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An aromatic ring current is an effect observed in 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 π electrons of the aromatic ring.〔''The induced magnetic field in cyclic molecules.'' Merino, G.; Heine, T.; Seifert, G. Chem. Eur. J.; 2004; 10; 4367-4371. 〕 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. Aromatic ring currents are relevant to NMR spectroscopy, as they dramatically influence the chemical shifts of 1H nuclei in aromatic molecules,.〔''Aromaticity and Ring Currents.'' Gomes, J. A. N. F.; Mallion, R. B. Chem. Rev.; (Review); 2001; 101(5); 1349-1384. 〕 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 because both fields are in opposite direction. This effect can be observed in cyclooctadecanonaene (()annulene) with 6 inner protons at −3 ppm. The situation is reversed in antiaromatic compounds. In the dianion of ()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. A similar effect is observed in three-dimensional fullerenes; in this case it is called a sphere current.〔''Sphere Currents of Buckminsterfullerene'', Mikael P. Johansson, Jonas Jusélius, and Dage Sundholm, Angew. Chem. Int. Ed., Vol. 44, No. 12, pp. 1843-1846, 2005 PMID 15706578〕 ==Relative aromaticity== Numerous attempts have been made to quantify aromaticity with respect to the observed ring current.〔''What is aromaticity?'' Paul von Ragué Schleyer and Haijun Jiao Pure & Appl. Chem., Vol. 68, No. 2, pp. 209-218, 1996 (Link )〕 One method is called diamagnetic susceptibility exaltation 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. Another measurable quantity is the chemical shift of lithium ions Li+ in complexes of lithium with aromats because lithium tends to coordinate to the face of the aromatic rings. Thus the lithium atom in cyclopentadienyl lithium (CpLi) has a chemical shift of −8.6 ppm (aromatic) and its Cp2Li− complex a shift of −13.1. Both methods suffer from the disadvantage that values depend on ring size. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Aromatic ring current」の詳細全文を読む スポンサード リンク
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