Solute-solvent interactions dipolar forces

TheD term accounts for part of the effects of solution enthalpy. Enthalpy of mixing results when the solute-solvent interaction force is different from the solute-solute and the solvent-solvent interactions. Intermolecularforces can be further characterized as dispersion, dipolar, and hydrogen-bond forces. In the mobile order solubility approach, dispersion and dipolar forces were not separated. The effects of these two forces on solubility were expressed in terms of modiLed solubility parameters, S andSj. The relationship between solubility and solubility parameters can be derived in the... 

There are two types of solute-solvent interactions which affect absorption and emission spectra. These are universal interaction and specific interaction. The universal interaction is due to the collective influence of the solvent as a dielectric medium and depends on the dielectric constant D and the refractive index n of the solvent. Thus large environmental perturbations may be caused by van der Waals dipolar or ionic fields in solution, liquids and in solids. The van der Waals interactions include (i) London dispersion force, (ii) induced dipole interactions, and (iii) dipole-dipole interactions. These are attractive interactions. The repulsive interactions are primarily derived from exchange forces (non bonded repulsion) as the elctrons of one molecule approach the filled orbitals of the neighbour. If the solute molecule has a dipole moment, it is expected to differ in various electronic energy states because of the differences in charge distribution. In polar solvents dipole-dipole inrteractions are important. 

The physical intermolecular solute-solvent interaction forces (88MI1) as well as the solute-solute interactions should be taken into account for reliable interpretation of physico-chemical data measured in solution. Further structural studies may enhance our understanding of these highly dipolar organic molecules through their role in noncovalent interactions both in liquid solution and in solid state. 

One reason for this at first sight unexpected result is the fact that probably 70... 90% of the solute/solvent interaction term is caused by London dispersion forces, which are more or less equal for the cis and trans isomers. Another important reason is that one has to take into account higher electric moments the trans isomer has a quadrupole moment, and the cis isomer also has moments of a higher order than two. Calculations of solute/solvent interactions of both diastereomers using a reaction field model led to the conclusion that the quadrupolar contribution of the trans isomer is comparable to the dipolar contribution of the cis isomer. It has been pointed out that the neglect of solute/solvent interactions implying higher electric moments than the dipole moment can lead to completely false conclusions [202],... 

As the data for the Menschutkin reactions indicate, the character of the solute-solvent interactions is more complex than described by Eq. (5-87). It is evident that functions of relative permittivity alone, as given in Eq. (5-87), are not useful for describing the solvent effect on reactions between dipolar reactants, except in certain special cases, such as when a mixture of two solvents is used. In addition to electrostatic forces, non-electrostatic interactions, such as dispersion forces and hydrogen-bonding, must also be involved in Menschutkin reactions. 

The additive contribution of the individual bonds to the anisotropic potential draws attention to the rule of the additivity of bond polarizabilities [122]. This strongly suggests that the average orientation of the substituted benzenes is directly related to their principal polarizabilities and that the anisotropic solute-solvent interaction is determined by London dispersion forces. Considering dispersion forces (in dipolar approximation) one obtains the following expression for the... 

When applied to solvents, this rather iU-defined term covers their overall solvation capabihly (solvation power) for solutes (i.e., in chemical equilibria reactants and products in reaction rates reactants and activated complex in light absorptions ions or molecules in the ground and excited state), which in turn depends on the action of aU possible, nonspecific and specific, intermolecular interactions between solute ions or molecules and solvent molecules, excluding such interactions leading to definite chemical alterations of the ions or molecules of the solute. Occasionally, the term solvent polarity is restricted to nonspecific solute/solvent interactions only (i.e., to van der Waals forces). With respect to this definition of polarity, when discussing dipolar molecules, the dipolarity (rather than polarity) of solvents should be considered. Molecules with a permanent dipole moment are dipolar (not polar). Molecules, which are lacking permanent dipole moment should be called apolar (or nonpolar). In literature, the terms polar and apo-lar are used indiscriminately to characterize a solvent by its relative permittivity as well as its permanent dipole moment, even though dipole moments and relative permittivities are not directly related to each other. 

In non-polar aprotic solvents, characterized by low dielectric constants, molecules interact by weak van der Waals forces. Non-polar solutes are readily soluble as the van der Waals forces between solute molecules in the solid phase are replaced by similar interactions with solvent molecules. Dipolar and polar protic solutes are generally found to have very low solubilities in these solvents except in cases where non-polar complexes are formed. 

Solvent-solute interactions may be stronger and more specific than simple dipolar forces there may be ordering of solvent molecules around specific parts of the molecule due to local dipoles, or there may be stronger interactions such as hydrogen bonding—which occurs, for example, in ammine and cyanide complexes and is discussed below. 

The term solvation refers to the surrounding of each dissolved molecule or ion by a shell of more or less tightly bound solvent molecules. This solvent shell is the result of inter-molecular forces between solute and solvent. For aqueous solutions the term used is hydration. Intermolecular interactions between solvent molecules and ions are particularly important in solutions of electrolytes, since ions exert specially strong forces on solvent molecules. Crude electrostatic calculations show that the field experienced by nearest neighbours of dissolved ions is 10. .. 10 V/cm. Fig. 2-7 shows a highly simplified picture of such an interaction between ions and dipolar solvent molecules. 

A second limitation of the Hughes-Ingold theory concerns the fact that the solvent is treated as dielectric continuum, characterized by one of the following its relative permittivity, e, the dipole moment, fi, or by its electrostatic factor, EF, defined as the product of and [27]. The term solvent polarity refers then to the ability of a solvent to interact electrostatically with solute molecules. It should be remembered, however, that solvents can also interact with solute molecules through specific inter-molecular forces like hydrogen bonding or EPD/EPA complexation cf. Section 2.2). For example, specific solvation of anionic solutes by pro tic solvents may reduce their nucleophilic reactivity, whereas in dipolar aprotic solvents solvation of anions is less,... 

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