The Magnetic Criterion

A special magnetic behaviour of aromatic compounds is believed to result from the occurrence of aromatic ring currents. The following magnetic criteria can be used to determine if a molecule is aromatic. 

Anomalous H chanical shifts manifest the deshielding of protons of aromatic cyclic molecules, caused by ring currents induced by an extanal magnetic field. For aromatic compounds, the protons located inside the ring are shifted upheld and those outside the ring are shifted downfield for antiaromatic compounds the direction is opposite Although this criterion is widely used , it is not general and many exceptions are known.  

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The magnetic criterion is particularly valuable because it provides a basis for differentiating sharply between essentially ionic and essentially electron-pair bonds Experimental data have as yet been obtained for only a few of the interesting compounds, but these indicate that oxides and fluorides of most metals are ionic. Electron-pair bonds are formed by most of the transition elements with sulfur, selenium, tellurium, phosphorus, arsenic and antimony, as in the sulfide minerals (pyrite, molybdenite, skutterudite, etc.). The halogens other than fluorine form electron-pair bonds with metals of the palladium and platinum groups and sometimes, but not always, with iron-group metals. 

Thus aromaticity in cyclopropenone cannot be detected by the magnetic criterion although it is suggested by other chemical and physical properties. UV, IR, and -NMR data of cyclopropenone are summarized in Table 6, but are discussed in later chapters. 

Complex Bond Orbitals The Magnetic Criterion for Bond Type... 

Many other nickel complexes, some of which have been shown to be planar by the methods mentioned in Section 5-3, have been found to satisfy the magnetic criterion. These include the nickel glyoximes,81... 

The main computational criteria used for determining aromaticity are (a) the geometric criterion, (b) the energetic criterion, (c) the magnetic criterion. These criteria are discussed below. 

The magnetic criterion is, however, not a unique indicator of aromaticity. The linear polymethinium ions 42... 

The Cu+ ion, for instance, has a Sd structure. It is diamagnetic in all its compounds. The Cu + ion, however, has a 3d structure and its paramagnetic moment corresponds to the presence of one free electron. Thus the oxidation state of copper in one of its compounds can be found by measuring its paramagnetic susceptibility. Indeed the magnetic criterion of unpaired spin is so well established that, unless a compound is paramagnetic, the existence of an unpaired electron can be discounted. 

It is evident that the 5f orbitals of the uranium metal centers play a key role in the bonding of the aforementioned f-block metal systems significantly contributing to the cyclic electron delocalization and the associated magnetic diatropic response. The aromaticity of the cyclo-U X (n = 3,4 X = OH, NH) and cycZo-U (/tt2-X) (n = 3, 4 X = C, CH, NH) clusters was verified by the magnetic criterion of... 

Pelloni S, Lazzeretti P (2013) Polygonal current model an effective quantified of aromaticity on the magnetic criterion. J Phys Chem A 117 9083-9092... 

Estimation of resonance energy by spin-coupled valence bond calculations [129] and the application of newly developed aromaticity indices considering the cyclic x-electron delocalization [142, 143] have confirmed that [3]- and [6]radialene are nonaromatic. As far as the magnetic criterion for aromaticity of conjugated... 

The stepwise transfer of electrons from alkali metals to Ti-conjugated systems enables the study of the relationship between the number of Ti-electrons and the aromatic/antiaromatic properties of the systems vis d vis the Hiickel rule. The magnetic criterion for aromaticity serves as a probe for the aromatic nature of the systems under study. 

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