Ab initio HF and DFT calculations of geometric structures and vibrational spectra of electrically conducting doped oligothiophenes J. Casado, V. Herna ´ndez, F.J. Ramı ´rez, J.T. Lo ´pez Navarrete * Departamento de Quı ´mica Fi ´ sica, Universidad de Ma ´laga. 29071-Ma ´laga, Spain. Abstract This article reports results of a theoretical study of the effect of ionization on the infrared spectra of a series of a,a 0 -dimethyl end-capped oligothiophenes with the purpose of obtaining a theoretical insight into the geometry and spectral changes ocurring on doping polythiophene. For that purpose we determined the structures, vibrational frequencies and infrared intensities of the neutral oligomers, their radical cations and their dications. Ab initio HF and DFT molecular orbital calculations were employed to assess the structural and spectral changes from neutral to charged states.  1999 Elsevier Science B.V. All rights reserved. Keywords: Oligothiophenes; Vibrational spectra; DFT 1. Introduction Polythiophene and its oligomers are amongst the most intensely studied conducting polymers because of their good enviromental stability in both the neutral and doped states [1,2]. Moreover, the a,a 0 -dimethyl substitution has recently proved very effective in enhancing chemical stability and crystallinity without damaging the electronic response of the resulting material [3]. The fundamental and applicative aspects of these materials require the accurate identification of their basic molecular dynamics and geometries. Of particular interest are the vibrational properties of these systems, because their alteration may be a prelude to new properties with potential technological applications. In this article the geometries and the infrared spectra of a series of a,a 0 -dimethyl end- capped oligothiophenes (a,a 0 -DMOT), from 2 to 6 thiophene units (see Fig. 1) are theoretically investi- gated in both neutral and positively charged states; the results are then compared with experimental data. 2. Methodology Density functional theory (DFT) using the 6-31G** basis set and the three-parameter compound function of Becke (B3LYP) has been used to calculate geome- tries, vibrational frequencies and infrared intensities. Calculations were also performed at the HF level for comparison purposes. We have used the independent particle model; the neutral and doubly ionized forms were treated as closed shell systems (RHF and B3LYP) and spin restricted (ROHF). The basis used in all calculations was the 6-31G**. We have initially limited our theoretical investigation to planar conformations. All computed structures were optimized under the constraints of (a) planarity and (b) all-anti conformations of the thio- phene units. Thus, the even-membered oligomers Journal of Molecular Structure (Theochem) 463 (1999) 211–216 THEOCH 5860 0166-1280/99/$ - see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S0166-1280(98)00416-3 * Corresponding author. Fax: +34-52-132000.