Crystal structure determinations of six beta-diketo-arylhydrazones have been accomplished with the aim of a detailed investigation of the nature 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 parameters, i.e. IR nu(NH) stretching frequencies, H-1 NMR chemical shifts, lambda(max) of the UV absorption of the charge-transfer (CT) band from the hydrazone to the carbonyl group, and the Hammett constants, sigma(X), of the substituent X on the N-phenyl group. Experiments show that the hydrogen bond is characterized by the following interrelated features: ((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 the proton resonance (14.53-14.93 ppm); and (v) UV lambda(max)(CT) in the range 370-406 nm. These findings are interpreted in terms of the RAHB (Resonance-Assisted Hydrogen Bonding) model [Gilli G., Bellucci F., Ferretti V., Bertolasi V., J. Am. Chem. Soc., 1989, 111, 1023]. We show that the magnitude of the strong hydrogen bond is essentially determined by the degree of pi-delocalization within the keto-hydrazone heteroconjugated system and modulated by all others factors that can affect the degree of pi-conjugation, including inductive effects and non-bonded intermolecular interactions.
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
BERTOLASI, Valerio;NANNI, Luca;GILLI, Paola;FERRETTI, Valeria;GILLI, Gastone;
1994
Abstract
Crystal structure determinations of six beta-diketo-arylhydrazones have been accomplished with the aim of a detailed investigation of the nature 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 parameters, i.e. IR nu(NH) stretching frequencies, H-1 NMR chemical shifts, lambda(max) of the UV absorption of the charge-transfer (CT) band from the hydrazone to the carbonyl group, and the Hammett constants, sigma(X), of the substituent X on the N-phenyl group. Experiments show that the hydrogen bond is characterized by the following interrelated features: ((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 the proton resonance (14.53-14.93 ppm); and (v) UV lambda(max)(CT) in the range 370-406 nm. These findings are interpreted in terms of the RAHB (Resonance-Assisted Hydrogen Bonding) model [Gilli G., Bellucci F., Ferretti V., Bertolasi V., J. Am. Chem. Soc., 1989, 111, 1023]. We show that the magnitude of the strong hydrogen bond is essentially determined by the degree of pi-delocalization within the keto-hydrazone heteroconjugated system and modulated by all others factors that can affect the degree of pi-conjugation, including inductive effects and non-bonded intermolecular interactions.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
