CNDO/2 analyses of the order of protonation of rodlike polybenzobisoxazoles (PBO)

Protonation of the rodlike polybenzobisoxazoles (PBO) and polybenzobisthiazoles and their model compounds in acidic media will have significant effects on their solubility, solution behavior, geometry, and conformational characteristics. Experimental studies on the cis-PBO model compound suggest that the aromatic heterocyclic group can accept up to four acidic protons, first at each of the two nitrogen atoms then at the two oxygen atoms. In this study geometry-optimized CNDO/2 calculations were carried out on the cis-PBO model compound to determine the order of successive protonation, estimate differences in proton binding energies, and provide information on the effects of successive protonation on the geometry and charge distribution. The results indicate that protonation occurs in the order nitrogen, nitrogen, oxygen, oxygen, which is consistent with the greater bascity of nitrogen relative to oxygen. The difference in energy between initial protonation at nitrogen versus oxygen is ca. 60.7 kcal mol^-1. Second protonation is preferred at the other nitrogen rather than at the more distant oxygen by nearly the same amount, suggesting that energetically the initial protonation at the nitrogen has a negligible effect on subsequent protonation and that the two oxazole moieties are highly insulated from each other. Repulsive Coulombic interactions between the acidic protons have a negligible influence on whether the second protonation occurs at the nitrogen or the oxygen considered. Coulombic effects will in fact slightly favor at the nitrogen since, despite its closer proximity to the nitrogen first protonated, protonation there yields a more delocalized and hence less repulsive charge distribution than at the oxygen.