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| {{about|aromatic chemistry|the meteorological effect|Ring current}}
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| [[Image:Aromatic-ring-current.png|thumb|right|250px|A diagram of an aromatic ring current. ''B''<sub>0</sub> is the applied magnetic field, the red arrow indicating its direction. The orange ring shows the direction of the ring current, and the purple rings show the direction of the [[electromagnetic induction|induced]] magnetic field.]]
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| An '''aromatic ring current''' is an effect observed in [[aromaticity|aromatic]] [[molecules]] such as [[benzene]] and [[naphthalene]]. If a [[magnetic field]] is directed [[perpendicular]] to the plane of the aromatic system, a ring current is induced in the delocalized [[pi-bond|π electrons]] of the aromatic ring.<ref>''The induced magnetic field in cyclic molecules.'' Merino, G.; Heine, T.; Seifert, G. [[Chem. Eur. J.]]; '''2004'''; 10; 4367-4371. {{DOI|10.1002/chem.200400457}}</ref> This is a direct consequence of [[Ampère's law]]; since the electrons involved are free to circulate, rather than being localized in bonds as they would be in most non-aromatic molecules, they respond much more strongly to the magnetic field.
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| Aromatic ring currents are relevant to [[NMR spectroscopy]], as they dramatically influence the [[chemical shift]]s of [[carbon-13|<sup>13</sup>C]] and [[hydrogen atom|<sup>1</sup>H]] [[atomic nucleus|nuclei]] in aromatic molecules,<ref>''Aromaticity and Ring Currents.'' Gomes, J. A. N. F.; Mallion, R. B. [[Chem. Rev.]]; (Review); '''2001'''; 101(5); 1349-1384. {{DOI|10.1021/cr990323h}}</ref> as well as in any organic or inorganic aromatic molecule. The effect helps distinguish these nuclear environments and is therefore of great use in molecular structure determination. In benzene, the ring protons experience [[deshielding]] because the induced magnetic field has the same direction as the external field and their [[chemical shift]] is 7.3 ppm compared to 5.6 to the [[vinylic]] proton in [[cyclohexene]]. In contrast any proton inside the aromatic ring experiences [[shielding (NMR)|shielding]] because both fields are in opposite direction. This effect can be observed in [[cyclooctadecanonaene]] ([18]annulene) with 6 inner protons at −3 ppm.
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| The situation is reversed in [[antiaromatic]] compounds. In the [[dianion]] of [18]annulene the inner protons are strongly deshielded at 20.8 ppm and 29.5 ppm with the outer protons significantly shielded (with respect to the reference) at −1.1 ppm. Hence a '''diamagnetic ring current''' or '''diatropic ring current''' is associated with aromaticity whereas a '''paratropic ring current''' signals antiaromaticity.
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| A similar effect is observed in three-dimensional [[fullerene]]s; in this case it is called a '''sphere current'''.<ref>''Sphere Currents of Buckminsterfullerene'', Mikael P. Johansson, Jonas Jusélius, and Dage Sundholm, [[Angew. Chem. Int. Ed.]], Vol. 44, No. 12, pp. 1843-1846, '''2005''' {{DOI|10.1002/anie.200462348}} PMID 15706578</ref>
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| [[Image:Benzene ring currents.png|thumb|center|610px|Magnetically induced probability current density vectors in benzene (C<sub>6</sub>H<sub>6</sub>) calculated explicitly using [[quantum chemistry|quantum chemical methods]]. ''B''<sub>0</sub> is set perpendicular to the molecular plane, in the left subfigure only vectors in the molecular plane are shown, in the right subfigure only vectors 1 a.u. (~52 pm) above the molecular plane are shown. Only vectors with a [[modulus]] between 0.01 and 0.1 nA/T are displayed. Contrasting the schematic picture, which gives in analogy to the [[electrodynamics|laws of classical electrodynamics]] only diatropic contributions, the full quantum mechanical picture also yield paratropic contributions, as counter-clockwise vortices in this diagram. These are located in benzene mainly in the molecular plane, inside the C<sub>6</sub> ring.]]
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| ==Relative aromaticity==
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| {|align="right" class="wikitable"
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| |colspan=2 align="center"|Selected '''NICS values'''<ref name=Schleyer/> / ppm
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| || [[Pyrrole]]||−15.1
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| ||[[Thiophene]]||−13.6
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| || [[Furan]]||−12.3
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| || [[naphthalene]]||−9.9
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| || [[Benzene]]||−9.7
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| || [[Tropylium]]||−7.6
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| || [[Cyclopentadiene]]||−3.2
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| || [[Cyclohexane]]||−2.2
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| || [[Pentalene]]||18.1
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| || [[Heptalene]]||22.7
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| || [[Cyclobutadiene]]||27.6
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| |}
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| Numerous attempts have been made to quantify [[aromaticity]] with respect to the observed ring current.<ref>''What is aromaticity?'' Paul von Ragué Schleyer and Haijun Jiao Pure & Appl. Chem., Vol. 68, No. 2, pp. 209-218, '''1996''' [http://www.iupac.org/publications/pac/1996/pdf/6802x0209.pdf Link]</ref> One method is called '''diamagnetic susceptibility exaltation''' [[Lambda|Λ]] defined as the difference between the measured [[magnetic susceptibility]] of a compound and a calculated value based on group additivity tables. [[Benzene]] is clearly aromatic (Λ = −13.4), [[borazine]] (Λ = −1.7) and [[cyclohexane]] (Λ = 1.1) are not aromatic and [[cyclobutadiene]] (Λ = +18) is antiaromatic.
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| Another measurable quantity is the [[chemical shift]] of [[lithium]] ions Li<sup>+</sup> in complexes of lithium with aromats because lithium tends to [[coordination chemistry|coordinate]] to the [[hapticity|face]] of the aromatic rings. Thus the lithium atom in [[cyclopentadienyl]] lithium (CpLi) has a chemical shift of −8.6 ppm (aromatic) and its Cp<sub>2</sub>Li<sup>−</sup> complex a shift of −13.1.
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| Both methods suffer from the disadvantage that values depend on ring size. The '''nucleus-independent chemical shift''' ('''NICS''') is a [[computational chemistry|computational method]] that calculates the absolute [[magnetic shielding]]s at the center of the ring taken with reversed sign.<ref name=Schleyer>''Nucleus-Independent Chemical Shifts: A Simple and Efficient Aromaticity Probe'' Paul von Ragué Schleyer, Christoph Maerker, Alk Dransfeld, Haijun Jiao, and Nicolaas J. R. van Eikema Hommes [[J. Am. Chem. Soc.]]; '''1996'''; 118(26) pp 6317-6318; (Communication) {{DOI|10.1021/ja960582d}}</ref> In this method negative NICS values indicate aromaticity and positive values antiaromaticity.
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| Yet another method called the '''harmonic oscillator model of aromaticity''' ('''HOMA''') <ref>''Definition of aromaticity basing on the harmonic oscillator model'' Tetrahedron Letters, Volume 13, Issue 36, '''1972''', Pages 3839-3842 J. Kruszewski and T. M. Krygowski {{DOI|10.1016/S0040-4039(01)94175-9}}</ref> is defined as a [[Normalization (statistics)|normalized sum]] of [[squared deviations]] of [[bond length]]s from the optimal value, which is assumed to be realized for a fully aromatic system.<ref>''How far is the π-electron delocalization of the [[phenanthrene]] [[Moiety (chemistry)|moiety]] modified in the aza-analogues and their N-oxides?'' Beata T. Stępień, Tadeusz M. Krygowski,a Michał K. Cyrański, Jacek Młochowski, Pierluigi Orioli, and Francesco Abbate [[Arkivoc]] 2003 [http://www.arkat-usa.org/?VIEW=MANUSCRIPT&MSID=893 Link]</ref> An aromat has HOMA value 1 whereas a non-aromatic compound has value 0. For all-carbon systems, a HOMA value is obtained making use of this equation:
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| :<math> \mathrm{HOMA} = 1- 257.7/n\sum^n_i(d_{\rm opt} - d_i)^2 \,,</math>
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| with the value of 257.7 the normalization value, n-number of CC bonds, ''d''<sub>opt</sub> the optimized bond length (1.388 [[angstrom]]) and ''d<sub>i</sub>'' the experimental or computed bond length.
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| ==References==
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| {{Reflist}}
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| [[Category:Nuclear magnetic resonance]]
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| [[Category:Electric current]]
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Hi there. Allow me begin by introducing the author, her title is Myrtle Cleary. The factor she adores most is body building and now she is trying to earn money with it. North Dakota is her beginning location but she will have to move 1 day or another. Bookkeeping is her day job now.
Feel free to surf to my site home std test (you can try these out)