Solvent property scales

The main physical implication of this condition is that good correlations involving all solvents will be limited to compounds and/or properties that are extremely closely related. This seems to be the main reason for the proliferation of solvent property scales. 

This AN solvent parameter scale is of interest to us in that, whereas in many other instances we have found that solvent property scales intended to serve as measures of solvent polarity, that is, 7r -equivalent, were in fact measures of combined polarity and HBD acidity properties, that is, equivalent to a linear combination of tt and a (as has been shown for r(30) and will be shown for Z, xr, and A ), here we have a property intended as an electrophilicity measure, that is, a-equivalent, which is also, in fact, a combined function of tt and a. 

Tfaeie have been a number of attempts to develop solvent parameter scales that could be used to correlate ttiermodynamic and kinetic results in terms of these patametois. Gutmann s Donor Numbers, discussed previously, are sometimes used as a solvent property scale. Kamlet and Taft and co-workers developed the solvatochromic parameters, Uj, B, and n that are related to the hydrogen bonding acidity, basicity and polarity, respectively, of the solvent. Correlations with these parameters also use the square of tte Hildebrand solubility parameter, (5, that gives the solvent cohesive energy density. Parameters for some common solvents are collected in Table 3.6. 

Above the critical temperature and pressure, a substance is referred to as a supercritical fluid. Such fluids have unusual solvent properties that have led to many practical applications. Supercritical carbon dioxide is used most commonly because it is cheap, nontoxic, and relatively easy to liquefy (critical T = 31°C, P = 73 atm). It was first used more than 20 years ago to extract caffeine from coffee dichloromethane, CH2C12, long used for this purpose, is both a narcotic and a potential carcinogen. Today more than 10s metric tons of decaf coffee are made annually using supercritical C02. It is also used on a large scale to extract nicotine from tobacco and various objectionable impurities from the hops used to make beer. 

Solvent properties and dipoles, 313 Sorbitol, 423 Sprensen pH scale, 190 Space, interstellar, 448 Spectrograph mass, 242 simple, 247 Spectroscopy, 187 infrared, 249 microwave, 249 X-ray, 248 Spectrum... 

Quantitative determination of solvent polarity is difficult, and quantitative methods rely on physical properties such as dielectric constant, dipole moment and refractive index. It is not possible to determine the solvent polarity by measuring an individual solvent property, due to the complexity of solute-solvent interactions, and for this reason empirical scales of solvent polarity based on chemical... 

From a practical viewpoint, Ej values are quickly and easily obtained, giving a very useful and convenient scale. However, a general polarity scale based on a single probe molecule has its limitations because a single compound cannot experience the diversity of interactions that the whole range of solvents can offer. The Kamlet-Taft parameters a, /3 and n tackle this problem by using a series of seven dyes to produce a scale for specific and nonspecific polarity of liquids [23], Whilst it undoubtedly gives a more detailed description of the solvents properties,... 

Miscible organic solutes modify the solvent properties of the solution to decrease the interfacial tension and give rise to an enhanced solubility of organic chemicals in a phenomenon often called cosolvency . According to theory, a miscible organic chemical such as a short chain alcohol will have the effect of modifying the structure of the water in which it is dissolved. On the macroscopic scale, this will manifest itself as a decrease in the surface tension of the solution [238,246]. 

The use of organic solvents in nonaqueous capillary electrophoresis not only increases the solubility of the solutes, but also allows one to control important characteristics of the separation. For instance, the solvent properties affect the acid-base behavior of the analytes on a wider scale than... 

The development of these various solvent parameters and scales has been accompanied by the realization that there are uncertainties in the physical property of the solvent that is correlated by a particular parameter in cases where systematic changes in solvent structure affect several solvent properties. Consider a reaction that shows no rate dependence on the basicity of hydroxylic solvents, and a second reaction that proceeds through a transition state in which there is a small transition state stabilization from a nucleophilic interaction with the hydroxyl group. The rate constants for the latter reaction will increase more sharply with changing solvent nucleophilicity than those for the former, and they should show a correlation with some solvent nucleophilicity parameter. This trend was observed in a comparison of the effects of solvent on the rate constants for solvolysis of 1-adamantyl and ferf-butyl halides, and is consistent with a greater stabilization of the transition state for reaction of the latter by interaction with nucleophilic solvents. ... 

Baker, S.N., Baker, G.A., Bright, F.V., Temperature-dependent microscopic solvent properties of dry and wet l-butyl-3-methylimidazolium hexafluo-rophosphate Correlation with ET(30) and Kamlet-Taft polarity scales. Green Chem., 4,165-169, 2002. 

A commonly nsed solnte concentration scale is the mole fraction one, x, which specifies directly the number of moles of solvent per mol of solute (1 - x)/x. This number is of particular interest in the more concentrated solntions, where a lack of solvent molecules required to surround a solute particle and separate solute particles from one another greatly affects the properties of the solution. However, this scale is useful for the entire composition range, from very dilute solution to such solutions, mixtures, where it is difficult to designate one component as the solute and the other as the solvent. This scale requires knowledge of the chemical nature of the solute and the solvent in view of the necessity to specify a mole of each. 

As discussed in Chapter 2, most force fields are validated based primarily on comparisons to small molecule data and moreover most comparisons involve what might be called static properties, i.e., structural or spectral data for computed fixed conformations. There are a few noteworthy exceptions the OPLS and TraPPE force fields were, at least for molecular solvents, optimized to reproduce bulk solvent properties derived from simulations, e.g., density, boiling point, and dielectric constant. In most instances, however, one is left with the question of whether force fields optimized for small molecules or molecular fragments will perform with acceptable accuracy in large-scale simulations. 

The difficulty in dealing with solvent influences on reaction rates is that the free energy of activation, AG, depends not only on the free energy of the transition state but also on the free energy of the initial state. It is therefore of considerable interest to dissect solvent influences on AG into initial-state and transition-state contributions. As far as electrophilic substitution at saturated carbon is concerned, the only cases for which such a dissection has been carried out are (a) for the substitution of tetraalkyltins by mercuric chloride in the methanol-water solvent system (see page 79), and (b) for the iododemetallation of tetraalkylleads in a number of solvents (see p. 173). Data on the latter reaction (6) are more useful from the point of view of the correlation of transition-state effects with solvent properties, and in Table 13 are listed values of AG (Tr), the free energy of transfer (on the mole fraction scale) of the tetraalkyllead/iodine transition states from methanol to other solvents. 

Those solutes for which the solvent shifts are particularly large have been used in the specification of solvent properties, such as electron-pair donation ability, Lewis basicity, or softness. For the former property, the solvent shifts of deuteromethanol or of phenol have served as suitable scales. For the latter property the solvent shifts of the symmetrical stretch of Hg-Br in the Raman spectrum of HgBr2 and of I-CN in the infrared spectrum of ICN have been so employed (see Chapter 4). 

The wide variety of possible solvent-solute interactions requires that any scale used to quantify solvent properties will be complex. Unfortunately, no universally accepted scale of solvating power has been devised. It does not seem reasonable to develop an entirely new scale for supercritical fluid solvents, especially since it is desirable to compare the solvent behavior of supercritical fluids with that of liquid solvents. 

By a quantitative structure-property relationship (QSPR) analysis of a total of 45 different empirical solvent scales and 350 solvents, the direct calculation of predicted values of solvent parameters for any scale and for any previously unmeasured solvent was possible using the CODESS A program [ie. comprehensive descriptors for structural and statistical analysis) developed by Katritzky et al. [244]. The QSPR models for each of the solvent scales were constructed using only theoretical descriptors, derived solely from the molecular solvent structure. This QSPR study enabled classification of the various solvent polarity scales and ultimately allowed a unified PCA treatment of these scales. This PCA treatment, carried out with 40 solvent scales as variables (each having 40 data points for 40 solvents as objects), allowed a rational classification and grouping... 

It has been stated that, when specific hydrogen-bonding effects are excluded, and differential polarizability effects are similar or minimized, the solvent polarity scales derived from UV/Vis absorption spectra Z,S,Ei 2Qi),n, Xk E- ), fluorescence speetra Py), infrared spectra (G), ESR spectra [a( " N)], NMR spectra (P), and NMR spectra AN) are linear with each other for a set of select solvents, i.e. non-HBD aliphatic solvents with a single dominant group dipole [263]. This result can be taken as confirmation that all these solvent scales do in fact describe intrinsic solvent properties and that they are to a great extent independent of the experimental methods and indicators used in their measurement [263], That these empirical solvent parameters correlate linearly with solvent dipole moments and functions of the relative permittivities (either alone or in combination with refractive index functions) indicates that they are a measure of the solvent dipolarity and polarizability, provided that specific solute/ solvent interactions are excluded. 

For a two-parameter treatment of solvent effects (with two independent solvent vectors), only two critical subsidiary conditions must be defined in order to force the two solvent parameters to represent physically significant solvent properties. Four other trivial arbitrary conditions have to be defined in order to fix zero reference points and scale-unit sizes. However, for a three-parameter treatment (with three independent solvent vectors), already six critical subsidiary conditions must be defined, in addition to the six trivial reference or scale-factor conditions. On the contrary, singleparameter treatments require no definition of critical subsidiary conditions, but only one reference (zero) condition and one standard (unit) condition, whose arbitrary assignment changes only the reference solvent and the scale-unit size (265, 276). 

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