Solvation of Polar Molecules

Summarizing this chapter briefly, available calculations generally concern very limited aspects of the solvation of polar molecules. We have to be aware of the fact that much remains to be learnt about the theoretical treatment of associated liquids and liquid mixtures. 

The dissolution of polar molecules in water is favored by dipole—dipole interactions. The solvation of the polar molecules stabilizes them in solution. Nonpolar molecules are soluble in water only with difficulty because the relatively high energy cost associated with dismpting and reforming the hydrogen-bonded water is unfavorable to the former occurring. 

The analysis of the transient fluorescence spectra of polar molecules in polar solvents that was outlined in Section I.A assumes that the specific probe molecule has certain ideal properties. The probe should not be strongly polarizable. Probe/solvent interactions involving specific effects, such as hydrogen-bonding should be avoided because specific solute/solvent effects may lead to photophysically discrete probe/solvent complexes. Discrete probe/solvent interactions are inconsistent with the continuum picture inherent in the theoretical formalism. Probes should not possess low lying, upper excited states which could interact with the first-excited state during the solvation processes. In addition, the probe should not possess more than one thermally accessible isomer of the excited state. 

This is of course the major solvation energy of polar molecules in non-polar solvents, but it can be an important term in polar solvents as well, especially in highly polarizable solvents such as aromatic derivatives. 

Polar molecules develop an electrical field around themselves, therefore they are marked by the ability to attract either mutually themselves or other molecules with unsymmetrical electron structures. In this way we can explain the association of polar molecules (of water for instance) or the solvation of ions (hydration in case of water) caused by drawing in of the dipoles of water molecules into the electrical field of the ions (to be discussed further on). 

The Mossbauer spectrum of ferrous Y-zeolite is somewhat similar to that of the reduced silica gel samples (103). The spectrum consists of two overlapping and partially resolved doublets with the inner doublet, 3 = 0.89 mm sec-1 and A = 0.62 mm sec-1, being attributed to the ferrous ion on the surface. In both the Y-zeolite and the reduced iron oxide on silica samples, the inner doublets representing surface ferrous states are the first to be affected by adsorption of polar molecules, but in the case of Y-zeolite the addition of excess amounts of water or ammonia causes the disappearance of the spectrum, and this has been interpreted in terms of "solvation of the ferrous ions by absorbate causing weakening of the bonding to the crystalline lattice. It is also possible that the spectrum is a composite representing a multiplicity of parameters. 

Moreover, ion-molecule adduct formation is observed in the case of polar molecules, a type of gas-phase solvation, for example... 

Simple electrostatic models can be used to interpret the activity coefficients of polar molecules in terms of just three parameters a radius, the dipole moment of the solute, and the dielectric constant of the solvent. The continuum model of the solvent can be used to deduce a value for the free energy of solvation of a spherical molecule of radius r containing a point dipole at its center. The value obtained by Kirkwood from electrostatic theory is... 

In this chapter, the recent progress in the understanding of the nature and dynamics of excess (solvated) electrons in molecular fluids composed of polar molecules with no electron affinity (EA), such as liquid water (hydrated electron, and aliphatic alcohols, is examined. Our group has recently reviewed the literature on solvated electron in liquefied ammonia and saturated hydrocarbons and we refer the reader to these publications for an introduction to the excess electron states in such liquids. We narrowed this review to bulk neat liquids and (to a much lesser degree) large water anion clusters in the gas phase that serve as useful reference systems for solvated electrons in the bulk. The excess electrons trapped by supramolecular structures (including single macrocycle molecules ), such as clusters of polar molecules and water pools of reverse micelles in nonpolar liquids and complexes of the electrons with cations in concentrated salt solutions, are examined elsewhere. 

How do water anion clusters in the gas phase relate to the solvated electrons observed in the bulk How does 2D electron localization in layers of polar molecules on metal and metal oxide surfaces relate to 3D localization in the bulk ... 

Equation (13) has been applied to rate data (Evans and Parker, 1966 Evans and Parker, unpublished work) for the 8 2 decomposition of trimethylsulphonium bromide (14). The dipolar transition state has °y( rCHsSMe,)+ = 35 7 y CHs8Mea)+ = 0 73 and ymrOHsSMej)+ = I O at 25°C in the solvents ethanol (E), dimethylaoetamide and nitromethane (N) respectively, relative to dimethylformamide. In other words, the polar transition state for (14) is more solvated by ca. 2 kcal mole in dipolar aprotic solvents than it is in the protic solvent ethanol. The value of the activity coefficient of the polar transition state, °yBrOH8SMe2t = 35-7, is Comparable with those of polar molecules in protic solvents, relative to DMF (cf. Table 3). 

The role of vibrational relaxation and solvation dynamics can be probed most effectively by fluorescence experiments, which are both time- and frequency-resolved,66-68 as indicated at the end of Sec. V. We have recently developed a theory for fluorescence of polar molecules in polar solvents.68 The solvaion dynamics is related to the solvent dielectric function e(co) by introducing a solvation coordinate. When (ai) has a Lorentzian dependence on frequency (the Debye model), the broadening is described by the stochastic model [Eqs. (113)], where the parameters A and A may be related to molecular... 

There is a somewhat similar phenomenon in which the presence of a dipole within a molecule induces a temporary dipole, either elsewhere in the molecule or in another molecule. The induced dipole is then attracted to the inducing charge or dipole, and another small attractive force comes into play that is not included in the molecular orbital picture at the most simple level of calculation, but is included when larger basis sets are used. Weak dipolar attractions like these, both the static and the induced, are not strong, and so nonpolar molecules are not well solvated by polar molecules the polar solvent molecules would rather solvate each other and the nonpolar molecules are left to their own devices. As it happens they do not repel each other as much as one might expect. 

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