Monomeric water molecule

Stevenson (141) has tried recently to estimate the number of monomers in liquid water by three different approaches. He concludes that the concentration of monomers (non-hydrogen bonded water molecules) is less than 1% between 0° and 100°C. This estimate contrasts with recent theoretical values (107, 112). However, more recent studies (67, 68, 86) give lower monomer concentrations than the earlier estimates but still considerably higher values than Stevensons. The only current theory of water which would imply small concentrations of monomers is Pople s. Stevenson s study may well be subject to some criticism—i.e., the assumption that water in carbon tetrachloride should represent freely rotating monomeric water molecules. Stevenson s thermodynamic considerations may also be questioned. 

In water-organic solvent mixtures the decrease in the water concentration is generally accompanied by degradation of the water associates and hence by the formation of smaller associates, which finally leads to the liberation of monomeric water molecules. It must be noted, however, that this process does not result in enhancement of the donor properties of the water in every system. For example, Moreau and Douheret [Mo 74] reported that, in a water-acetonitrile solvent mixture, the breakdown of the water structure as a consequence of the addition of acetonitrile is not associated with an increase in the solvation of the proton. 

Regarding the first condition, there is no general rule that guarantees that the frozen-in term is normal. In the simplified ad hoc TSMs, this assumption is quite natural. For instance, if one species is ice-like and the second one is, say, monomeric water molecules, then it is natural to assume that each of the partial molar volumes has a normal temperature dependence. 

The idea that a solute changes the structure of the solvent is very old. As an example of the application of this idea, we refer to Chad well s explanation of the following puzzling observation (Chadwell, 1927) The addition of solutes such as ether or methyl acetate to water was found to decrease the compressibility of the system, in spite of the fact that the compressibilities of these pure solutes are about three times larger than the compressibility of pure water. It has been postulated that water contains two species, say monomeric water molecules and polymers of water molecules. Addition of a solute causes a shift toward the component that has a lower compressibility hence a qualitative explanation of the experimental observation is provided. Similar attempts to explain the effect of solute on viscosity, dielectric relaxation, self-diffusion, and many other properties have been suggested in the literature. 

Nevertheless, these experiments also indicate that it is possible to cover the entire concentration range from almost pure water containing IL ions at infinite dilution to pure ILs containing monomeric water molecules dispersed in their nanostructured liquid phases. 

There is much discussion on the nature of the aluminum species present in slightly acidic and basic solutions. There is general agreement that in solutions below pH 4, the mononuclear Al " exists coordinated by six water molecules, ie, [ ( 20) ". The strong positive charge of the Al " ion polarizes each water molecule and as the pH is increased, a proton is eventually released, forming the monomeric complex ion [A1(0H)(H20) ]. At about pH 5, this complex ion and the hexahydrated Al " are in equal abundance. The pentahydrate complex ion may dimerize by losing two water molecules... 

See Nakahara, M. Wakai, C. Chem. Lett., 1992, 809 for a discussion of monomeric and cluster states of water molecules in organic solvents due to hydrogen bonding. 

EPR and proton ENDOR measurements on Cu(II)-doped zinc acetate have been reported by Atherton and Horsewill215). In contrast to the pure copper salt, which consists of copper pairs, zinc acetate has a monomeric structure in which two water molecules are coordinated to each zinc ion216), as illustrated in Fig. 40. The magnetic parameters are summarized in Table 13. Both the g and ACu tensor obtained from single crystal and... 

Monitoring of acetaldehyde-induced polymerization of catechin and epicatechin by HPLC-MS demonstrated the formation of several methylmethine-linked flavanol dimers, trimers, and tetramers. Detection of the intermediate ethanol adducts confirmed the mechanism postulated by Timberlake and Bridle, which involves protonation of acetaldehyde in the acidic medium, followed by nucleophilic attack of the resulting carbocation by the flavan unit. The ethanol adduct then loses a water molecule and gives a new carbocation that undergoes nucleophilic attack by another flavanol molecule. Four dimers (C6-C6, C8-C8, and C6-C8, R and S) were formed from each monomeric flavanol. When both epicatechin and catechin units were present, additional isomers containing both types of units were... 

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