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All cases of strong (2.50 I d ( 0 --0 ) I 2.65 A) and very strong ( d ( 0 --0 ) < 2.50 A) 0-H---0 hydrogen bonds whose geometries are known from accurate neutron or X-ray diffraction studies are reviewed and classified in chemical classes belonging to three fundamental types: (A) -0-H---e-, or negative charge assisted hydrogen bonding, (-)CAHE (B) =0---H+--O=, or positive charge assisted hydrogen bonding, (+)CAHB; and ( C ) -0-H---O=, where the two oxygens are interconnected by a system of *-conjugated double bonds, or resonanceassisted hydrogen bonding, RAHB. An empirical model is discussed where the hydrogen bond energy is expressed as EHB = ECOV + EEL + EREP, Ecov being the energy of the covalent three-center-four-electron 0. -.H---:O bond, and EEL and EREP, the electrostatic attraction and interoxygen repulsion energies, respectively. By means of a detailed analysis of bond and contact distances, it is shown that, while the 0-0 distance is shortened from 2.80 to 2.40 A, the hydrogen bond is transformed from a dissymmetrical 0-H---0 electrostatic interaction to a covalent and symmetrical 0--H--0 bond. It is suggested that such behavior is common to all homonuclear hydrogen bonds (0-H---0, N-H---N, F-H---F), while heteronuclear ones (e.g. N-H---0) can only give weaker bonds of mostly electrostatic nature.M.

Unlike normal chemical bonds, hydrogen bonds (H-bonds) characteristically feature binding energies and contact distances that do not simply depend on the donor (D) and acceptor (:A) nature. Instead, their chemical context can lead to large variations even for a same donor-acceptor couple. As a striking example, the weak HO-H...OH(2) bond in neutral water changes, in acidic or basic medium, to the 6-fold stronger and 15% shorter [H(2)O...H...OH(2)](+) or [HO...H...OH](-) bonds. This surprising behavior, sometimes called the H-bond puzzle, practically prevents prediction of H-bond strengths from the properties of the interacting molecules. Explaining this puzzle has been the main research interest of our laboratory in the last 20 years. Our first contribution was the proposal of RAHB (resonance-assisted H-bond), a new type of strong H-bond where donor and acceptor are linked by a short pi-conjugated fragment. The RAHB discovery prompted new studies on strong H-bonds, finally leading to a general H-bond classification in six classes, called the six chemical leitmotifs, four of which include all known types of strong bonds. These studies attested to the covalent nature of the strong H-bond showing, by a formal valence-bond treatment, that weak H-bonds are basically electrostatic while stronger ones are mixtures of electrostatic and covalent contributions. The covalent component gradually increases as the difference of donor-acceptor proton affinities, DeltaPA, or acidic constants, DeltapK(a), approaches zero. At this limit, the strong and symmetrical D...H...A bonds formed can be viewed as true three-center-four-electron covalent bonds. These results emphasize the role PA/pK(a) equalization plays in strengthening the H-bond, a hypothesis often invoked in the past but never fully verified. In this Account, this hypothesis is reconsidered by using a new instrument, the pK(a) slide rule, a bar chart that reports in separate scales the pK(a)'s of the D-H proton donors and :A proton acceptors most frequently involved in D-H...:A bond formation. Allowing the two scales to shift so to bring selected donor and acceptor molecules into coincidence, the ruler permits graphical evaluation of DeltapK(a) and then empirical appreciation of the D-H...:A bond strength according to the pK(a) equalization principle. Reliability of pK(a) slide rule predictions has been verified by extensive comparison with two classical sources of H-bond strengths: (i) the gas-phase dissociation enthalpies of charged [X...H...X](-) and [X...H...X](+) bonds derived from the thermodynamic NIST Database and (ii) the geometries of more than 9500 H-bonds retrieved from the Cambridge Structural Database. The results attest that the pK(a) slide rule provides a reliable solution for the long-standing problem of H-bond-strength prediction and represents an efficient and practical tool for making such predictions directly accessible to all scientists.

The resonance-assisted hydrogen bond (RAHB) is a model of synergistic interplay between π-delocalization and hydrogen-bond (H-bond) strengthening originally introduced (Gilli, G.; Bellucci, F.; Ferretti, V.; Bertolasi, V. J. Am. Chem. Soc. 1989, 111, 1023; Bertolasi, V.; Gilli, P.; Ferretti, V.; Gilli, G. J. Am. Chem. Soc. 1991, 113, 4917) for explaining the abnormally strong intramolecular O−H···O bonds formed by the ···OC−CC−OH··· β-enolone fragment I which are typical of β-diketone enols. The applicability of this model to the intramolecular N−H···O hydrogen bonds formed by a number of heteroconjugated systems (···OC−CC−NH···, β-enaminones II; ···OC−CN−NH···, ketohydrazones III; and ···ON−CC−NH···, nitrosoenamines IV) is investigated. The X-ray crystal structures of five molecules which close a six-membered ring by an intramolecular N−H···O bond through the resonant ···OX−CX−NH··· (X = C, N) fragments II−IV are compared to those of two other molecules closing the same ring through the nonresonant ···OC−C−C−NH··· β-aminone moiety V. Experimental findings are complemented by a CSD (Cambridge Structural Database) search of all compounds forming intramolecular N−H···O bonds through the molecular fragments II−V and by a comprehensive analysis of the IR νNH stretching frequencies and 1H NMR δNH chemical shifts available for compounds of these classes of known crystal structure. It is shown that all the descriptors of H-bond strength [d(N···O) shorthening, decrease of νNH, increase of δNH, and increase of π-delocalization within the heteroconjugated fragment] are mutually intercorrelated according to RAHB rules, which can then account for the strength of heteronuclear N−H···O bonds in II−IV as well as for that of the homonuclear O−H···O bonds in I. Heteronuclear N−H···O bonds appear, however, to have distinctive features. In particular, their strength turns out to be partially hampered by the proton affinity difference (ΔPA) between the N and O atoms, so that very strong H-bonds (2.65 ≥ d(N···O) ≥ 2.48 Å, 3200 ≥ νNH ≥ 2340 cm-1, 13 ≤ δNH ≤ 18 ppm) can occur only when the π-delocalization of the heterodienic moiety is associated with proper electron-attracting substituents which are able to decrease this ΔPA by increasing the NH acidity. Moreover, at variance with strong O−H···O RAHBs, whose protons are mostly found in nearly symmetrical positions, even the strongest N−H···O RAHBs are highly dissymmetric, despite the very similar changes undergone by both IR and 1H NMR spectra in O−H···O and N−H···O H-bonded systems. Specificities of heteronuclear H-bonds are shown to be interpretable by the electrostatic-covalent H-bond model (ECHBM) which was previously developed for the homonuclear case (Gilli, P.; Bertolasi, V.; Ferretti, V.; Gilli, G. J. Am. Chem. Soc. 1994, 116, 909). The conclusions drawn are corroborated by extended DFT quantum-mechanical calculations at the B3LYP/6-31+G(d,p)//B3LYP/6-31+G(d,p) level of theory and by full geometry optimization carried out on 27 variously substituted heterodienes II−...

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