CONFORMATIONS AND ROTATIONAL BARRIERS OF AROMATIC POLYESTERS

An integrated approach employing ab initio, semiempirical (AM1), and force field (CVFF) methods to study torsional barriers in conjugated aromatic molecular systems is presented. It is the first time that such an attempt including full geometry optimization up to the ab initio level is reported. We have focused on monomer-like units of poly(p-hydroxybenzoic acid) (PHBA) and poly(ethylene terephthalate) (PET). Coupling between the torsional motions was studied with the semiempirical AM1 method as well as with the consistent valence force field. Molecular dynamics simulations were carried out on single chains; the relation between MD results and chain flexibility is discussed as well as the consequences of the uncertainty in barrier heights for the MD results and the calculated persistence length. The numerical results are compared with experimental data as far as such data are available. Large differences between various theoretical results are observed, indicating that there are problems with the modeling of rotational barriers in aromatic moieties containing substituents which are pi-conjugated to the aromatic ring. This as well as the lack of sufficient and accurate experimental data hampers progress in modeling the properties of the conjugated aromatic molecules investigated here.