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Issue 5, 1997
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IntramolecularO–H · · · Ohydrogenbonds assisted by resonance. Correlation between crystallographic dataand 1H NMR chemical shifts

Abstract

A number of crystal structures of molecules where the π-conjugated · · · O [double bond, length as m-dash]C–C[double bond, length as m-dash]C–OH · · · β-diketone enol group is found to form intramolecular O–H · · · O hydrogen bonds and for which 1H 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 1H NMR chemical shift, δ(OH), and increased π-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 β-diketone enol fragment and to be extensible to other hydrogen-bonded conjugated compounds such as · · · O[double bond, length as m-dash]C–C [double bond, length as m-dash]C–C[double bond, length as m-dash]C–OH · · · δ-diketone enols and · · · O[double bond, length as m-dash]C –C[double bond, length as m-dash]N–OH · · · α-keto-oximes. The proton chemical shifts, δ(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 ⩽ d(O  · · · O) ⩽ 2.64 Å] to 14.9–19.0 ppm for the strongest resonant hydrogen bonds [2.41 ⩽ d(O  · · · O) ⩽ 2.55 Å]. Comparison with 1H NMR data obtained in the solid-state shows a strictly similar dependence of δ(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.

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Article type: Paper
DOI: 10.1039/A606862F
Citation: J. Chem. Soc., Perkin Trans. 2, 1997,0, 945-952
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    Intramolecular O–H · · · O hydrogen bonds assisted by resonance. Correlation between crystallographic data and 1H NMR chemical shifts

    V. Bertolasi, P. Gilli, V. Ferretti and G. Gilli, J. Chem. Soc., Perkin Trans. 2, 1997, 0, 945
    DOI: 10.1039/A606862F

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