A number of crystal structures of molecules where the pi-conjugated ... O=C-C=C-OH ...beta-diketone enol group is found to form intramolecular O-H ... O hydrogen bonds and for which H-1 NMR spectroscopic data were known are discussed, Five of these structures, determined by X-ray diffraction techniques, are reported and the other 42 were retrieved from the Cambridge Structural Database, It is shown that all the descriptors of hydrogen-bond strength [d(O ... O) shortening, increased enolic H-1 NMR chemical shift, delta(OH), and increased pi-delocalization of the hydrogen-bonded heteroconjugated fragment] are mutually and linearly intercorrelated according to the rules defined by RAHB (resonance-assisted hydrogen bonding), Such a model is found to be of general applicability to all intramolecular O-H ... O bonds observed in a variety of molecules of different complexity embedding the simple beta-diketone enol fragment and to be extensible to other hydrogen-bonded conjugated compounds such as ... O=C-C-C=C-C-OH ...beta-diketone enols and ... O=C-C-N-OH ...alpha-keto-oximes, The proton chemical shifts, delta(OH), measured in CDCl3 solutions are found to depend strongly on the O ... O contact distances going from 8.6-10.1 ppm for weak non-resonant [2.59 less than or equal to d(O ... O) less than or equal to 2.64 Angstrom] to 14.9-19.0 ppm for the strongest resonant hydrogen bonds [2.41 less than or equal to d(O ... O) less than or equal to 2.55 Angstrom], Comparison with H-1 NMR data obtained in the solid-state shows a strictly similar dependence of delta(OH) on d(O ... O), irrespective of the very different experimental conditions and in spite of the fact that solution and solid-state values concern intramolecular and intermolecular hydrogen bonds, respectively.
Intramolecular O-H...O hydrogen bonds assisted by resonance. Correlation between crystallographic data and 1H NMR chemical shifts
BERTOLASI, Valerio;GILLI, Paola;FERRETTI, Valeria;GILLI, Gastone
1997
Abstract
A number of crystal structures of molecules where the pi-conjugated ... O=C-C=C-OH ...beta-diketone enol group is found to form intramolecular O-H ... O hydrogen bonds and for which H-1 NMR spectroscopic data were known are discussed, Five of these structures, determined by X-ray diffraction techniques, are reported and the other 42 were retrieved from the Cambridge Structural Database, It is shown that all the descriptors of hydrogen-bond strength [d(O ... O) shortening, increased enolic H-1 NMR chemical shift, delta(OH), and increased pi-delocalization of the hydrogen-bonded heteroconjugated fragment] are mutually and linearly intercorrelated according to the rules defined by RAHB (resonance-assisted hydrogen bonding), Such a model is found to be of general applicability to all intramolecular O-H ... O bonds observed in a variety of molecules of different complexity embedding the simple beta-diketone enol fragment and to be extensible to other hydrogen-bonded conjugated compounds such as ... O=C-C-C=C-C-OH ...beta-diketone enols and ... O=C-C-N-OH ...alpha-keto-oximes, The proton chemical shifts, delta(OH), measured in CDCl3 solutions are found to depend strongly on the O ... O contact distances going from 8.6-10.1 ppm for weak non-resonant [2.59 less than or equal to d(O ... O) less than or equal to 2.64 Angstrom] to 14.9-19.0 ppm for the strongest resonant hydrogen bonds [2.41 less than or equal to d(O ... O) less than or equal to 2.55 Angstrom], Comparison with H-1 NMR data obtained in the solid-state shows a strictly similar dependence of delta(OH) on d(O ... O), irrespective of the very different experimental conditions and in spite of the fact that solution and solid-state values concern intramolecular and intermolecular hydrogen bonds, respectively.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
