Theoretical and Spectroscopic Analysis of <i>N</i>,<i>N</i>′‑Diphenylurea and <i>N</i>,<i>N</i>′‑Dimethyl‑<i>N</i>,<i>N</i>′‑diphenylurea Conformations

Abstract

Structural organization of macromolecules is highly dependent on the conformational propensity of the monomer units. Our goal is to systematically quantify differences in the conformational propensities of aromatic oligourea foldamer units. Specifically, we investigate the conformational propensities of <i>N</i>,<i>N</i>′-diphenylurea and <i>N</i>,<i>N</i>′-dimethyl-<i>N</i>,<i>N</i>′-diphenylurea in different media using a combination of theoretical methods, and infrared and nuclear magnetic resonance spectroscopies. Our results show variation in the conformational behavior upon adding methyl substituents on <i>N</i>,<i>N</i>′-diphenylurea, and varying the environments surrounding the compounds. Our energetic analyses and conformational distributions in the gas phase show predominance of the <i>cis</i>–<i><i>trans</i></i> and <i>trans</i>–<i>trans</i> conformations for <i>N</i>,<i>N</i>′-diphenylurea, while <i>cis</i>–<i>cis</i> conformation is favored for <i>N</i>,<i>N</i>′-dimethyl-<i>N</i>,<i>N</i>′-diphenylurea. In solution, our results support the <i>trans</i>–<i>trans</i> conformer as the predominant conformer for <i>N</i>,<i>N</i>′-diphenylurea, whereas the <i>cis</i>–<i>cis</i> and <i>cis</i>–<i>trans</i> forms are favored in <i>N</i>,<i>N</i>′-dimethyl-<i>N</i>,<i>N</i>′-diphenylurea. <i>N</i>,<i>N</i>′-Dimethyl-<i>N</i>,<i>N</i>′-diphenylurea also exhibits a more dynamic conformational behavior in solution, with constant fluctuations between <i>cis</i>–<i>cis</i> and <i>cis</i>–<i>trans</i> conformations. Our detailed quantitative analyses are an important aspect in fine-tuning desired conformations and dynamic properties of this class of oligomers by providing a molecular basis for the behavior at the monomeric level