Effects of protonation on the conformational characteristics and geometry of the rod‐like benzobisoxazole polymers

Recent experiments on model compounds suggest that rodlike polybenzobisoxazole (PBO) and polybenzobisthiazole (PBT) chains are protonated when dissolved in highly acidic solvent. The PBO model compound can exist as a 2H⁺ ion, with one proton on each nitrogen atom, or (depending on the acidity of the medium) as a 4H⁺ ion, with two additional protons on the oxygen atoms. The PBT model compounds generally form 2H⁺ ions, owing to the lower basicity of sulfur atoms relative to oxygen atoms. In the present study, geometry‐optimized CNDO/2 calculations have been carried out in an attempt of predict the effect of protonation on the conformational characteristics and geometry of PBO model compounds. Values of the conformational energy vs. rotation of the endphenylenes about the heterocyclic group are calculated for cis‐PBO model compounds in the unprotonated form and as 2H⁺ and 4H⁺ ions. All three species prefer the coplanar conformation with maximum barriers, occurring at the perpendicular conformation, of approximately 8.4, 33.6, and 84.0 kJ mol⁻¹ for the unprotonated form, the 2H⁺ ion, and the 4H⁺ ion, respectively. Steric arguments would suggest that repulsions between the acidic protons and the ortho hydrogens on the phenylenes would render the coplanar conformation more repulsive than other orientations. However, detailed analysis of the optimized geometries reveals that the rotatable bond shortens with protonation, indicating an increased bond strength and, hence, increased conjugation energy.