Magnetic criteria

Magnetic susceptibility anisotropy has been used for the estimation of relative aromaticity of some azines in comparison with benzene (77JCS(P2)897). If the extent of ir-electron delocalization for benzene is taken as 1.0, the corresponding values for azines are pyridine 0.7, pyridazine 0.7, pyrimidine 0.5, and 1,3,5-triazine 0.3. 

One of the most obvious examples is strong deshielding of the a-protons in the series pyridine (8 8.29 ppm), phosphabenzene (8.61), arsabenzene (9.68), stibabenzene (10.94), and bismabenzene (13.25), although other data unambiguously point to a falling off of the aromaticity in this sequence. Here the contribution by crAringcurr is mostly obscured by local effects connecting with nonuniform distribution of the electron density and by the anisotropy of the heavier heteroatoms. 

Finally one can conclude that despite still remaining uncertainties there exist some firmly established aromaticity regularities in series of six-membered heterocycles. They are (1) Increase in the electronegativity of the heteroatom leads to decrease of aromaticity (2) Aromaticity falls with increasing the number of pyridine-like nitrogens in the ring (3) Transition from simple azines to their benzoderivatives is also accompanied by a decrease of aromaticity. 

For more detailed discussion on problems of heteroaromaticity see (85KGS867), (91H127), (93AHC(56)303), (98JOC5228). 

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Most of the known borides are compounds of the rare-earth metals. In these metals magnetic criteria are used to decide how many electrons from each rare-earth atom contribute to the bonding (usually three), and this metallic valence is also reflected in the value of the metallic radius, r, (metallic radii for 12 coordination). Similar behavior appears in the borides of the rare-earth metals and r, becomes a useful indicator for the properties and the relative stabilities of these compounds (Fig. 1). The use of r, as a correlation parameter in discussing the higher borides of other metals is consistent with the observed distribution of these compounds among the five structural types pointed out above the borides of the actinides metals, U, Pu and Am lead to complications that require special comment. 

The observed planarity and bond length equalization in 1,3,2-diazaphospholenium cations likewise suggest that these compounds have substantial n-electron delocalization and possess possibly aromatic character. Several studies were undertaken to quantify the degree of n-delocalization by computational calculations using the interpretation of population analyses, ELF calculations, evaluation of magnetic criteria [nucleus independent chemical shift (NICS) values], and the... 

As with the above geometric and energetic criteria, the relatively high aromaticity of 1,2,5-thiadiazole was supported by magnetic criteria (Table 9). 

Table 9 Aromaticity based on magnetic criteria (TDA=thiadiazole) <2002JOC1333>...

Aromaticity, defined as a structural feature, was used as a predictive tool for compounds that had not been prepared previously, whereas reactivity, bond length, or magnetic criteria had to await the isolation of a compound and its experimental investigation. Only recently has the development of quantum-chemical methods reached the point where one can predict with sufficient accuracy the magnetic properties, the bond lengths, and the reactivity patterns of aromatics. The multidimensional character and the definition and measurement of aromaticity generated confusion and conflicts.43 A recent review discussed the multidimensional character of aromaticity and theoretical and experimental approaches to aromatic structures and their predictions, and references are indicated extensively.66... 

The aromaticity of the phosphorus compounds with the A—D bonding type (shown in Scheme 1) will be discussed below, using structural, energetic, and magnetic criteria. The chemical reactivity—in relation to the aromatic stabilization—will be mentioned as well. 

Magnetic criteria of aromaticity are based on the model of interatomic teratomic ring currents induced in conjugated cyclic molecules by external magnetic fields. 

Important conclusions about the interrelationship among aromaticity indices drawn from energetic, structural, and magnetic criteria stem from principal component analysis of the problem (89JA7). The scheme of principal components is given by... 

The material presented in Section II warrants, apparently, the conclusion that the main test of aromaticity and antiaromaticity is represented by the energetic criterion realizable within the framework of various schemes for calculating resonance energies. In most cases it correlates with structural and magnetic criteria moreover, it often accords well with a manifestation of numerous properties of compounds, which, being regarded as attributes of aromaticity, make its very concept substantially broader. Indeed, the concept of aromaticity claims an increasing number of types of compounds and requires a more and more sophisticated classification. 

D.A. (1993) Rock magnetic criteria for the detection of biogenic magnetite. Earth Planetary Sci. Letters 120 283-300 Moukassi, M. Gougeon, M. Steinmetz, P. Dupre, B. Gleitzer, C. (1984) Hydrogen reduction of single crystals of wiistite doped with Mg, Mn, Ca, A1 and Si. Met. Trans. 15B 383-390... 

Optimization at the restricted Hartree-Fock (RHF) and DFT (B3LYP) levels in the 6-31G basis set has also contributed to structural clarification of [l,2,5]thiadiazolo[3,4-f][l,2,5]thiadiazole 6 both approaches successfully predict that 6 ought to have planar equilibrium geometry. Direct a3 initio ipsocentric calculations of the total 7t-current density map for 6 permits classification as aromatic according to magnetic criteria. It exhibits a strong and uniform diatropic perimeter circulation, of comparable intensity to the parent monocycle 11 and similar to that of naphthalene <2004JA11202>. 

Magnetic criteria have received wide application mainly as a qualitative test for aromaticity and antiaromaticity. The values of the exaltation of diamagnetic susceptibility (in 10-6A cm-3 mol-1), and therefore aromaticity, decrease in the sequence thiazole (17.0) > pyrazole (15.5) > sydnone (14.1). The relative aromaticity of heterocycles with a similar type of heteroatom can be judged from values of the chemical shifts of ring protons. The latter reveals paramagnetic shifts when Tr-electron delocalization is weakened. For example, in the series of isomeric naphthoimidazoles aromaticity decreases in the sequence naphthof 1,2-djimidazole (8 = 7.7-8.7 ppm) > naphtho[2,3- perimidine (8 = 6.1-7.2 ppm). This sequence agrees with other estimates, in particular with energetic criteria. 

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. 

D. Interrelations between the Geometric, Energetic and Magnetic Criteria... 

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