We have designed, synthesized, and crystallized 36 compounds, each containing an azide group and an oxygen atom separated by three bonds. Crystal structure analysis revealed that each of these molecules adopts a conformation in which the azide and oxygen groups orient syn to each other with a short O⋅⋅⋅Nβ contact. Geometry-optimized structures [using M06-2X/6-311G(d,p) level of theory] also showed the syn conformation in all 36 of these cases, suggesting that this is not merely a crystal packing effect. Quantum topological analysis using Bader's Atoms in Molecules (AIM) theory revealed bond paths and bond critical points (BCP) in these structures suggesting its nature and energetics to be similar to weak hydrogen bonding. The NCI-RDG plot clearly revealed the attractive interaction consisting of electrostatic or dispersive components in all the 36 systems. NBO analysis suggested a weak orbital-relaxation (charge-transfer) contribution of energy for a few (sp2) O-donor systems. Natural population analysis (NPA) and molecular electrostatic potential mapping (MESP) of these crystal structures further revealed the existence of favorable azide-oxygen interaction. A CSD search indicated the frequent and consistent occurrence of this interaction and its role dictating the syn conformation of azide and oxygen in molecules where these groups are separated by 2-4 bonds.
Keywords: azide-oxygen interaction; conformation; crystal engineering; non-covalent interactions; supramolecular chemistry.
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