of a highly reducing neutral “extended viologen” and the isostructural hydrocarbon 4,4<nowiki>''''</nowiki>-di-n-octyl-p-quaterphenyl"  Journal of the American Chemical Society, volume 127, issue 47, pages 16559–16566 {{doi|10.1021/ja053084q}}
</ref>

<ref name=Casado>
  J. Casado, S. Patchkovskii, M. Zgierski, L. Hermosilla, C. Sieiro, M. Moreno Oliva, and J. López Navarrete (2008), "Raman Detection of “ambiguous” conjugated biradicals: Rapid thermal singlet-to-triplet intersystem crossing in an extended viologen." Angewandte Chemie International Edition, volume 47, pages 1443–1446. {{doi|10.1002/anie.200704398}}
</ref>

<ref name=Ueda>
  A. Ueda, S. Nishida, K. Fukui, T. Ise, D. Shiomi, K. Sato, T. Takui, K. Nakasuji, and Y. Morita (2010), "Three-dimensional intramolecular exchange interaction in a curved and nonalternant π-conjugated system: Corannulene with two phenoxyl radicals." Angewandte Chemie International Edition, volume 49, pages 1678–1682. {{doi|10.1002/anie.200906666}} PMID 20108294
</ref>

<ref name=Ziessel>
  ''Strong Exchange Interactions between Two Radicals Attached to Nonaromatic Spacers Deduced from Magnetic, EPR, NMR, and Electron Density Measurements'' Raymond Ziessel Christophe Stroh, Henrike Heise, Frank H. Köhler, Philippe Turek,Nicolas Claiser, Mohamed Souhassou, and Claude Lecomte J. Am. Chem. Soc., 2004, 126 (39), pp 12604–12613 {{DOI|10.1021/ja0305959}}
</ref>

<ref name=Kubo>
  Takashi Kubo, Akihiro Shimizu, Mikio Uruichi, Kyuya Yakushi, Masayoshi Nakano, Daisuke Shiomi, Kazunobu Sato, Takeji Takui, Yasushi Morita, Kazuhiro Nakasuji (2007), "Singlet biradical character of phenalenyl-based kekulé hydrocarbon with naphthoquinoid structure"  Org. Lett., volume 9, issue 1, pages 81–84 {{doi|10.1021/ol062604z}}
</ref>

<ref name=Konishi>
  Akihito Konishi, Yasukazu Hirao, Masayoshi Nakano, Akihiro Shimizu, Edith Botek, Benot Champagne, Daisuke Shiomi, Kazunobu Sato, Takeji Takui, Kouzou Matsumoto, Hiroyuki Kurata and Takashi Kubo (2010), "Synthesis and characterization of teranthene: A singlet biradical polycyclic aromatic hydrocarbon having Kekulé structures."  Journal of the American Chemical Society, volume 132, issue 32, pages 11021–11023 {{doi|10.1021/ja1049737}}
</ref>

<ref name=Lambert>
  Lambert, C. (2011), "Towards polycyclic aromatic hydrocarbons with a singlet open-shell ground state." Angewandte Chemie International Edition, volume 50, pages 1756–1758. {{doi|10.1002/anie.201006705}}
</ref>

<ref name=Ichino>
  Takatoshi Ichino, Stephanie M. Villano, Adam J. Gianola, Daniel J. Goebbert, Luis Velarde, Andrei Sanov, Stephen J. Blanksby, Xin Zhou, David A. Hrovat, Weston Thatcher Borden, and W. Carl Lineberger (2009), "The lowest singlet and triplet states of the oxyallyl diradical"  Angewandte Chemie International Edition volume 48, pages 8509–8511 {{doi|10.1002/anie.200904417}}
</ref>

</references>

{{DEFAULTSORT:Non-Kekule molecule}}
[[Category:Organic chemistry]]
[[Category:Free radicals]]