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-enamid e; (3) p-methoxyacetoacetanilide; (4) dibenzamide; (5) 3-methyl-4-(3'- methyl-1'-phenylpyrazol-5'-yl)-1- phenyl-4-pyrazolin-5-one; (6) 3-anil ino-2-chloro-1,4-naphthoquinone; (7) 3 methyl-5H-pyrazolo[4,3-d]-1,2,3 -triazin-4(3N)-one. 32 other structures of related compounds are surve yed in order to understand the reasons for the N-H ... O bond presenti ng a range of N ... O distances as wide as 2.70-3.05 Angstrom. Data in dicate that neutral intermolecular N-H ... O hydrogen bonds shorter th an 2.90 Angstrom can occur only when the NH and C=O groups are connect ed by a pi-conjugated system, which turns out to be increasingly deloc alized while the N ... O distance becomes shorter. All present finding s 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 t reatment of the strong intermolecular N-H ... O bond.