The N—H···X (X = Cl, Br, and I) Hydrogen-Bonding Pattern in Aromatic Amides: A Crystallographic and ¹H NMR Study

Abstract

This paper addresses the stability of the intramolecular five- and six-membered N—H···X (X = Cl, Br, and I) hydrogen bonds in aromatic amides. The crystal structures of 11 chloro-, bromo-, or iodo-substituted aromatic amides are reported. It is revealed that five-membered N—H···X hydrogen bonding occurs for all the halogen atoms in the designed compounds. Introduction of a large aryl or alkyl unit increases the stability of the hydrogen bonding because its existence weakens the competitive intermolecular N—H···O═C hydrogen bonding. A stable six-membered N—H···Cl hydrogen bond is also exhibited in the crystal structure of N-triphenylmethyl 2-chlorobenzamide. However, its bromo- and iodo-substituted counterparts display only intermolecular N—H···O═C hydrogen bonding. 1H NMR experiments in CDCl3 for 18 halogenated isomeric N -phenyl benzamides indicate that in solution five-membered N—H···X (X = Cl, Br, and I) hydrogen bonds can also be formed, but no evidence for the existence of the six-membered one is observed. The results show that the five-membered N—H···X (X = Cl, Br, and I) hydrogen bond is generally more stable than its corresponding six-membered counterpart.