Intermolecular N–H...O hydrogen bonds assisted by resonance. Heteroconjugated systems as hydrogen-bond-strengthening functional groups

The crystal structures of seven compounds forming intermolecular N--H..-O hydrogen bonds connected by 7r-conjugated systems are reported: (1) 2-methyl-Nphenylprop-2-enamide; (2) 2-methyl-N-benzylprop-2-enamide; (3) p-methoxyacetoacetanilide; (4) dibenzamide; (5) 3-methyl-4-(3'-methyl- l'-phenylpyrazol-5'-yl)- 1 phenyl-4-pyrazolin-5-one; (6) 3-anilino-2-chloro-l,4naphthoquinone; (7) 3-phenethyl-7-methyl-5H-pyrazolo[4,3-d]-l,2,3-triazin-4(3H)-one. 32 other structures of related compounds are surveyed in order to understand the reasons for the N--H...O bond presenting a range of N--.O distances as wide as 2.70-3.05 A. Data indicate that neutral intermolecular N--H...O hydrogen bonds shorter than 2.90,~, can occur only when the NH and C---O groups are connected by a 7r-conjugated system, which turns out to be increasingly delocalized while the N...O distance becomes shorter. All present findings are in agreement with the RAHB (resonanceassisted hydrogen bonding) model originally developed to explain strong hydrogen-bond formation in /3diketone enols and contribute to extending this model to the treatment of the strong intermolecular N--H...O bond.