INTRAMOLECULAR N-H-CENTER-DOT-CENTER-DOT-CENTER-DOT-O HYDROGEN-BONDING ASSISTED BY RESONANCE .2. INTERCORRELATION BETWEEN STRUCTURAL AND SPECTROSCOPIC PARAMETERS FOR 5 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-conjuga ted ring via strong intramolecular N-H...O hydrogen bonding. The N...O hydrogen bond distances are correlated with the resonance entity with in the ring and with spectroscopic data such as nu(NH)IR frequencies d elta(NH) H-1 NMR chemical shifts and lambda(max) UV absorption bands o f charge transfer from hydrazone to carbonyl group. The structural and spectroscopic variations of the hydrogen bond parameters are modulate d by the electronic properties of substituents on the aryl group in th e sense that electron donating groups produce the strongest hydrogen b onds. 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 res onance and hydrogen bond magnitude, which we have called Resonance Ass isted Hydrogen Bonding (RAHB), is a general phenomenon in the whole cl ass of 1,3-diketone aryihydrazones.