Diamagnetic susceptibility exaltation and

A theoretical study was carried out to evaluate some bicyclic heterocyclic systems related to the bicyclo[3.2.11-octane skeleton for evidence of neutral homoaromaticity <2005JOC1998>. Included in the study were phosphines 17 and 18. Consideration of a series of factors including NICS values, diamagnetic susceptibility exaltations, and stabilization energies led to the conclusion that, while bicyclic compound 17 was nonhomoaromatic, the analogue 18 displayed evidence of homoaromaticity. 

Significant relationships between aromaticity measured by diamagnetic susceptibility exaltation and derivatives of the total molecular valence for molecule and HOMO orbital, calculated ab initio in the framework the Density Function Theory, were also found [Balawender, Komorowski et al., 1998]. 

The sydnones may be represented by structures (123a-d), of which the zwitterionic structure (123a) most clearly implies an aromatic sextet. The diamagnetic susceptibility exaltation for Af-phenylsydnone of ll.Ox 10 cm moP is comparable with the corresponding value for pyrrole (10.2x10 ). 3-p-Bromophenylsydnone (123 R = H, R = p-bromophenyl) is essentially planar however, the O—N bond and 0(1)—C(5) bond lengths are not very different from normal single bond distances. 

The usefulness of magnetic susceptibility and diamagnetic susceptibility exaltation measurements as a means of assessing aromaticity has been outlined elsewhere (68JA811, 74AHC(17)255, B-75MI22206). 

Comparison of the magnitude of diamagnetic susceptibility exaltations of different systems must take into account the area of the unsaturated cyclic system. In the case of benzene and cycloheptatrienes (homobenzene) the areas of the cyclic 7i-systems are similar. 

Aromatic compounds display magnetic properties related to a ring current. These include unusual chemical shifts, especially of protons, large magnetic anisotropies, and large diamagnetic susceptibility exaltations. 

Positive values reveal the presence of ring currents and therefore of benzenoid aromatic systems. The diamagnetic susceptibility exaltation highly depends on the ring size. 

The first derivatives of the molecular valency index were also proposed as quantum descriptors [Balawender, Komorowski et al., 1998], where the left-hand-side derivative describes the effect of the electrophilic attack and the right-hand-side derivative measures reactivity toward a nucleophilic attack. This last one is also related to the aromatic character of a molecule, measured by the —> diamagnetic susceptibility exaltation. 

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