Intramolecular O–H · · · O hydrogen bonds assisted by resonance. Correlation between crystallographic data and 1H NMR chemical shifts

Abstract

A number of crystal structures of molecules where the π-conjugated · · · O C–CC–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 · · · OC–C C–CC–OH · · · δ-diketone enols and · · · OC –CN–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.