Intramolecular N-H---O Hydrogen Bonding Assisted by Resonance. Part 2. Intercorrelation between Structural and Spectroscopic Parameters for Five 1,3-Diketone Arylhydrazones Derived from Dibenzoylmethane.

The crystal structures of five propane-1,2,3-trionearylhydrazones are reported. All molecules are chelated to form a six-membered pi-conjugated ring via strong intramolecular N-H...O hydrogen bonding. The N...O hydrogen bond distances are correlated with the resonance entity within the ring and with spectroscopic data such as nu(NH)IR frequencies delta(NH) H-1 NMR chemical shifts and lambda(max) UV absorption bands of charge transfer from hydrazone to carbonyl group. The structural and spectroscopic variations of the hydrogen bond parameters are modulated by the electronic properties of substituents on the aryl group in the sense that electron donating groups produce the strongest hydrogen bonds. The intercorrelation between N...O hydrogen bond strength and pi delocalization in all the structures of this class retrieved from the Cambridge Structural Database shows that the interplay between pi resonance and hydrogen bond magnitude, which we have called Resonance Assisted Hydrogen Bonding (RAHB), is a general phenomenon in the whole class of 1,3-diketone aryihydrazones.