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Molecular geometries of benzene and its 18 monosubstituted derivatives were optimized at B3LYP/6-311+G** level of theory. The changes of pi-electron delocalization of the benzene fragment were estimated by use of aromatic stabilization energies (ASE) based on different homodesmotic reaction schemes, geometry-based HOMA model, magnetism-based NICS, NICS(1), NICS(1)zz, and an electronic delocalization index, PDI, derived from the AIM theory. Apart from aromatic stabilization energies the other descriptors of aromaticity vary to a very small extent, indicating high resistance of the pi-electron structure to the substituent effect. This is somewhat analogous to a tendency of benzene systems to retain their initial pi-electron structure during the reaction course that leads to aromatic substitution.

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Fulvene derivatives, 20 mono- and 14 di-exocyclically substituted, were optimized at the B3LYP/6-311+G level of theory. Aromaticity indices include aromatic stabilization energy (ASE), Schleyer's Nucleus Independent Chemical Shift (NICS), (3)He chemical shifts, anisotropy and exaltation of magnetic susceptibility, and the Harmonic Oscillator Model of Aromaticity (HOMA), which is a geometry-based descriptor. These indices were used to estimate the extent of a cyclic pi-electron delocalization due to the substituent effect. A dramatic variation of these indices was found, indicating great sensitivity of the pi-electron structure of the ring. Except for anisotropy of magnetic susceptibility, all other indices exhibited perfect equivalence.

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Molecular geometries of phenol and its 17 p-substituted derivatives were optimized at the B3LYP/6-311þG** level of theory. Three homodesmotic and three isodesmotic reaction schemes were used to estimate aromatic stabilization energies (ASE) and the substituent effect stabilization energy (SESE). Other descriptors ofelectron delocalization (HOMA and NICS, NICS(1) and NICS(1) zz ) were also estimated. The SESE and ASE values correlated well with one another as well as with substituent constants. Much worse correlations with substituent constants were found for other aromaticity indices. The NICS(1) zz values are the most negative for unsubstituted phenol, indicating its highest aromaticity; that was not the case for NICS(1) and NICS.

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Eleven mono-and eight di-exocyclically substituted heptafulvene derivatives were optimized at the B3LYP/6-311G** level of theory. The aromaticity indices REC (ring energy content), Schleyer's NICS, 3 He NMR chemical shift and anisotropy of magnetic susceptibility and the geometry based descriptor HOMA were used to estimate the extent of cyclic p-electron delocalization due to the substituent effect. A dramatic variation of these indices was found for electron-accepting substituents indicating great sensitivity of the p-electron structure of the ring. In the case of monosubstituted derivatives all indices exhibit a perfect or at least very good equivalence, whereas for disubstituted species the mutual correlations are markedly worse.