Aromatic ring current
An aromatic ring current is an effect observed in
The ring current creates its own magnetic field. Outside the ring, this field is in the same direction as the externally applied magnetic field; inside the ring, the field counteracts the externally applied field. As a result, the net magnetic field outside the ring is greater than the externally applied field alone, and is less inside the ring.
Relevance to NMR spectroscopy
Aromatic ring currents are relevant to
The situation is reversed in
A similar effect is observed in three-dimensional fullerenes; in this case it is called a sphere current.[3]
Relative aromaticity
Numerous attempts have been made to quantify aromaticity with respect to the observed ring current.[4] 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. Large negative values are aromatic, for example, benzene (Λ = −13.4). Values close to zero are non-aromatic, for example, borazine (Λ = −1.7) and cyclohexane (Λ = 1.1). Large positive values are antiaromatic, for example, cyclobutadiene (Λ = +18).
Another measurable quantity is the
Both methods suffer from the disadvantage that values depend on ring size.
Chemical | ppm |
---|---|
Pyrrole | −15.1 |
Thiophene | −13.6 |
Furan | −12.3 |
Naphthalene | −9.9 |
Benzene | −9.7 |
Tropylium |
−7.6 |
Cyclopentadiene | −3.2 |
Cyclohexane | −2.2 |
Pentalene | 18.1 |
Heptalene | 22.7 |
Cyclobutadiene | 27.6 |
Nucleus-independent chemical shift
The nucleus-independent chemical shift (NICS) is a
Harmonic oscillator model of aromaticity
Yet another method called the harmonic oscillator model of aromaticity (HOMA)
where V=257.7 Å−2 is the normalization value, n is the number of carbon–carbon bonds, and d are bond lengths (dopt=1.388 Å is the optimum value and di are the observed or computed values).
References
- PMID 15706578
- ^ What is aromaticity? Paul von Ragué Schleyer and Haijun Jiao Pure Appl. Chem., Vol. 68, No. 2, pp. 209-218, 1996 Link
- ^
- ISSN 1434-193X.
- ^ How far is the π-electron delocalization of the phenanthrene 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 Link