INTRAMOLECULAR N-H...O=C HYDROGEN-BONDING ASSISTED BY RESONANCE - INTERCORRELATION BETWEEN STRUCTURAL AND SPECTROSCOPIC DATA FOR 6 BETA-DIKETO-ARYLHYDRAZONES DERIVED FROM BENZOYLACETONE OR ACETYLACETONE

Crystal structure determinations of six beta-diketo-arylhydrazones hav e been accomplished with the aim of a detailed investigation of the na ture of the intramolecular hydrogen bond formed by the HN - N = C - C = O fragment in these compounds. The N...O hydrogen bond distances are found to be correlated with the values of the spectroscopic parameter s, i.e. IR nu(NH) stretching frequencies, H-1 NMR chemical shifts, lam bda(max) of the UV absorption of the charge-transfer (CT) band from th e hydrazone to the carbonyl group, and the Hammett constants, sigma(X) , of the substituent X on the N-phenyl group. Experiments show that th e hydrogen bond is characterized by the following interrelated feature s: ((i) very short N...O distances (2.551-2.600 angstrom); (ii) strong delocalization in the keto-hydrazone fragment; (iii) lowering of the nu(NH) frequencies (3002-3117 cm-1); (iv) H-1 NMR downfield shift of t he proton resonance (14.53-14.93 ppm); and (v) UV lambda(max)(CT) in t he range 370-406 nm. These findings are interpreted in terms of the RA HB (Resonance-Assisted Hydrogen Bonding) model [Gilli G., Bellucci F., Ferretti V., Bertolasi V., J. Am. Chem. Soc., 1989, 111, 1023]. We sh ow that the magnitude of the strong hydrogen bond is essentially deter mined by the degree of pi-delocalization within the keto-hydrazone het eroconjugated system and modulated by all others factors that can affe ct the degree of pi-conjugation, including inductive effects and non-b onded intermolecular interactions.