Solvation hydrogen-bond formation

The purpose of this research was to compare the effect on the conformational equilibrium for the hydroxylated compounds listed in Table II of changing the solvent from dimethyl sulfoxide, which is expected to minimize intramolecular hydrogen bonding, to 1,2-dichloroethane which should promote such bonds. These solvation effects on conformational equilibria were then to be compared with those of water which can serve as a hydrogen donor and hydrogen acceptor in hydrogen bond formation. As will be seen, the conformational equilibria generally appear similar for water and dimethyl sulfoxide but often different from those in 1,2-dichloroethane. 

Modifications of the tautomeric equilibrium and therefore the pXa value, through hydrogen-bond formation and the electrostatic solvation effects of imidazole, are... 

Radical reactions could be classified in different ways. First of all, they could be selected according to the phase they are studied in. This review deals almost entirely with liquid-phase reactions. Certain important comparisons can actually be made between reactions proceeding in gas and liquid phases. In discussing Arrhenius parameters, the limited liquid phase data are complemented by those taken from gas phase reactions. On the other hand, special attention is given to solvation effects, in general, and to hydrogen bond formation, in particular. 

Solvent effects on the reaction of 0—H bonds and carbon radicals could, at least partly, be accounted for by considering initial and final state solvation. Since hydrogen-bond formation with an electron donor (but not proton donor) solvent can exist in the initial state, may exist in the transition state but cannot exist in the final state, a rate-retarding effect of hydrogen bonding is to be expected. In reactions of 0—H bonds with alkoxy or peroxy radicals, however, the solvent effect may be an indication for the existence of a long-lived intermediate. 

Figure 3 Overview of the receptor-ligand binding process. All species involved are solvated by water (symbolized by gray spheres). The binding free energy difference between the bound and unbound state is a sum of enthalpic components (breaking and formation of hydrogen bonds, formation of specific hydrophobic contacts), and entropic components (release of water from hydrophobic surfaces to solvent, loss of conformational mobility of receptor and ligand).

All of the nucleic acid bases have multiple sites for hydrogen bond formation. Explicit microsolvation may be appropriate for describing the important interactions between the base and its first solvation shell. The complicating factor, as is always true when attempting to carry out microsolvation studies, is how many solvent molecules to place about the substrate and what configurations should be sampled. For the nucleic acid bases, the many tautomeric forms available further add to the number of potential geometry optimizations that need to be performed. 

The strong anion-solvating power of water, methanol, and many other hydroxylic solvents is due to hydrogen bond formation. 

The above presentation shows that the detailed analysis of the dependence of AGfr on the mixed solvent composition and on the stability and concentration of various solvated reactant species may be quite complicated, especially in the case of labile ions. The free energy of ions is dependent not only on their interaction with the solvent and with other components of the mixture [252] (for instance, ions of background electrolyte), but also on the change in solution structure [252] and on the change in hydrogen bond formation [253]. 

TTie solution phase ionization potentials of Br in 16 solvents have been determined by photoeiectron emission spectroscopic technique. The values obtained as the threshold energy E for Br" in various solvents are found to be correlated well with the Mayer-Gutmann acceptor number of solvent. The reorganization energy AC, of solvent after the photoionization of Br" has been obtained from the E value. The AG, values are well reproduced by using a simple model which incorporates the dipole-dipole repulsion and the hydrogen-bond formation in the first solvation layer. The solvation structures of Br" determined by EXAFS are used for the AG, calculation. 

When liquids contain dissimilar polar species, particularly those that can form or break hydrogen bonds, the ideal liquid solution assumption is almost always invalid. Ewell, Harrison, and Berg provided a very useful classification of molecules based on the potential for association or solvation due to hydrogen bond formation. If a molecule contains a hydrogen atom attached to a donor atom (0, N, F, and in certain cases C), the active hydrogen atom can form a bond with another molecule containing a donor atom. The classification in Table... 

Figure 1.101 Illustration of hydrophobic effect, (a) water forms an imperfect ordered solvent cage around two hydrophobic entities (blue). Each water of solvation is prevented from forming four hydrogen bonds with neighbouring water molecules for steric reasons. Waters of solvation are excluded upon association of two hydrophobic entities (b). These increase system entropy by entering bulk solution and release enthalpy through enhanced hydrogen bond formation. Short range Van der Waals interactions between hydrophobic entities may also contribute to system enthalpy.

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