N-H…X (X = F, Cl, Br, and I) hydrogen bonding in aromatic amide derivatives in crystal structures

Abstract: Intramolecular N-H…X (X = F, Cl, Br, and I) hydrogen bonding patterns of aromatic amides in the solid state are summarized. It is revealed that the key for the formation of this kind of weak intramolecular hydrogen bonding in X-ray crystal structures is to suppress the competition of strong intermolecular N-H…O-C hydrogen bonding of the amide unit. For amides with identical backbones, the bonding capacity of halogen atoms as hydrogen bonding acceptors is in the order of F>Cl>Br>I, which is in accordance with their electronegativity strength. Generally, the five-membered hydrogen bonding is easier to form than the six-membered one.