Intermolecular N-H...O Hydrogen Bonds Assisted by Resonance. Heteroconjugated Systems as Hydrogen-Bond-Strengthening Functional Groups

摘要

The crystal structures of seven compounds forming intermolecular N-H...O hydrogen bonds connected by -conjugated systems are reported: (1) 2-methyl-N-phenylprop-2-enamide; (2) 2-methyl-A/-benzylprop-2-en-amide; (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,4-naphthoquinone; (7) 3-phenethyl-7-methyl-5//-pyrazo-lo[4,3-d]-l,2,3-triazin-4(3#)-one. 32 other structures of related compounds are surveyed in order to understand the reasons for the N-H- o 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 A can occur only when the NH and C=O groups are connected by a -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 f5-diketone enols and contribute to extending this model to the treatment of the strong intermolecular N-H- o O bond.