Internal rotations of aromatic polyamides: a density functional theory study

Internal rotations of benzanilide (BA) and 4-(4'-aminobenzamido)benzoic acid (AA) were investigated by density functional theory (DFr) calculations. B3LYP/6-31G* optimization for both BA and AA structures gives non-planar trans structures as the most stable conformers with lower energy of 4.60 and 5.08 kcal/mol than cis ones, respectively. The amide bond and aniline moiety are found to be coplanar in trans BA, while in trans phenyl benzoate (PB) the ester bond and benzoyl moiety are coplanar. The relaxed potential energy surface (PES) scans were then carried out with rotations of three single bonds, i.e. amide bond and both adjacent bonds. The discontinuous point is found on the relaxed PES for the amide bond rotation. This indicates that inversion of a pyramidal amino group is involved with the amide bond rotation. Therefore, two transition states (TSs) arise for rotation around the amide bond. Two TS structures (TS-1 and TS-2) were optimized for both BA and AA, and their activation energies were estimated as 14.34 kcal/mol (TS-1) and 16.27 kcal/mol (TS-2) for BA, and 12.20 kcal/mol (TS-1) for AA, respectively. The TS-2 structure for AA failed to be optimized. The activation energy for the amide bond rotation, which is larger than that of 7.90 kcal/mol for PB, as well as the coplanarity in aromatic amide is ascribed to the partial double bond character of amide bond. This is also confirmed by the Wiberg bond index (bond order). The chain persistence length for poly(4-benzamide) was estimated by the rotation matrix formalism using the calculated structural parameters of trans AA. The estimated value of 1131 angstrom is longer than our previously calculated value of corresponding aromatic polyester, 364 angstrom for poly(p-hydroxybenzoic acid) [T. Imase, S. Kawauchi, J. Watanabe, Macromol. Theory Simul. 10 (2001) 434]. (c) 2005 Elsevier B.V. All rights reserved.