Covalent versus Electrostatic Nature of the Strong Hydrogen Bond: Discrimination among Single, Double, and Asymmetric Single-Well Hydrogen Bonds by Variable-Temperature X-ray Crystallographic Methods in β-Diketone Enol RAHB Systems

β-Diketone enols are known to form intramolecular ···OC−CC−OH··· resonance-assisted
hydrogen bonds (RAHBs) with O···O distances as short as 2.39−2.44 Å. However, even the most accurate
diffraction studies have not been able to assess with certainty whether these very strong hydrogen bonds
(H-bonds) are to be described as proton-centered O···H···O bonds in a single-well (SW) potential or as
the dynamic or static mixing of two O−H···O ⇌ O···H−O tautomers in a double-well (DW) one. This
contribution reexamines the problem and shows that diffraction methods are fairly able to assess the SW
or DW nature of the H-bond formed and, in the second case, its dynamic or static nature, provided a
Bayesian approach is used which associates a number of experimental techniques (X-ray crystallography
at variable temperature, difference Fourier maps, least-squares refinement of proton populations, Hirshfeld's
rigid-bond test) with a reasonable prior, that is the full set of possible proton-transfer (PT) pathways for the
O−H···O system derived from theoretical calculations. The method is first applied to three β-diketone enols,
whose crystal structures were determined in the interval of temperatures 100−295 K and then generalized
to the interpretation of a much wider set of β-diketone enol structures derived from the literature, making
it possible to establish a general relationship between chemical structure (symmetric or dissymmetric
substitution, steric compression or stretching, increased π-bond delocalizability), H-bond strength, and the
shape of the PT-barrier. Final results are interpreted in terms of simplified VB theory and state-correlation
(or avoided-crossing) diagrams.