Ab initio studies on heterocyclic conjugated polymers: structure and vibrational spectra of thiophene, oligothiophenes and polythiophene

We report the results of a series of ab initio calculations on the structure and on the vibrational spectra of thiophene and thiophene oligomers as performed with double-zeta quality basis sets. Polarized basis sets and the averaged coupled pair functional (ACPF) method were applied for the thiophene monomer. We also calculated the torsional potential of 2,2'-bithiophene at the SCF level and characterized three stationary points including full vibrational analysis within the harmonic approximation. Geometry optimizations were also performed for the planar all-anti-trimer and the planar all-anti-tetramer. The in-plane force field of the all-anti-trimer was also calculated. The computed vibrational frequencies of the thiophene molecule, the 2,2'-bithiophene confonners and of terthiophene are compared to experimental data. The force fields of the oligomers and the trends in their chain length dependence were taken as the basis for the calculation of the vibrational frequencies and the phonon dispersion curves of the infinitely extended polythiophene.