Intermolecular N-H center dot center dot center dot 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 pi-conjugated systems are reported: (1) 2-methyl-N-phenylprop-2-enamide; (2) 2-methyl-N-benzylprop-2-enamide; (3) p-methoxyacetoacetanilide; (4) dibenzamide; (5) 3-methyl-4-(3'-methyl-1'-phenylpyrazol-5'-yl)-1- phenyl-4-pyrazolin-5-one; (6) 3-anilino-2-chloro-1,4-naphthoquinone; (7) 3 -phenethyl-7-methyl-5H-pyrazolo[4,3-d]-1,2,3-triazin-4(3N)-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 Angstrom. Data indicate that neutral intermolecular N-H ... O hydrogen bonds shorter than 2.90 Angstrom can occur only when the NH and C=O groups are connected by a pi-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 (resonance-assisted hydrogen bonding) model originally developed to explain strong hydrogen-bond formation in beta-diketone enols and contribute to extending this model to the treatment of the strong intermolecular N-H ... O bond.