Substituent effects in 1,4-disubstituted benzene and cyclohexadiene: Olefinic vs aromatic electron shift pathway of the substituent effect

Substituent effects in 1-X-4-Y-substituted benzene and cyclohexadiene derivatives were studied for X = OH or NO2, and Y = BH2, B(OH)2, CCH, CH3, CHO, Cl, CN, COCH3, COCl, CONH2, COOCH3, COOH, F, H, NH2, N(CH3)2, NO, NO2, OCH3, OH. All systems were optimized at the B3LYP/6-311+G∗∗ DFT level of theory and the resulting data of the optimized systems were used to estimate various characteristics of the interactions between the fixed (the same in a given series of calculations) groups X (OH or NO2) and the substituents. The following characteristics were used: substituent effect stabilization energy (SESE), harmonic oscillator model of aromaticity (HOMA) calculated for three CC bonds (1–4 bond aromatic or olefinic pathway in the rings), occupations of 2pz orbitals at C1C2C3C4 named pEDA, changes in CN or CO bond lengths, NPA charges at OH and NO2, delocalization index (DI) between atoms C1 and C4, magnetic susceptibility. In all cases (except the last one) sensitivity of the pathway via C1C2C3C4 bonds in cyclohexadiene derivatives was significantly greater than in the case of the pathway via aromatic system.