Theoretical\nand 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\nis highly dependent on\nthe conformational propensity of the monomer units. Our goal is to\nsystematically quantify differences in the conformational propensities\nof aromatic oligourea foldamer units. Specifically, we investigate\nthe 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\nusing a combination of theoretical methods, and infrared and nuclear\nmagnetic resonance spectroscopies. Our results show variation in the\nconformational behavior upon adding methyl substituents on <i>N</i>,<i>N</i>′-diphenylurea, and varying the\nenvironments surrounding the compounds. Our energetic analyses and\nconformational distributions in the gas phase show predominance of\nthe <i>cis</i>–<i><i>trans</i></i> and <i>trans</i>–<i>trans</i> conformations\nfor <i>N</i>,<i>N</i>′-diphenylurea, while <i>cis</i>–<i>cis</i> conformation is favored\nfor <i>N</i>,<i>N</i>′-dimethyl-<i>N</i>,<i>N</i>′-diphenylurea. In solution,\nour 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\nconformational behavior in solution, with constant fluctuations between <i>cis</i>–<i>cis</i> and <i>cis</i>–<i>trans</i> conformations. Our detailed quantitative\nanalyses are an important aspect in fine-tuning desired conformations\nand dynamic properties of this class of oligomers by providing a molecular\nbasis for the behavior at the monomeric level.