The relative aromaticity of pyrrole, furan, thiophene, and their aza-derivatives has been examined using TRE (topological resonance energy), MRE (magnetic resonance energy), ring current (RC), and ring current diamagnetic susceptibility (χG) methods. The results obtained were compared with results obtained by others who used the energetic method ASE (aromatic stabilization energy), the geometric method HOMA (harmonic oscillator model of aromaticity), and the magnetic method NICS(1) (nucleus-independent chemical shift). The impact of nitrogen atoms on the aromaticity of the aza-derivatives of pyrrole, furan, and thiophene is discussed. An excellent correlation was found between the energetic (TRE, MRE) and magnetic (RC and χG) criteria of aromaticity for all compounds. It was expected that inclusion of a heteroatom would decrease the aromaticity relative to the cyclopentadienyl anion. Our results show that the type of the first heteroatom, which donates two electrons to the system, as well as the number of nitrogen atoms and their positions in the molecule have a strong effect on aromaticity. In general, aromaticity is enhanced when the nitrogen atom is adjacent to the first heteroatom. The magnitude of aromaticity is related closely with the uniformity of distribution of π-electrons in the molecule.

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The global aromaticity of the heterofullerene C 24-2n B n N n (n D 3 or 6) and C 12 N 12 isomers formed from the initial C 24 fullerene with D 6 symmetry, has been investigated using both the topological resonance energy (TRE) and the percentage topological resonance energy (%TRE) methods. The local aromaticity was studied using the bond resonance energy (BRE) method. Analysis was made of the effects of the types, the numbers, and the arrangements of heteroatoms on the global and local aromaticity of the molecules under consideration. Our results obtained by the TRE and %TRE methods were compared with the nucleus independent chemical shift (NICS (0)) values at the cage center. We found that NICS(0) values were not suitable for estimating the global aromaticity of these compounds. It was expected that the inclusion of heteroatoms would decrease the antiaromaticity relative to the C 24 (D 6 ) cage. Our BRE results show that the antiaromaticity is primarily related to the existence of highly reactive antiaromatic bonds in the molecule. We found that when a local structure within the cage had two nitrogen and one boron atom in the same pentagon ring, it played an important role in the global aromaticity of these compounds. Finally, we predicted the kinetic stability using the minimum BRE method.

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