A comparative ab initio and DFT study of neutral aniline oligomers

Comparative ab initio restricted Hartree-Fock (RHF) and density functional theory (DFT) calculations were performed to investigate the geometric and electronic structures of various neutral aniline oligomers. These oligomers were selected to model polyaniline (PANI) in different intrinsic oxidation states, with an aim to study the applicability and extendibility of the theoretical methods to conjugated polymers. In general, we found that DFT calculations produce results that are in good agreement with observations from x-ray diffraction, ultraviolet-visible absorption, ultraviolet photoelectron and x-ray photoelectron spectroscopy. The DFT method has reproduced well the similar to 4.0 eV pi-pi* transition of leucoemeraldine base and the similar to 2.0 eV Peierls gap transition of pernigraniline base. The valence band structure and the similar to 1.2 eV energy separation of nitrogen core levels of emeraldine base are also correctly predicted. On the other hand, large discrepancies with experimental measurements are predicted by the RHF method. Single-point MP2 calculations show that the DFT-optimized structures are all at lower energy than the RHF-optimized ones. (C) 2000 American Institute of Physics. [S0021-9606(00)30323-3].