Water solvation shell

Figure 3.46 Time dependence of the nuclear kinetic energy of an azo chromophore after ri tt excitation, (a) Azomethane in water solution, (b) Azobenzene attached to two cyclopeptide rings, with one water solvation shell. The kinetic energy is given as the absolute temperature (see text).

Unique properties of mosaic carboaerosils were pointed out in the further studies on carbon-silica adsorbents by H NMR [56]. Thickness of benzene and water solvation shells near the surface of three adsorbents (Table 2) differing in adsorption site topography and their chemical nature was studied. 

The FI2O molecules of these aquo-complexes constitute the iimer solvation shell of the ions, which are, in turn, surrounded by an external solvation shell of more or less uncoordinated water molecules fomiing part of the water continuum, as described in section A2.4.2 above. Owing to the difference in the solvation energies,... 

Consider now the aquo-complexes above, and let v be the distance of the centre of mass of the water molecules constituting the iimer solvation shell from the central ion. The binding mteraction of these molecules leads to vibrations... 

Modem understanding of the hydrophobic effect attributes it primarily to a decrease in the number of hydrogen bonds that can be achieved by the water molecules when they are near a nonpolar surface. This view is confirmed by computer simulations of nonpolar solutes in water [15]. To a first approximation, the magnimde of the free energy associated with the nonpolar contribution can thus be considered to be proportional to the number of solvent molecules in the first solvation shell. This idea leads to a convenient and attractive approximation that is used extensively in biophysical applications [9,16-18]. It consists in assuming that the nonpolar free energy contribution is directly related to the SASA [9],... 

The substituent effects on the H-bonding in an adenine-uracil (A-U) base pair were studied for a series of common functional groups [99JPC(A)8516]. Substitutions in the 5 position of uracil are of particular importance because they are located toward the major groove and can easily be introduced by several chemical methods. Based on DFT calculation with a basis set including diffuse functions, variations of about 1 kcal/mol were found for the two H-bonds. The solvent effects on three different Watson-Crick A-U base pairs (Scheme 100) have been modeled by seven water molecules creating the first solvation shell [98JPC(A)6167]. 

Micelles and reversed micelles are able to solubilize substances which are insoluble in the bulk phase of the system considered. This solubilization is due to a solvation by the amphiphile and concomitantly a change in the order of the solubilized molecules may occur as a consequence of its modified solvation shell. In this sense reversed micelles of detergents in hydrophobic solvents with solubilized water in the core are... 

The mode of extraction in these oxonium systems may be illustrated by considering the ether extraction of iron(III) from strong hydrochloric acid solution. In the aqueous phase chloride ions replace the water molecules coordinated to the Fe3+ ion, yielding the tetrahedral FeCl ion. It is recognised that the hydrated hydronium ion, H30 + (H20)3 or HgO,, normally pairs with the complex halo-anions, but in the presence of the organic solvent, solvent molecules enter the aqueous phase and compete with water for positions in the solvation shell of the proton. On this basis the primary species extracted into the ether (R20) phase is considered to be [H30(R20)3, FeCl ] although aggregation of this species may occur in solvents of low dielectric constant. 

Thus the coefficient can be evaluated from and aY- The approximate value of p (= 9.4 X 10 molecules m ) is obtained from the density of bulk water by assuming that the thickness of the first solvation shell equals the diameter (0.28 nm) of a water molecule,... 

The chemical shieldings were then recalculated in this same system using the QM/MM method [10], To this end, each molecule was considered individually. The water molecule of interest and its first solvation shell were treated quantum mechanically, whereas the surrounding water molecules were taken into account with an empirical force field representation (MM molecules). The first solvation shell was defined via a distance criterion on the oxygen-oxygen distance. As a threshold, the first minimum of the O-O pair correlation function was taken this occurs at 3.5 A [93]. All... 

These QM/MM calculations are in contrast to a standard evaluation of chemical shielding for gas phase water clusters where the classical point charge environment is omitted entirely. The same solvation shell criterion as before was applied, and the system was treated purely quantum mechanically. 

For a water molecule with its first solvation shell only, chemical shielding differs considerably from the complete ab initio calculation. Although this discrepancy is not completely removed by the inclusion of the electrical field of the remaining molecules through the QM/MM approach, it is strongly reduced. 

The vertical excitation energies were calculated for different configurations of a QM/MM trajectory using the approximate ROKS [27] method as well as TDDFT. The effect of the size of the quantum region was tested systematically by including (i) only the solute in the quantum region or (ii) the solute and its first solvation shell (defined as the 12 water molecules closest to the acetone molecule). 

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