Molecular orbital study of the initial reaction paths in the electrochemical polymerization of aniline

Abstract

The initial reaction paths in the electrochemical polymerization of aniline were studied by the molecular orbital method within the MNDO and AM1 approximations. The spin populations of the aniline cation, calculated by the AM1-UHF method, suggested a tail-to-tail coupling as well as a head-to-tail coupling. These were confirmed by the Bader-Pearson theory. This predicts that the tail-to-tail coupling is symmetry-allowed under C i and C 2 symmetry of the cation dimer (when two benzene rings exist in the same plane), whereas the head-to-tail coupling is symmetry-allowed under C s symmetry of the cation dimer (when one benzene ring is perpendicular to the other). The potential curve for the tail-to-tail coupling suggests the formation of a more or less stable intermediate. However, the coupling might not lead to the formation of benzidine, since a relatively large activation energy should be required for the elimination of protons. This result may endorse an experimental fact obtained previously. For the head-to-tail coupling, on the other hand, the MO calculation indicated that protonated p-aminodiphenylamine was formed after the elimination of a proton connected to the para-carbon atom. It was also shown that protonated p-aminodiphenylamine was more stable than its neutral form, the latter being reported to be produced in aqueous solutions.