Steric Effect Determines the Formation of Lactam–Lactam Dimers or Amide C═O···NH (Lactam) Chain Motifs in N-Phenyl-2-hydroxynicotinanilides

A series of N-phenyl-2-hydroxynicotinanilides (PHNAs) have been synthesized, and their crystal structures have been investigated. The identity of these compounds is revealed to be the tautomer of PHNAs, i.e., 2-oxo-1,2-dihydropyridine-3-carboxylic acid phenylamides, compounds with both amide and lactam functionalities. Among these compounds, those with no or negligible steric hindrance take a near planar conformation and form lactam–lactam dimers, not the widely observed amide NH···O═C hydrogen bond chains, while those with steric hindrance take a twisted conformation and form an amide NH···O═C (lactam) hydrogen bond. A succinct theoretic study combining conformational analysis and molecular electrostatic potential surfaces (MEPS) survey provided a reasonable explanation of the formation of these two different hydrogen bond motifs.