Evidence for resonance-assisted hydrogen bonding. 4. Covalent nature of the strong homonuclear hydrogen bond. Study of the O-H--O system by crystal structure correlation methods

Abstract

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.

A neutron scattering study of strong-symmetric hydrogen bonds in potassium and cesium hydrogen bistrifluoroacetates: Determination of the crystal structures and of the single-well potentials for protons

“…In particular, current controversies are focused on very strong hydrogen bonds ͑VSHBs͒ OHO whose lengths are shorter than Ϸ2.45 Å. [6][7][8][9][10] These bonds exhibit exceptional physical and chemical properties. In addition, the possible role of VSHBs in enzymatic catalysis broadens their potential significance.…”

“…Gilli and co-workers, [8][9][10] or Tian and Li 23 have suggested a resonance between covalent structures ͑e.g., O u H¯O ↔ O¯H u O͒, but Sørensen et al 24 have opposed a counterexample to this view.…”

How resonance assists hydrogen bonding interactions: An energy decomposition analysis

“…15,17 LBHBs are assumed to be largely covalent. 18 DHBs are designated as XÀ ÀHÁÁÁHÀ ÀM where X is an electronegative atom such as O or N, and M is a transition metal or boron. DHBs have been found in new types of complexes, posing challenging problems in chemistry, and their strength is often larger than 10 kcal/mol.…”

DFT studies on tautomeric preferences of 1-(pyridin-2-yl)-4-(quinolin-2-yl)butane-2,3-dione in the gas phase and in solution

“…Ring A is fully aromatic both in the OEa and OOa molecules. Relatively high HOMA values for the quasirings B (0) prove that intramolecular hydrogen bonds in OEa, OOa, and EEa are of Resonance-Assisted Hydrogen Bond (RAHB) type [20][21][22][23].…”