Competition between OH⋯O and multiple halogen-dipole interactions on the formation of intramolecular three-centred hydrogen bond in 3-acyl coumarins

This paper describes the synthesis and structural study in solution, by NMR, and in the solid state, by X-ray analysis, of 6-substituted (H, NO ₂, OCH₃, Cl, Br) 2-oxo-2H-chromene-3-carboxylic acid (2-hydroxy-ethyl) amides. The results were supported by ab initio calculations at RHF-631G** level of theory. The crystal structures of compounds 6-Cl and 6-Br show deviations from the predicted theoretical conformation and also from that observed in solution, pointing to the influence exerted by intermolecular interactions on the molecular structure and on intramolecular three-centred hydrogen bond formation (O2⋯H12⋯O1). The significance of the plethora of non-covalent interactions [C H⋯A (A = O, X, π), CO⋯CO, CO⋯π, C X⋯OC (X = halogen), Br⋯Br and ππ stacking] is discussed.