Synthesis, Structural Investigations, Hydrogen−Deuterium Exchange Studies, and Molecular Modeling of Conformationally Stablilized Aromatic Oligoamides
journal contributionposted on 2010-04-28, 00:00 authored by Yan Yan, Bo Qin, Changliang Ren, Xiuying Chen, Yeow Kwan Yip, Ruijuan Ye, Dawei Zhang, Haibin Su, Huaqiang Zeng
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.