Ab initio Hartree-Fock energy band structure calculations on polyaniline

Ab initio crystal orbital calculations on the oxidized form of polyaniline with alternating benzoid-quinoid structure are reported. For two possible configurations, the trans and the cis isomer, the twist angle between nearest neighbor rings is optimized with respect to the total energy per elementary cell. It turns out that the perpendicular orientation of the two rings leads to the most stable structure in agreement with ¹³C-NMR spectroscopic data. Analogous investigations are performed on the salt formed between oxidized polyaniline and hydrogen chloride. In these systems again the cis configuration with the perpendicular conformation of adjacent rings is the energetically preferred geometry. The protonation of one of the two nitrogen atoms in the elementary cell leads to a band representing the N-H bond at lower energy than the band standing for the lone pair orbital in the unprotonated form. The three highest filled bands can be attributed to the chloride ion. The energy gap in the protonated oxidized polyaniline is smaller than for the neutral form which may be responsible for the conductive properties of the salt form.