Geometries and band gaps of polyaniline oligomers up to decamer have been systematically calculated and analyzed using various computational techniques such as molecular mechanics, semiempirical, and ab initio methods. On the basis of fully optimized geometries of neutral and charged forms of polyaniline oligomers, excitation energies are calculated at the semiempirical ZINDO (INDO/S) level and extrapolated to the band gap value of the infinite chain. Band gaps are also approximated by extrapolating the HOMO/LUMO difference calculated at the density functional level (B3LYP/6-31G*). The ZINDO//AM1 band gaps in the reduced and oxidized form of polyaniline (4.3 and 2.7 eV) are in good agreement with experimental values (3.8 ( 2 and 1.8 ( 3 eV, respectively). The doped form of polyaniline (two positive charges per four aniline units) has been computed with a spin-unrestricted method (UAM1) and the band gap approximated from an extrapolation of the tetramer and octamer. The calculated band gap of 1.3 eV (UZINDO//UAM1) is in good agreement with experiment (1.5 eV). The influence of ring torsional angle and interchain interaction on the band gap of the polyaniline system are also discussed.

The molecular structures and the intramolecular hydrogen bonding for 1,2-dihydroxybenzene and 2-hydroxythiophenol have been precisely investigated by an ab initio method and density functional theory (DFT). We have considered the several possible conformations with different types of intramolecular hydrogen bonding in given molecules to understand the nature of the hydrogen bonding among these conformers. The optimized geometrical parameters for conformer 1a at the B3LYP levels as well as the computed 1H NMR chemical shifts for conformer 2b at the RHF/6-31+G*//B3LYP/6-31+G* level are in good agreement with previous experimental results. It is confirmed from these results that the inclusion of electron correlation is crucial to elucidate molecular properties for the intramolecular hydrogen bonding systems. We have also compared the molecular energies between two different conformations both with a hydrogen bond and with no hydrogen bond of a given molecule. In 1,2-dihydroxybenzene, the energy stabilized by hydrogen bonding is about 4 kcal/mol at both ab initio and DFT methods. However, the hydrogen bonding energies are different according to types of hydrogen bonding in 2-hydroxythiophenol:  about 1 kcal/mol for conformer 2a with the common type hydrogen bonding and 4 kcal/mol for conformer 2b with the π type hydrogen bonding.