Crystal structure analysis of eight 1,3-diaryl-1,3-propanedione enols has been accomplished with the aim of investigating in more detail the intramolecular hydrogen bond formed by the H-C=C-C=O fragment characterizing beta-diketone enols; structural data were correlated with spectroscopic parameters, that is IR nu(OH) stretching frequencies and H-1 NMR chemical shifts of the enolic proton. Present experimental data show that this hydrogen bond is characterized by the following interrelated features: (i) very short O- -O distances (2.432-2.554 angstrom); (ii) strong delocalization in the heteroconjugated fragment; (iii) lengthening of the O-H bond (to 1.20 angstrom); (iv) lowering of the nu(OH) frequencies (2566-2675 cm-1) and downfield shift of the enolic proton resonance (15.3-17.0 ppm). The data can bc interpreted by (and are in support of) the RAHB (resonance assisted hydrogen bond) model previously suggested (Gilli, G.; Bellucci, F.; Ferretti, V.; Bertolasi, V. J. Am. Chem. Soc. 1989, 111, 1023) for explaining the unusual features the hydrogen bond displays in these compounds. It is shown that the model is able to interpret fine structural details such as the general weakening of the intramolecular hydrogen bond caused by electron-donating 1,3-substituents or additional hydrogen bonds accepted by the carbonyl and the preference displayed by the proton for dwelling on the carbonyl oxygen having the smaller negative charge induced both by the nature of 1,3-substituents or by intermolecular hydrogen bonds or short contacts.
EVIDENCE FOR RESONANCE-ASSISTED HYDROGEN-BONDING .2. INTERCORRELATION BETWEEN CRYSTAL-STRUCTURE AND SPECTROSCOPIC PARAMETERS IN 8 INTRAMOLECULARLY HYDROGEN-BONDED 1,3-DIARYL-1,3-PROPANEDIONE ENOLS
BERTOLASI, Valerio;GILLI, Paola;FERRETTI, Valeria;GILLI, Gastone
1991
Abstract
Crystal structure analysis of eight 1,3-diaryl-1,3-propanedione enols has been accomplished with the aim of investigating in more detail the intramolecular hydrogen bond formed by the H-C=C-C=O fragment characterizing beta-diketone enols; structural data were correlated with spectroscopic parameters, that is IR nu(OH) stretching frequencies and H-1 NMR chemical shifts of the enolic proton. Present experimental data show that this hydrogen bond is characterized by the following interrelated features: (i) very short O- -O distances (2.432-2.554 angstrom); (ii) strong delocalization in the heteroconjugated fragment; (iii) lengthening of the O-H bond (to 1.20 angstrom); (iv) lowering of the nu(OH) frequencies (2566-2675 cm-1) and downfield shift of the enolic proton resonance (15.3-17.0 ppm). The data can bc interpreted by (and are in support of) the RAHB (resonance assisted hydrogen bond) model previously suggested (Gilli, G.; Bellucci, F.; Ferretti, V.; Bertolasi, V. J. Am. Chem. Soc. 1989, 111, 1023) for explaining the unusual features the hydrogen bond displays in these compounds. It is shown that the model is able to interpret fine structural details such as the general weakening of the intramolecular hydrogen bond caused by electron-donating 1,3-substituents or additional hydrogen bonds accepted by the carbonyl and the preference displayed by the proton for dwelling on the carbonyl oxygen having the smaller negative charge induced both by the nature of 1,3-substituents or by intermolecular hydrogen bonds or short contacts.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
