Conformations and rotational barriers of aromatic polyesters

This paper is concerned with the calculation of the rotational barriers in parahydroxybenzoic acid and polyethylene terephthalate. To compare the calculations with accurate experimental results, experimental gas-phase data of a series of representative aromatic fragments were used to test the theoretical methods employed. AM1 and MNDO-PM3 calculations are presented for these species; ab initio methods were tested by means of extensive calculations on benzaldehyde. It is concluded that in order to obtain satisfactory agreement with experimental data, the required level of sophistication of the ab initio calculations, e.g. calculations at the MCSCF level, becomes intractable for practical modelling purposes. Although the AM1 results are systematically too low, it is possible to introduce a scaling factor for the most difficult class of barriers to be modelled and reproduce the experimental results within 15%. Although this is a pragmatic approach rather than a fundamental one, it yields a very important and practical result for the modelling of polymers.