Dipole moments and electron distribution of conjugated molecules; para derivatives of benzene

Dipole moments of 24 1,4-bis derivatives of benzene with conjugated substituents were calculated at two levels of the DFT theory, B3LYP/6-311+G(d,p)//B3LYP/6-311+G(d,p) and B3LYP/AUG-cc-pVTZ//B3LYP/6-311+G(d,p). The results at the two levels were slightly different but very tightly correlated and were also proportional to the experimental apparent dipole moments determined from the permittivity of benzene solutions. The interaction dipole moments μint were obtained by subtracting the moments of the two pertinent mono derivatives and were interpreted in terms of resonance structures. This interpretation was supported by the values of bond lengths C2-C3 and by the energies of interaction obtained by subtracting the energies of mono derivatives. Interpretation by resonance seems to be correct and in agreement with the classical concept in typical examples: conjugation of the acceptors NO2, CN, B(CH3)2, CF3 with the donors NH2 or N(CH3)2. It is also acceptable in the case of donors OH and OCH3, although the direction of the dipole moment is less certain. In the case of weak donors F or Cl, the values of μint are near to zero and resonance cannot be proven.