Dynamical hydrogen bond

Reaction of [Ir( -Cp )Cl2(/A-Cl)2]2 and pyrazole in the presence of potassium hydroxide leads to complex 105 characterized by the dynamic hydrogen bond between three pyrazole nuclei (86AGE1114). 3,5-Dimethylpyrazole in identical conditicHis produces 106 where two pyrazole nuclei share a proton and one is unprotonated. Addition of tetrafluoroboric acid to 106 yields 107, where the second proton is bonded to the nonchelated ligand. Addition of the third proton causes formation of [Ir( -Cp )(Hpz )3](BF4)2. 

From the thermodynamics of such dynamical hydrogen bonds , one may actually expect an activation enthalpy of long-range proton diffusion of not more than 0.15 eV, provided that the configuration O—H "0 is linear, for which the proton-transfer barrier vanishes at 0/0 distances of less than 250 pm. However, the mobility of protonic defects in cubic perovskite-type oxides has activation enthalpies on the order of 0.4—0.6 eV. This raises the question as to which interactions control the activation enthalpy of proton transfer. 

Water has C2v symmetry. In the gas phase, the measured O-H bonds are 0.957 A, and the H-O-H angle is 104.5° (12). Liquid water and ice have stmctures controlled by the formation of hydrogen bonds. These bonds make it possible for hydrogen ions to exchange among water molecules on the millisecond to picosecond time scale (13), depending on pH. The extensive and dynamic hydrogen bond networks account for many unusual properties of water and hydrated biomolecules (12). 

Ishi-i, T. Crego-Caiama, M. Timmerman, P. Reinhoudt, D.N. Shinkai, S. Bnantioselective formation of a dynamic hydrogen-bonded assembly based on chiral memory concept. J. Am. Chcm. Soc. 2002, 124 (49), 14631-14641, and references cited therein. 

Cardullo. F. Crego Calama, M. Snellink-Ruel, B.H.M. Weidmann, J.-L. Bielejewska, A. Fokkens. R. Nibber-ing. N.M.M. Timmerman. P. Reinhoudt, D.N. Covalent capture of dynamic hydrogen-bonded assemblies. Chem. Commun. 21000. 367-368. 

To these criteria may be added a dynamic hydrogen bonding one, considering for how long before a given moment and after it two water molecules fulfilled the geometric and energetic criteria. If the criteria are met only very transiently, a HB is not considered to exist (Malenkov et al. 1999). This criterion may be formulated as ... 

Prins, L.J., P. Timmerman, and D.N. Reinhoudt. 2001. Amplification of chirality The sergeants and soldiers principle applied to dynamic hydrogen-bonded assemblies, f Am Chem Soc 123 10153. 

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