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Despite their structural similarities, ortho-phenylenes and 2,3-quinoxalinylenes (i.e., poly(quinoxaline-2,3-diyl)s), well known as foldamers and helical polymers, respectively, exhibit distinctly different conformational behavior. o-Phenylenes tend to fold into compact helices with every fourth ring stacked, whereas 2,3-quinoxalinylenes favor extended helices with no backbone stacking. To understand this difference, we have studied short o-arylenes with different sequences of benzene and pyrazine units. Through a combination of crystallography, variable-temperature NMR spectroscopy, and DFT calculations, we find that pyrazines favor extended helical conformations as a result of two effects. First, within an o-arylene architecture, pyrazines experience weaker arene–arene interactions. Cofacial packing of the rings is therefore less favorable. Second, bipyrazine units lead to an increase in vibrational entropy for extended conformers. Consequently, at higher temperatures (including room temperature), extended helices are favored for the heterocycle-containing systems. 

Chemical reactions that mimic the function of ATP hydrolysis in biochemistry are of current interest in nonequilibrium systems chemistry. The formation of transient bonds from these reactions can drive molecular machines or generate materials with time-dependent properties. While the behavior of these systems can be complicated, the underlying chemistry is often simple: they are therefore potentially interesting topics for undergraduate introductory organic chemistry students, combining state-of-the-art advances in systems chemistry with straightforward reactions. Here, a teaching experiment has been developed that explores the transient assembly of benzoic acid derivatives driven by carbodiimide hydration. Working in teams, students examine the formation and decomposition of anhydrides from two benzoic acids using a carbodiimide “fuel”. The students examine classic reaction kinetics of anhydride hydrolysis using two independent methods, NMR and IR spectroscopies. They then explore how the amount of carbodiimide affects the lifetimes of precipitates of benzoic anhydride as a simple example of out-of-equilibrium self-assembly.