Barrier to phenyl rotation in poly( p-phenylene benzobisthiazole) from ab initio molecular orbital calculations

Abstract

Optimal molecular geometry and the barrier to internal phenyl rotation were obtained from ab initio molecular orbital calculations of model compounds of poly( p-phenylene benzobisthiazole). Bond lengths and angles are in good agreement with those found from a previous X-ray crystallographic study of a similar model compound. Based on the calculated torsional potential, the discrepancy between theoretical and experimental phenyl torsions is attributed to crystal packing forces. The calculated torsion barrier is three to five times greater than in previous semi-empirical AM1 molecular orbital calculations. Calculations were performed with the 6 – 31 G ∗ basis set and the rotation barrier was corrected for electron correlation employing second-order Møller-Plesset perturbation theory.