Hydrogen-bonded systems mechanisms

E. R. Lippincott The proposed model is certainly empirical. However, the internuclear potential function used for the terms V1 and F2 may be derived from a quantum mechanical model which lends support to their use in such a treat-ment of hydrogen bond systems. Professor Pauling is quite right in suggesting that the terms Vx and F2 may include some electrostatic contribution, since it is known that the internuclear potential function used correlates properties fairly well for partial polar bonds. Nevertheless the fact that additional terms of the electrostatic type are not needed to describe a number of the important properties of hydrogen bond systems, suggests that the covalent, repulsion and dispersions energy contributions are more important than the electrostatic contribution. 

Precise quantum-mechanical calculations on real hydrogen-bonded systems are difficult but by various approximate methods useful calculations can now be done on model systems. They confirm our supposition of the importance of delocalization in short O-H O bonds. 

Within the quadrupolar hydrogen-bonding systems, three aspects are significant (1) the action of the association in diluted liquid solution, (2) the ordering in the solid state, most of all the mechanical properties of the associates, and (3) methods to investigate and prove the ordering process. The latter is strongly related to electric- and optical properties. 

Within the last 5 years a number of quantum mechanical calculations have been carried out, in particular for hydrogen-bonded systems such as HgO, NHg, and HF as solvents. Contrary to electrostatic models, MO calculations allow for the possibility of covalent bond formation and, consequently, constitute a fundamentally better approach to ion-molecule interactions. Some interesting results of recent model calculations, although qualitative in nature, are discussed in Section V. 

II. Quantum Mechanical Descriptions of Hydrogen-Bonded Systems... 

II. QUANTUM MECHANICAL DESCRIPTION OF HYDROGEN-BONDED SYSTEMS... 

Up to now there is still no complete information on the mechanisms of v(XH) stretching vibration band broadening in the infrared spectra of hydrogen-bonded systems. Among these systems the most intriguing seem to be those with strong... 

The EVB method proposed by Warshel [6] is an elegant and computationally very efficient method of describing the entire BO surface, thus allowing treatment of chemical reactions such as proton transfer in hydrogen bonds. It can also be used in vibrational analysis. In conjunction with the environment described at the molecular mechanics level it was the first QM/MM method. Vibrational analyses of hydrogen bonded systems and of enzymatic catalysis have a lot in common. 

Acyl-transfer of j8-lactam and acyclic substrates to active site Ser of the f-lactamase of Enterobacter cloacae P99 occurs with a unique mechanism for nucleophilic activation in which the phenolate of Tyr , stabilized in free enzyme by interaction with both the hydroxyl of Ser and by ionic interaction with protonated Ly , acts as general-base [25]. The unusual hydrogen bonding system which exists in free enzyme is thought to manifest as a ground state fractionation factor that is less than unity and account for the inverse solvent isotope effects around 0.7 that have been observed on kc/Km [25-27]. 

ZuNDEL, G., Meez, H., On the role of hydrogen bonds and hydrogen-bonded systems with large proton polarizability for mechanisms of proton activation and conduction in bacteriorhodopsin, Progr. Clin. Biol. Res., 1984, 164, 153-164. 

The most important component of the water trimer nonadditive energy is the induction interaction of the second order in V. Its simple mechanism is shown in Fig. 33.3 a permanent multipole moment on monomer A induces multipole moments on monomer B which in turn interact with the permanent multipole moments of monomer C. Higher orders involve interactions between induced moments. The nonadditive induction energy is in general the most important nonadditive component for hydrogen-bonded systems. As already mentioned, it is the only term used—and only in the asymptotic approximation, i.e. neglecting charge-overlap effects—in the polarizable empirical potentials. 

Fig. 17. Schematic diagram illustrating the hydrogen bond system of the zinc-bound water molecule, Ser-48 and His-51 and a possible mechanism for the proton release induced by NAD binding.

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