Solutions solvent

The solute-solvent interaction in equation A2.4.19 is a measure of the solvation energy of the solute species at infinite dilution. The basic model for ionic hydration is shown in figure A2.4.3 [5] there is an iimer hydration sheath of water molecules whose orientation is essentially detemiined entirely by the field due to the central ion. The number of water molecules in this iimer sheath depends on the size and chemistry of the central ion ... 

This is no longer the case when (iii) motion along the reaction patir occurs on a time scale comparable to other relaxation times of the solute or the solvent, i.e. the system is partially non-relaxed. In this situation dynamic effects have to be taken into account explicitly, such as solvent-assisted intramolecular vibrational energy redistribution (IVR) in the solute, solvent-induced electronic surface hopping, dephasing, solute-solvent energy transfer, dynamic caging, rotational relaxation, or solvent dielectric and momentum relaxation. 

Specific solute-solvent interactions involving the first solvation shell only can be treated in detail by discrete solvent models. The various approaches like point charge models, siipennoleciilar calculations, quantum theories of reactions in solution, and their implementations in Monte Carlo methods and molecular dynamics simulations like the Car-Parrinello method are discussed elsewhere in this encyclopedia. Here only some points will be briefly mentioned that seem of relevance for later sections. 

Considering, for simplicity, only electrostatic interactions, one may write the solute-solvent interaction temi of the Hamiltonian for a solute molecule surrounded by S solvent molecules as... 

Kramers solution of the barrier crossing problem [45] is discussed at length in chapter A3.8 dealing with condensed-phase reaction dynamics. As the starting point to derive its simplest version one may use the Langevin equation, a stochastic differential equation for the time evolution of a slow variable, the reaction coordinate r, subject to a rapidly statistically fluctuating force F caused by microscopic solute-solvent interactions under the influence of an external force field generated by the PES F for the reaction... 

RRKM fit to microcanonical rate constants of isolated tran.s-stilbene and the solid curve a fit that uses a reaction barrier height reduced by solute-solvent interaction [46],... 

Aguilar M A and Olivares del Valle F J 1989 Solute-solvent interactions. A simple procedure for constructing the solvent capacity for retaining a molecular solute Ohem. Rhys. 129 439-50... 

Schroeder J 1996 The role of solute-solvent interactions in the dynamics of unimolecular reactions in compressed solvents J. Phys. Condens. Matters 9379... 

Prediction of pKaS of Titratable Residues in Proteins Using a Poisson-Boltzmann Model of the Solute-Solvent System... 

The nature of soliite-solnte and solute-solvent in teraction s is dependent on the solvent environment. Solvent influences the hydrogen-bon ding pattern, solute surface area, and hydrophilic and hydrophobic group exposures. 

The variation of chemical shifts as a function of dilution could be accounted for only qualitatively (235) because of the large diversity of solute-solvent interactions resulting from the nature and the shape of the solvent molecule (Table 1-34). 

To ensure that reaction occurs m homogeneous solution solvents are chosen that dis solve both the alkyl halide and the ionic salt The alkyl halide substrates are soluble m organic solvents but the salts often are not Inorganic salts are soluble m water but alkyl... 

Caution For ionic reactions in solution, solvent effects can play a significant role. These, of course, are neglected in calculations on a single molecule. You can obtain an indication of solvent effects from semi-empirical calculations by carefully adding water molecules to the solute molecule. 

Other Organic Processes. Solvent extraction has found appHcation in the coal-tar industry for many years, as for example in the recovery of phenols from coal-tar distillates by washing with caustic soda solution. Solvent extraction of fatty and resimic acid from tall oil has been reported (250). Dissociation extraction is used to separate y -cresol fromT -cresol (251) and 2,4-x5lenol from 2,5-x5lenol (252). Solvent extraction can play a role in the direct manufacture of chemicals from coal (253) (see Eeedstocks, coal chemicals). 

Q are the absorbance and wavenumber, respectively, at the peak (center) of the band, p is the wavenumber, and y is the half width of the band at half height. Liquid band positions ate usually shifted slightly downward from vapor positions. Both band positions and widths of solute spectra are affected by solute—solvent interactions. Spectra of soHd-phase samples are similar to those of Hquids, but intermolecular interactions in soHds can be nonisotropic. In spectra of crystalline samples, vibrational bands tend to be sharper and may spHt in two, and new bands may also appear. If polarized infrared radiation is used, both crystalline samples and stressed amorphous samples (such as a stretched polymer film) show directional effects (28,29). 

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