Effect of the Basic Strength of Attached Groups on Light Absorption Spectra II: Near Ultraviolet Absorption Spectra of Para-substituted Aniline Derivatives

Abstract

The absorption spectra of para-substituted aniline derivatives of the type of A_iC₆H₄B, where A_i is one of the electron donating groups such as NH₂, NHCH₃, NHC₂H₅, N(CH₃)₂, N(C₂H₅)₂ etc. and B is one of the electron attracting groups such as COOH, COCH₃, CHO, NO₂ etc., change with the basic strength of the group A_i. The first absorption maxima of these molecules between 2800 and 4000 A are originated from the second absorption maxima of unsubstituted molecules (in 2600∼2900 A region) under the influence of the group A_i. This is explained by semi-empirical MO theory with perturbation. The effects of B groups are taken into consideration by the delocalization of the molecular orbitals of benzene molecule, while the effects of A_i groups are calculated through the method of electron migration, in which the non-bonding π electrons on the nitrogen atom of A_i migrate to the parent molecule. The induction effect of the substituted alkyl group in A_i, which determines the degree of the electron migration, can be estimated from the relative basic strength of the end group, i.e. dissociation constant of amine NHR₂. The calculated absorption energies are in good agreement with the observed values if we take the solvent effect into consideration.