Some Specific Solvent Effects

Having dealt with the excluded volume effect arising from the first term, AG, on the rhs of Eq. (9.4.2), we now examine a few other solvent effects associated with the remaining terms on the rhs of this equation. These are referred 

Clearly, since these effects depend on the type and distribution of the FGs, there is no general method of dealing with an arbitrary binding system. We shall therefore treat one, relatively simple example. The extension to any other specific system should become clear by generalization of the procedure performed in this example. 

In the following model example, we assume that each species involved in the binding process has a spherical shape and that the FGs on its surface are distributed in such a way that each pair of FGs on the surface (i.e., exposed to the solvent) is independently solvated. In other words, the conditional solvation Gibbs energy of the ith FG (given the hard core H) is independent of the presence or absence of any other FGs. Formally, this is equivalent to taking only the first sum over i in the expansion on the rhs of Eq. (9.4.2). 

As in the previous section, we shall discuss each of the three types of solvent effects separately. 

The quantity AG is the solvation Gibbs energy of the hard part of the interaction and has been dealt with in the previous section. The second expression is the conditional solvation Gibbs energy of the kth FG given that the hard part of the interaction has already been solvated. The conditional probability density is 

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The quantity A i (( )) is the solvation Gibbs (or Helmholtz) energy for the molecule with occupation number i (i = 0, 1,2) and specific angle (j). Although we believe some specific solvent effects might contribute significantly to the correlation function (see also Chapter 9), we did not include solvent effects in the present calculations (apart from the dielectric constants and Dj, as indicated above). 

Because the key operation in studying solvent effects on rates is to vary the solvent, evidently the nature of the solvation shell will vary as the solvent is changed. A distinction is often made between general and specific solvent effects, general effects being associated (by hypothesis) with some appropriate physical property such as dielectric constant, and specific effects with particular solute-solvent interactions in the solvation shell. In this context the idea of preferential solvation (or selective solvation) is often invoked. If a reaction is studied in a mixed solvent. 

When we perform experiment in such way that there is no interference of H-bonds or these bonds are stable and structure of solvent also does not varies essentially, solvatochromic plot demonstrates very good linearity as shown, for example, for some naphthylamine derivatives in ethanol-water mixtures. The linearity of solvatochromic plots is often regarded as an evidence for the dominant importance of nonspecific universal intermolecular interaction in the spectral shifts. Specific solvent effects lead to essential deviation of measured points from this linear plot. 

A difficulty which arises in some of the correlations is that there are specific solvent effects which must be brought into the problem empirically since there is no available theory for dealing with them. 

Before we examine some specific solvation effects on cooperativity we must first consider various aspects of the solvation Gibbs energy of a macromolecule a. We present here one possible decomposition of AG which will be useful for our purposes. Consider a globular protein a which, for simplicity, is assumed to be compactly packed so that there are no solvent molecules within some spherical region to which we refer as the hard core of the protein. The interaction energy between a and the fth solvent molecule (the solvent is presmned to be water, w) is written as... 

As pointed out in Chapter III, Section 1 some specific diluent effects, or even remnants of the excluded volume effect on chain dimensions, may be present in swollen networks. Flory and Hoeve (88, 89) have stated never to have found such effects, but especially Rijke s experiments on highly swollen poly(methyl methacrylates) do point in this direction. Fig. 15 shows the relation between q0 in a series of diluents (Rijke assumed A = 1) and the second virial coefficient of the uncrosslinked polymer in those solvents. Apparently a relation, which could be interpreted as pointing to an excluded volume effect in q0, exists. A criticism which could be raised against Rijke s work lies in the fact that he determined % in a separate osmotic experiment on the polymer solutions. This introduces an uncertainty because % in the network may be different. More fundamentally incorrect is the use of the Flory-Huggins free enthalpy expression because it implies constant segment density in the swollen network. We have seen that this means that the reference dimensions excluded volume effect. 

Ketones, Ketones have been studied systematically by many workers, and not all of this work will be described. The shifts brought on by H bonding solvents are only a little larger than those attributed to less specific solvent effects. This has led some workers to ignore H bond formation (175). Josien and co-workers made extensive studies of the... 

The slope of this correlation is about 50% larger than predicted by equation (31). This contrast may arise from an underestimate of the external radii of the complexes, or (more likely) some specific solvation effects and the inadequacies of the dielectric continuum model on which equation (31) is based. Equation (55) seems to be a good basis for estimating the contributions of Xs. provided the same procedure is used to estimate rmean- Relatively elaborate solvation models, in which the solvent structure near to the reactants plays an important role, are being explored. ... 

A class of previously unknown compounds, spiro[3H-indole-3,2 -[4H] pyrido [3,2-e]-l,3-thiazine]-2,4 -(lH) diones, can be synthesized by the reaction of in situ generated 3-indolylimine with 2-mercaptonicotinic acid under microwaves in the absence of solvent. Both neat reactions or reactions on solid supports such as silica, alumina etc., are effective in promoting the reaction, while reaction under thermal heating failed to proceed (Scheme 2.2-33) thus indicating some specific microwave effect [99]. 

Specific solvent effects have also been obseived for other commouly used fluoropfaores. Examples are shown in Figures 6.21 and 6.22 for l-aminonaphthalene and 9-metbyl anthroate, respectively. In both instances, the Stokes shifts are approximately proportioaal to the orientation polarizability for some solvents, but other solvents produced larger spectral shifts. Larger spectral shifts were found in those solvents capable of forming hydro-... 

For vibrational properties, solute—solvent short-range interactions not only can induce a shift in the frequency but they can also modify the normal modes. This effect can be taken into account only by including some specific solvent molecules within the QM portion of the system. This supermolec-ular approach, however, introduces some additional aspects that make the analysis more complex. First of aU, the selection of the number and the position of the solvent molecules to be included in the QM part is not unequivocal moreover, as now the solvent enters as a QM component, we cannot easily dissect the response of the solute from that of the solvent molecules. 

On the other hand, the specific donor-acceptor interactions arising in dipolar solvents cannot be calculated with such a simple, essentially electrostatic approximation. If such complexes containing coordinated solvent molecules are regarded as supermolecules , attempts may be made to calculate the solvation effect using the Monte Carlo method [Ko 72]. Even this comparatively intricate method, with considerable computer requirements, has proved successful only for the quantitative description of relatively simple systems. Such specific solvent effects, however, can be well described by means of some empirical solvent parameter or an experimental (e.g., spectroscopic) parameter characteristic of the stability or possibly of the electronic structure of the complex. 

Table 2.20 presents some data on the NMR spectra of diamagnetic homo- and heteroleptic metal alkoxides. In comparing chemical shifts it should be kept in mind that the 8 values of some metal alkoxides are subject to specific solvent effects. As expected the 8 values of the protons on a-carbon atoms are shifted considerably down-field relative to protons on /5-carbons. Since the H/ C chemical shift is the resultant of several contributing factors, there are no obvious correlations with metal oxidation state, atomic radius, or co-ordination number. However, metal NMR studies have proved to be of considerable importance in indicating the coordination environment of the central metal atom. 

Due to the nature of some excited and ground states, specific solvent effects may be observed in some cases.Thus for example in states arising from excitations,... 

The complexity of an exact treatment of the solvent effects on intramolecular electron transfer has precluded such an analysis until now. Thus one has to use, for some time still further, macroscopic models such as the dielectric continuum model. Such models have indeed good predictive properties but they fail to describe specific solvent effects such as the donor ability or the H bonding ability. Even the more sophisticated quantum model, in its present form, is an oversimplification since the solvent motion is described by a single vibrational mode. The quantum model has some success because the vibronic levels corresponding to solvent modes are so closely spaced that in fact they can be approximated by a continuum. There is no doubt however that the progress in computing ability will allow in the future the simulation of the exact behaviour of the solvent in these reactions. 

With regard to differences in polymer behavior in solution versus the bulk state, several points must be made. Clearly, it is now well-established that the choice of theta solvent can affect chain dimensions to some extent [42-44, 46, 47]. Hence, only the chain in an amorphous melt of identical neighbors can be considered to be in the unperturbed state. Particularly striking are some of the differences noted in temperature coefficients measured by different techniques. Is it possible that the thermal expansion of a polymer molecule is fundamentally different in the bulk and in solution Can specific solvent effects exist and vary in a systematic way within a series of chemically similar theta solvents Does the different range of temperatures usually employed in bulk versus solution studies affect K Are chains in the bulk (during SANS and thermoelastic experiments) allowed adequate time to completely relax to equilibrium All of these issues need further attention. Other topics perhaps worthy of consideration include the study of the impact of deuterium labelling on chain conformation (H has lower vibrational energy than does H ) and the potential temperature dependence of the Flory hydrodynamic parameter . 

As demonstrated in the two previous sections, TRIR spectroscopy can be used to provide direct structural information concerning organic reactive intermediates in solution as well as kinetic insight into mechanisms of prodnct formation. TRIR spectroscopy can also be used to examine solvent effects by revealing the inflnence of solvent on IR band positions and intensities. For example, TRIR spectroscopy has been used to examine the solvent dependence of some carbonylcarbene singlet-triplet energy gaps. Here, we will focns on TRIR stndies of specific solvation of carbenes. 

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