Aromatic ring current
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If you are looking for the meteorological effect, it is at ring current.
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 observed. That is, the delocalized electrons form a current traversing the boundary of the aromatic ring. This is a direct consequence of Ampère's law: since the electrons involved are free to move around, rather than being constrained in bonds as they would be in a non-aromatic molecule, they can react to the magnetic field.
A similar effect is observed in three-dimensional fullerenes; in that case, it is called a sphere current.
Aromatic ring currents are relevant to NMR spectroscopy, as they dramatically influence the chemical shifts of 13C and 1H nuclei in aromatic molecules [1]. 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. On the other hand any proton inside the inner experiences shielding because both fields are in opposite direction. This effect can be observed in cyclooctadecanonaene ([18]annulene) with 6 inner protons at -3 ppm (at the right of the internal standard).
The situation is reverted in antiaromatic compounds. In the dianion of [18]annulene the inner protons are located downfield at 20.8 ppm and 29.5 ppm with the outer electrons upfield with -1.1 ppm. Hence a diamagnetic ring current or diatropic ring current is associated with aromaticity whereas a paratropic ring current signals antiaromaticity.
[edit] Relative aromaticity
| NICS values | |
| Pyrrole | -15.1 |
| Thiophene | -13.6 |
| Furan | -12.3 |
| Naphtalene | -9.9 |
| Benzene | -9.7 |
| Tropylium | -7.6 |
| Cyclopentadiene | -3.2 |
| Cyclohexane | -2.2 |
| Pentalene | 18.1 |
| Heptalene | 22.7 |
| Cyclobutadiene | 27.6 |
| Some selected values [2] (in ppm) | |
Numerous attempts have been made to quantify aromaticity with respect to the observed ring current [3]. 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. The nucleus-independent chemical shift (NICS) is a computational method that calculates the chemical shift of a hypothetical lithium ion positioned directly inside the ring. [2]. In this method negative NICS values indicate aromaticity and positive values antiaromaticity.
[edit] References
- ^ Aromaticity and Ring Currents Gomes, J. A. N. F.; Mallion, R. B. Chem. Rev.; (Review); 2001; 101(5); 1349-1384. DOI:10.1021/cr990323h
- ^ a b Nucleus-Independent Chemical Shifts: A Simple and Efficient Aromaticity ProbePaul 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
- ^ What is aromaticity? Paul von Rage Schleyer and Haijun Jiao Pure & Appl. Chem., Vol. 68, No. 2, pp. 209-218, 1996Link
