frame|right|Examples of non-Kekulé (a) polyenes, (b) quinodimethanes, and (c) polynuclear aromatics
A '''non-Kekulé molecule''' is a conjugated hydrocarbon that cannot be assigned a classical Kekulé structure{{definition needed|date=January 2023}}.
Since non-Kekulé molecules have two or more formal charges or radical centers, their 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 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 biradicals. 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 diradicals.<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/>
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 ground state.
Another classic biradical was synthesised by Aleksei Chichibabin 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" |200px|Tschitschibabin biradical (1907) ||||180px|Yang biradical (1960) |||| 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 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 simplest member of this class is triangulene. After unsuccessful attempts by Erich Clar in 1953, trioxytriangulene was synthesized by Richard J. Bushby in 1995, and kinetically stabilized triangulene by Kazuhiro Nakasuji in 2001. However, in 2017 a project led by David Fox and Anish Mistry from the University of Warwick in collaboration with IBM synthesized and imaged triangulene.<ref>{{Cite journal|last1=Pavliček|first1=Niko|last2=Mistry|first2=Anish|last3=Majzik|first3=Zsolt|last4=Moll|first4=Nikolaj|last5=Meyer|first5=Gerhard|last6=Fox|first6=David J.|last7=Gross|first7=Leo|date=2017-02-13|title=Synthesis and characterization of triangulene|journal=Nature Nanotechnology|language=en|volume=advance online publication|issue=4|pages=308–311|doi=10.1038/nnano.2016.305|pmid=28192389|issn=1748-3395|bibcode=2017NatNa..12..308P|url=http://wrap.warwick.ac.uk/86722/1/WRAP_ch-030317-wrap_-cusersdavid_foxdesktop40triangulene_complete_accepted.pdf}}</ref> In 2019, larger homologues of triangulene, consisting of ten ([4]triangulene)<ref>{{Cite journal|last1=Mishra|first1=Shantanu|last2=Beyer|first2=Doreen|last3=Eimre|first3=Kristjan|last4=Liu|first4=Junzhi|last5=Berger|first5=Reinhard|last6=Gröning|first6=Oliver|last7=Pignedoli|first7=Carlo A.|last8=Müllen|first8=Klaus|last9=Fasel|first9=Roman|last10=Feng|first10=Xinliang|last11=Ruffieux|first11=Pascal|date=2019-07-10|title=Synthesis and Characterization of π-Extended Triangulene|journal=Journal of the American Chemical Society|volume=141|issue=27|pages=10621–10625|doi=10.1021/jacs.9b05319|pmid=31241927|s2cid=195696890 |issn=0002-7863}}</ref> and fifteen fused six-membered rings ([5]triangulene)<ref>{{Cite journal|last1=Su|first1=Jie|last2=Telychko|first2=Mykola|last3=Hu|first3=Pan|last4=Macam|first4=Gennevieve|last5=Mutombo|first5=Pingo|last6=Zhang|first6=Hejian|last7=Bao|first7=Yang|last8=Cheng|first8=Fang|last9=Huang|first9=Zhi-Quan|last10=Qiu|first10=Zhizhan|last11=Tan|first11=Sherman J. R.|date=2019-07-01|title=Atomically precise bottom-up synthesis of π-extended [5]triangulene|journal=Science Advances|language=en|volume=5|issue=7|article-number=eaav7717|doi=10.1126/sciadv.aav7717|pmid=31360763|pmc=6660211|issn=2375-2548|doi-access=free}}</ref> were synthesized in 2019. In 2021, synthesis of the hitherto largest triangulene homologue, consisting of twenty-eight fused six-membered rings ([7]triangulene)<ref>{{Cite journal|last1=Mishra|first1=Shantanu|last2=Xu|first2=Kun|last3=Eimre|first3=Kristjan|last4=Komber|first4=Hartmut|last5=Ma|first5=Ji|last6=Pignedoli|first6=Carlo|last7=Fasel|first7=Roman|last8=Feng|first8=Xinliang|last9=Ruffieux|first9=Pascal|date=2021-01-07|title=Synthesis and characterization of [7]triangulene|url=https://pubs.rsc.org/en/Content/ArticleLanding/2021/NR/D0NR08181G|journal=Nanoscale|language=en|volume=Accepted Manuscript|issue=3|pages=1624–1628|doi=10.1039/D0NR08181G|pmid=33443270|s2cid=231605335 }}</ref> was achieved. Scanning tunneling microscopy experiments on triangulene spin chains have revealed the clearest proof yet of the existence of Haldane gap and fractional edge states predicted for spin-1 Heisenberg chain.<ref>{{cite journal |last1=Mishra |first1=Shantanu |last2=Beyer |first2=Doreen |last3=Eimre |first3=Kristjan |last4=Ortiz |first4=Ricardo |last5=Fernández‐Rossier |first5=Joaquín |last6=Berger |first6=Reinhard |last7=Gröning |first7=Oliver |last8=Pignedoli |first8=Carlo A. |last9=Fasel |first9=Roman |last10=Feng |first10=Xinliang |last11=Ruffieux |first11=Pascal |title=Collective All‐Carbon Magnetism in Triangulene Dimers |journal=Angewandte Chemie International Edition |date=13 July 2020 |volume=59 |issue=29 |pages=12041–12047 |doi=10.1002/anie.202002687 |pmid=32301570 |pmc=7383983 |arxiv=2003.00753 }}</ref><ref>{{cite journal |last1=Mishra |first1=Shantanu |last2=Catarina |first2=Gonçalo |last3=Wu |first3=Fupeng |last4=Ortiz |first4=Ricardo |last5=Jacob |first5=David |last6=Eimre |first6=Kristjan |last7=Ma |first7=Ji |last8=Pignedoli |first8=Carlo A. |last9=Feng |first9=Xinliang |last10=Ruffieux |first10=Pascal |last11=Fernández-Rossier |first11=Joaquín |last12=Fasel |first12=Roman |title=Observation of fractional edge excitations in nanographene spin chains |journal=Nature |date=13 October 2021 |volume=598 |issue=7880 |pages=287–292 |doi=10.1038/s41586-021-03842-3|pmid=34645998 |arxiv=2105.09102 |bibcode=2021Natur.598..287M |s2cid=234777902 }}</ref> A related class of biradicals are para-benzynes.
Other studied biradicals are those based on pleiadene,<ref name=Kolc/> extended viologens,<ref name=Porter/><ref name=Casado/> corannulenes,<ref name=Ueda/> nitronyl-nitroxide,<ref name=Ziessel/> bis(phenalenyl)s <ref name=Kubo/> and teranthenes.<ref name=Konishi/><ref name=Lambert/>
{|align="center" class="wikitable" |400px|Teranthene biradical ||||370px|Bisphenalenyl biradical |- |'''Teranthene biradical''' ''Singlet. max. 3 stabilizing 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" |600px|Pleiadene generation |- |'''Pleiadene generation and dimerization''' |- |}
===Oxyallyl=== The oxyallyl diradical (OXA) is a trimethylenemethane molecule with one methylene group replaced by oxygen. This reactive intermediate is postulated to occur in ring opening of cyclopropanones, allene oxides and in the Favorskii rearrangement. The intermediate has been produced by reaction of oxygen radical anions with acetone and studied by photoelectron spectroscopy.<ref name=Ichino/> The experimental electron affinity of OXA is 1.94 eV.
==Classification== 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 orbitals (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 repulsion by filling one orbital with two electrons. Therefore, such molecules with ''non-disjoint'' NBMOs are expected to prefer a 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 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>{{cite journal | last1 = Tschitschibabin | first1 = A. E. | year = 1907 | title = Über einige phenylierte Derivate des p, p-Ditolyls | url = https://zenodo.org/record/1426231| journal = Berichte der Deutschen Chemischen Gesellschaft | volume = 40 | issue = 2| pages = 1810–1819 | doi = 10.1002/cber.19070400282}}</ref>
<ref name=Mont>{{cite journal | last1 = Montgomery | first1 = Lawrence K. | last2 = Huffman | first2 = John C. | last3 = Jurczak | first3 = Edward A. | last4 = Grendze Jr | first4 = Martin P. | year = 1986 | title = The molecular structures of Thiele's and Chichibabin's hydrocarbons | journal = Journal of the American Chemical Society | volume = 108 | issue = 19| pages = 6004–6011 | doi = 10.1021/ja00279a056 | pmid = 22175364}}</ref>
<ref name=Yang>{{cite journal | last1 = Yang | first1 = N. C. | last2 = Castro | first2 = A. J. | year = 1960 | title = Synthesis of a stable biradical" n P. Grendze Jr. (1986), "The molecular structures of Thiele's and Chichibabin's hydrocarbons | journal = Journal of the American Chemical Society | volume = 82 | issue = 23| page = 6208 | doi = 10.1021/ja01508a067}}</ref>
<ref name=Coppi62>{{cite journal | last1 = Coppinger | first1 = G. M. | year = 1962 | title = A stable phenoxy radical inert to oxygen | journal = Tetrahedron | volume = 18 | issue = 1| pages = 61–65 | doi = 10.1016/0040-4020(62)80024-6}}</ref>
<ref name=Coppi64>{{cite journal | last1 = Coppinger | first1 = G. M. | year = 1964 | title = Inhibition Reactions of Hindered Phenols | journal = 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>{{cite journal | last1 = Kolc | first1 = Jaroslav | last2 = Michl | first2 = Josef | year = 1973 | title = π,π-Biradicaloid hydrocarbons. Pleiadene family. I. Photochemical preparation from cyclobutene precursors | journal = Journal of the American Chemical Society | volume = 95 | issue = 22| pages = 7391–7401 | doi = 10.1021/ja00803a030}}</ref>
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<ref name=Kubo>{{cite journal | last1 = Kubo | first1 = Takashi | last2 = Shimizu | first2 = Akihiro | last3 = Uruichi | first3 = Mikio | last4 = Yakushi | first4 = Kyuya | last5 = Nakano | first5 = Masayoshi | last6 = Shiomi | first6 = Daisuke | last7 = Sato | first7 = Kazunobu | last8 = Takui | first8 = Takeji | last9 = Morita | first9 = Yasushi | last10 = Nakasuji | first10 = Kazuhiro | year = 2007 | title = Singlet biradical character of phenalenyl-based kekulé hydrocarbon with naphthoquinoid structure | journal = Org. Lett. | volume = 9 | issue = 1| pages = 81–84 | doi = 10.1021/ol062604z | pmid = 17192090}}</ref>
<ref name=Konishi>{{cite journal | last1 = Konishi | first1 = Akihito | last2 = Hirao | first2 = Yasukazu | last3 = Nakano | first3 = Masayoshi | last4 = Shimizu | first4 = Akihiro | last5 = Botek | first5 = Edith | last6 = Champagne | first6 = Benot | last7 = Shiomi | first7 = Daisuke | last8 = Sato | first8 = Kazunobu | last9 = Takui | first9 = Takeji | last10 = Matsumoto | first10 = Kouzou | last11 = Kurata | first11 = Hiroyuki | last12 = Kubo | first12 = Takashi | year = 2010 | title = Synthesis and characterization of teranthene: A singlet biradical polycyclic aromatic hydrocarbon having Kekulé structures | journal = Journal of the American Chemical Society | volume = 132 | issue = 32| pages = 11021–11023 | doi = 10.1021/ja1049737 | pmid = 20698663}}</ref>
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{{DEFAULTSORT:Non-Kekulé molecule}} Category:Organic chemistry Category:Free radicals