Synthesis, Structural Investigations, Hydrogen−Deuterium Exchange Studies, and Molecular Modeling of Conformationally Stablilized Aromatic Oligoamides

Abstract

Biasing the conformational preferences of aromatic oligoamides by internally placing intramolecular hydrogen bonds has led to a series of stably folded molecular strands. This article presents the results from extensive solid-state, solution, and computational studies on these folding oligomers. Depending on its backbone length, an oligoamide adopts a crescent or helical conformation. Surprisingly, despite the highly repetitive nature of the backbone, the internally placed, otherwise very similar intramolecular hydrogen bonds showed significantly different stabilities as demonstrated by hydrogen-deuterium exchange data. It was also observed that the hydrogen-bonding strength can be tuned by adjusting the substituents attached to the exterior of the aromatic backbones. Examining the amide hydrogen-deuterium exchange rates of trimers revealed that a six-membered hydrogen bond nearing the ester end is the weakest among all the four intramolecular hydrogen bonds of a molecule. This observation was verified by ab initio quantum mechanical calculations at the level of B3LYP/6-31G*. Such a "weak point" creates the "battle of the bulge" where backbone twisting is centered, which is consistently observed in the solid-state structures of the four trimer molecules studied. In the solid state, the oligomers assemble into interesting one-dimensional structures. A pronounced columnar packing of short oligomers (i.e., dimers, trimers, and tetramer) and channel-like, potentially ion-conducting stacks of longer oligomers (i.e., tetramer, pentamer, and hexamer) were observed.