Internal rotation of ester linkage in phenyl benzoate and hydroxybenzoic acid dimer as models of aromatic polyesters using density functional theory

The internal rotation of the ester linkage was reinvestigated more quantita tively by using the density functional theory (DFT) in order to understand the characteristic stiffness and extendedness of polymer chain found in aro matic polyesters. Phenyl benzoate (PB) and p-hydroxybenzoic acid (HBA) dime r (HB) were selected as models of aromatic polyesters. The relaxed potentia l energy surface (PES) scan was carried out along the internal rotation of three bonds (denoted as R, S and T, respectively) of the aromatic esters by using the hybrid DFT (B3LYP) with 6-31G* basis set. The rotation of S bond , which mainly determines the linearity of the molecule, leads to the trans - and cis-conformers of PB. Since the cis-conformer of PB is 7.69 kcal . mo l(-1) higher than the trans-conformer, the cis-conformer has little populat ion at standard condition. HE does not have the cis-conformer. In addition, the chain persistence length of 364 Angstrom is obtained by the rotation m atrix formalism using the structural parameters of HE. These agree with the experimental tal understanding that poly(p-hydroxybenzoic acid) is the cla ss of stiff and extended polymer.