Electrostatic effects Volume

In the models discussed thus far in this section, emphasis has been placed on electrostatic effects and solvent polarity. An alternative view that to some extent takes other forces into account begins with the idea that, in order to dissolve a solute molecule in a solvent, energy is required to create a cavity in the solvent the solute is then inserted into this cavity. In Section 8.2 we saw that the energy to create a cavity can be expressed as a product of the surface area of the cavity and the surface tension of the solvent. An equivalent expression is obtained as the product of the volume of the cavity and the pressure exerted by the solvent, and we now explore this concept. 

Bashford D (1997) Scientific Computing in Object-Oriented Parallel Environments Lecture Notes in Computer Science, volume 1343, chapter An object-oriented programming suite for electrostatic effects in biological molecules. Springer, Berlin, pp 233-240. 

The calculation of the energy for the interaction between the solvent and solute is more complicated. Formally, the free energy for the process, AC/ini, is composed of two chemical parts and an essentially entropic term. The chemical terms are associated with van der Waals interactions AGvdw> electrostatic effects, AG, between solute and solvent. The entropic term measures the free volume i.e., the volume a molecule explores before encountering another, which is assumed to be proportional to the molar volume of the solvent. Thus, the free energy change associated with solute-solvent interactions at temperature T is given by... 

In 1993, Fuhr et al. (159) correlated electrostatic and volume descriptors for a series of quinolone antibacterials with their percentage of inhibition effect of caffeine 3-demethylation in CYP1A2. The descriptors were derived with the SYBYL (207) and ALCHEMY II (208) software packages. It was shown that... 

Combining the highest electronegativity (4,0) with a rather small polarizability volume which amounts to not more than 0.5 makes fluorine a unique element. Its incorporation into hydrocarbon frameworks results in different electrostatic effects, which are sometimes rarely predictable. However, the influence of fluorine substituents on the acidity of nearby functional groups such as OH, NH,... 

The electrostatic effects are influenced by the micelle concentration. This effect can be viewed as a micelle-micelle interaction mediated by counterions. The most direct way for modelling the finite micelle concentration is to confine the volume per micelle by an outer radius if of finite size298 300). This is called the cell model. 

This reaction was followed by a second slower reaction that was not characterized in the report. An analysis of the overall results showed that the influence of steric or electrostatic effects on kinetic parameters is not necessarily minor. The dynamics of binding of NO to the diaqua species is mainly tuned by modulation of electron density on the iron center by the porphyrin macrocycle. A volume profile for NO binding based on values ofAV on and A V 0ff of + 1.5 and + 9.3 cm3/mol, respectively, maybe interpreted as an interchange mechanism for the on reaction, as the Fenl-H20 bond is decidedly stabilized (see Figure 7.15a). The volume of activation for the off reaction indicates a less dissociative mode of activation compared with NO release from other porphyrins. 

For electrostatically stabiUzed suspensions, this maximum volume fraction will depend primarily on the dimensionless double layer thickness, Ka, and the electrostatic interaction energy through the dimensionless surface potential, T g(= etpelksT). Because the dimensionless double layer thickness can be large (i.e., 10-100) when the salt concentration is low, the effective volume fraction at which the maximum volume fraction is reached can be very small. 

Liu, J., Kelley, C.P., Goren, A.C., Marenich, A.V., Cramer, C.J., Truhlar, D.G., Zhan, C.G. Free energies of solvation with surface, volume, and local electrostatic effects and atomic surface tensions to represent the first solvation shell. J. Chem. Theory. Comput. 2010, 6(4), 1109-1117. 

The influence of the pore size distribution of carbon on NOM uptake has been recognized by several researchers [57, 63]. Likewise, Karanfil and coworken [64] and Kilduffand coworkers [65] concluded that the adsorption of humic substances was largely governed by molecular size distribution in relation to pore size. Moreover, a good linear relationship (Fig. 25.6) was foundbetween the amount adsorbed by different carbons and their pore volume between 0.8 and 50 nm [61, 66] when the adsorption was carried out at pH 3. This is because electrostatic effects are minimized under these experimental conditions and nonelectrostatic interactions predominate. The adsorption mechanism would be due to hydrophobic and/or TT-TT-electron interactions, and in this case as with other electrolytes (see above). 

The description of the metal is improved considerably if metallic structure is introduced by accounting for the local attractive force of the metal atoms on the free electron gas. This corresponds to the jellium model with pseudopotentials. Each metal atom in the lattice is pictured as being surrounded by a spherical volume Fc in which electrostatic effects may be ignored. Outside of the sphere the metal atom behaves as a point charge of charge number n. Thus, it has a pseudopotential < )(., where... 

FIG. 14 Dependence of the second virial coefficient on added-salt concentration. NaPSS in NaCl solutions. Results by light scattering from Ref. 41 ( ), Ref. 30 (A) and by osmometry from Ref. 41 (o). The dashed line shows calculation of the second virial coefficient by incorporating electrostatic excluded volume effects after Ref. 30. 

The solvent dependence of conformational energies has its origin primarily in two effects, an electrostatic contribution, no doubt the most important one, and a contribution from the variation in internal solvent pressure, which may affect the conformational equilibrium if the effective volumes of... 

Lombardo P, Obach RS, Shalaeva MY and Gao E, Prediction of human volume of distribution values for neutral and basic drugs. 2. Extended data set and leave-class-out statistics, J. Med. Chem., 47,1242-1250 (2004). Ref. 297 Irvin JL and Irvin EM, Apparent ionization exponents of homologs of quinacrine Electrostatic effects, JACS, 72, 2743-2749 (1950). 

---