Ab initio studies on heterocyclic conjugated polymers: Structure and vibrational spectra of pyrrole, oligopyrroles, and polypyrrole

We report the results of a series of ab initio calculations on the structure and on the vibrational spectra of pyrrole and pyrrole oligomers performed with double zeta quality basis sets. More extended basis sets and the averaged coupled pair functional method were applied for the pyrrole monomer. The effect of basis set extension and the influence of electron correlation on the computed equilibrium geometry and on the harmonic force constants is discussed. We calculated the torsional potential of 2,2′-bipyrrole at the self-consistent field level and characterized three stationary points including full vibrational analysis within the harmonic approximation. Geometry optimizations were also performed for the planar all-antitrimer and the planar all-antitetramer. The in-plane force field of the all-antitrimer was calculated too. The computed vibrational frequencies of the pyrrole molecule, of the 2,2′-bipyrrole conformers, and of terpyrrole 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 polypyrrole.