General Papers ARKIVOC 2008 (xi) 24-45
Aromatic zones
This chapter presents examples of “aromatic zones”, defined according to TOPAZ.
All aromatic rings in Fig. 5 represent unique cyclic aromatic zones, including all the bonds between the heavy atoms of the analyzed specie.
In naphthalene (A = 608), anthracene (A = 531), [9] helicene (A = 445), fullerene C60 (A = 252), indole (A = 448), for instance, the TOPAZ algorithm identifies unique polycyclic aromatic zones that include all the bonds between the heavy atoms.
Application of the TOPAZ algorithm leads to the conclusion that the aromaticity of the unique aromatic zone in naphthalene paramagnetic cation radical (A = 483) and anion radical (A = 479) is lower than that of their diamagnetic parent (A = 608), contrary to the conclusions based on application of the HOMA formula.73 Both TOPAZ and HOMA bi-dimensional index40-46 measure deviations of chemical bonds from the "reference value" and deviations from the single / double alternating type. However, HOMA formula uses "bond length", while TOPAZ algorithm uses "bond order".
In azulene we can identify a unique aromatic zone (A= 442), that coincides with the peripheral topological path. Similarly, in pentalene dication, we can identify a unique aromatic zone (A= 449), that is the peripheral topological path.
Unique aromatic zones, with complex shapes, that also include all bonds between the heavy atoms, are identified, for instance, in cyanuric acid (A = 219), substituted cyclopropene C3H2S in Fig. 3 (A = 209), thiotropone (A = 391), pyromeconic acid cation (A = 277), phenalenyl radical (A = 475), benzyl radical (A = 699) and triphenylmethane radical (A = 556).
Almost all properties of aromatic cyclic and non-cyclic structures (exception is "magnetic properties") are similar. Therefore, the application of aromaticity computation, related only to cyclic structures, seems to be rather unproductive. For about 25 years are being studied the noncyclic, "Y-shaped", aromatic species.62-71 TOPAZ identifies as "Y-shaped" aromatic molecules, for instance, the urea (A = 255), thiourea (A = 248) and guanidine (A = 479). In these molecules, the algorithm we have proposed here identifies a unique aromatic zone including all the heavy atoms. The unique aromatic zone in trimethylene-methane dication (CH2)3C2+, the parent specie for the "Y-aromaticity" research field, exhibits a low aromaticity A = 325. The dianions CX32- are other examples of “Y-shaped" ions, computed as aromatic. These ions result, theoretically, from the molecules with the structure of X = C (XH)2, by the loss of two protons. For instance, CO32- is formed from the carbonic acid, and C(NH)32-had guanidine as a parent compound. In these anions, the algorithm proposed here identifies a unique aromatic zone containing all the heavy atoms. The aromaticity calculated for these aromatic zones is low: A = 261 (X = CH2), A = 212 (X = O), A = 275 (X = NH) and A = 229 (X = S). In benzene trianion in Fig. 4 we can identify three CNO2 "Y-shaped" aromatic zones (A = 304). The unique aromatic zone, branched as double Y, in [S-C(NH2)=CH-C(NH2)=S]-anion, containing all the heavy atoms, exhibits a low aromaticity A = 232. The unique aromatic zone in trivinyl-methane “Yshaped" radical exhibits a low aromaticity A = 346.
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