Abstract
Correlations between 13C nuclear magnetic resonance protonation shifts and protonation-induced charge densities on the carbon atoms as calculated by the INDO-MO method have been examined for mono- and dicationic species of 4-aminoazobenzene. The paramagnetic shielding constants have been calculated with the average excitation energy approximation. It is concluded that the deviation from the correlation between 13C protonation shifts and protonation-induced charge densities on some carbon atoms (Cl, C3, 5 etc.) is mainly due to variations in the average excitation energy term which are attributable to delocalization of lone-pair electrons assisted by resonance structure which makes the molecule planar.
