Nonpolar solvents 380 INDEX

The dispersion interaction is nicely isolated from other factors by examining the solutes in mixtures of heptane and carbon disulfide nonpolar solvents of similar dielectric constant, but quite different refractive index. Under these conditions there is a better than 0.95 correlation between VM-h and the McRae term as illustrated for 27ii9Sn H in Fig. 7. 

These expressions appear more applieable to nonpolar solvents or mixtures than to polar solvents. The nature of the solvation process (and the radii and so forth of the solvated reactants) may stay approximately constant in the first situation but almost certainly will not in the seeond. The function (E>op A ) features in the reorganisation term Xq which is used for estimating rate constants for redox reactions (Eqn. 5.23). is the optical dielectric constant and Dj the static dielectric constant (= refractive index ). 

The Kamlet-Taft u polarity/polarizability scale is based on a linear solvation energy relationship between the n it transition energy of the solute and the solvent polarity ( 1). The Onsager reaction field theory (11) is applicable to this type of relationship for nonpolar solvents, and successful correlations have previously been demonstrated using conventional liquid solvents ( 7 ). The Onsager theory attempts to describe the interactions between a polar solute molecule and the polarizable solvent in the cybotatic region. The theory predicts that the stabilization of the solute should be proportional to the polarizability of the solvent, which can be estimated from the index of refraction. Since carbon dioxide is a nonpolar fluid it would be expected that a linear relationship... 

In conclusion, we have successfully demonstrated that, by using a fluorous label and a fluorous solvent, we can affect the phase transfer of gold and CdSe nanoparticles from an aqueous or hydrocarbon medium to the fluorous phase. Single-walled carbon nanotubes and ZnO nanorods can be solubilized in a fluorous solvent after interaction with a fluorous amine. Phase transfer of the nanostructures to a fluorous solvent represents solubilization in a highly nonpolar solvent, accompanied by purification. The high nonpolarity of the fluorocarbon makes it possible to study the optical and other properties of nanostructures in a medium of very low refractive index. Since the fluorocarbon extracts only the species attached to the fluorous label, the process enables one to obtain solely one product in the pure state. We believe that fluorous chemistry may have practical utility in carrying out studies of nanostructures. 

Nonpolar solute in a nonpolar solvent. In this case, only dispersion forces contribute to the solvation of the solute. Dispersion forces, operative in any solution, invariably cause a small bathochromic shift, the magnitude of which is a function of the solvent refractive index n, the transition intensity, and the size of the solute molecule. The function (n — l)/(2n - -1) has been proposed to account for this general red shift [69, 70]. Corresponding linear correlations between this function of n and Av have been observed for aromatic compounds e.g. benzene [22], phenanthrene [71]), polyenes e.g. lycopene [23], y9-carotene [464]), and symmetrical polymethine dyes e.g. cyanines [26, 27, 292, 293]). 

Here, n is the refractive index of the solvent. Both equations have been widely used and tested for a large number of compounds [136-144] and have proved to be vahd only in a very limited range in dilute solutions of nonpolar solvents, where specific... 

Properties A viscous, pale-yellow liquid. Refr index 1.4825-1.4855 (25C). Soluble in most nonpolar solvents. Combustible. 

The influence of temperature on the mean dipole moment of polybutyl methacrylate dissolved in carbon tetrachloride is shown in Table II. The average moments were calculated from Frohlich s equation (see Section VI) taking unity as the most probable value of the correlation factor. Since the Onsager theory makes use of the refractive index of the solute, for which only approximate values can be found, results obtained by Onsager s and FrOhlich s theories for solutions are not identical even in a nonpolar solvent like carbon tetrachloride. The moments given in Table II are not comparable to those given in Table I, especially as they are not extrapolated to infinite dilution. 

In nonpolar solvents, Eu chelates studied in the present work exhibited luminescence kinetics closely following the single exponential decay law over the intensity range of about three orders of magnitude. In agreement with the previous observations [5, 6], an increase in the refractive index of the nonpolar medium leads to a systematic decrease in the excited-state lifetime of the Eu ion. This effect is accounted for by the well-known refractive-index dependence of the radiative rate y where and are radiative rates in a dielectric medium and in vacuo, respectively, n is the refractive index of the medium, and/[ ) is the local-field correetion factor [4, 5]. 

For fluorescent molecules in nonpolar solvents of the same refractive index, solvatochromic shifts are governed by the change of the molecule dipole moment Apge and by a solvent polarity function f D), which depends on the static dielectric constant D of the medium. 

For an exhaustive study on the use of dielectric and refractive index functions to correlate solvent effects, see, e.g., F. W. Fowler, A. R. Katritzky, and R. J. D. Rutherford, 7. Chem. Soc. (B), 460 (1971). We are not considering here the contribution from dispersion forces as discussed in Refs. S8a and 586 and by Longuet-Higgins and Pople [H. C. Longuet-Higgins and J. A. Pople, J. Chem. Phys., 27, 192 (1957)]. These forces will introduce a red shift (relative to the gas phase) even in the case of nonpolar solutes in nonpolar solvents. 

Experimentally, the dielectric constant and density of the substance are measured. Either the dielectric constant is measured at several temperatures or the refractive index must be measured the dipole moment is then calculated. The dielectric constant is measured in a conductance cell on the vapor or, more often, in dilute solution in a nonpolar solvent such as benzene.J For accurate work, the dielectric constant is measured at several concentrations to allow extrapolation to infinite dilution. ... 

Window materials for microscopy Typical window materials for microscopy include barium fluoride (BaF2) for use with polar solvents (including water), potassium bromide (KBr) for solids and nonpolar solvent use, zinc selenide (ZnSe) with its high refractive index for use in diamond cell filler (background measurement), and diamond for compression cell work in which higher pressures are required. 

Fig. 4.26 Dependence of the transition energy of the long-wavelength (Qy) absorption band of bacteriochlorophyll-a on the refractive index (n) in nonpolar solvents. Experimental data from Limantara et al. [83] are replotted as a function of (n - l)/(2n + 1). Extrapolating ton = 1 (Oon the abscissa) gives 13,810 cm for the transition energy in a vacuum. A similar plot of the data vs (n — l)/(n + 2) gives 13,600 cm ...

As the spectral shifts in hydrocarbons represent a susbstantial part as compared with the other solvents (excepting water and alcohols) we consider that the dispersion forces of the London [25] type have an important contribution to the solvation energies, and then to the red shiftj because the polarizability of solute molecule in the excited state increases [26], and an instantaneous redistribution of the electric charge will take place. From the McRae s [27] theory results a formula giving the spectral shift under the solvent influence (in terms of solute polarizability and dipole moment of the solute molecule and in terms of the refractive index and dielectric constant of the solvent), which, for nonpolar solvents, reduces to ... 

The polarity index is a measure of the polarity of the solvent, which is often the most important factor in the solvent choice for the particular application. In extraction processes, the tenet that like dissolves like (and conversely, opposites do not attract ) is the primary consideration in choosing the solvent for extraction, partitioning, and/or analytical conditions. For example, hexane often provides a selective extraction for nonpolar analytes, and toluene may provide more selectivity for aromatic analytes. 

Liquids with equal solubility parameters are miscible, there is no heat of mixing. With increasing difference of <5, two phases coexist, which become miscible at elevated temperature, at the critical consolute temperature Tc. Tc increases with the difference of the <5 s and with the mean molar volume of the two liquids. Another polarity scale was recently introduced by Middleton and co-workers13 based on the bathochromic shift of UV-visible 2max. The obtained spectral polarity index ranks the solvents at one end of the scale is the nonpolar perfluorohexane and at the opposite the highly polar and acidic l,l,2,3,3.3-hexafluoropropan-2-ol. The latter is much more polar than its hydrocarbon analog. 

It was shown that the solvent effect is generally significant and that it therefore needs to be taken into account properly. For nonpolar structures such as the bare tt backbone of TSB such an effect has been found to follow closely the refraction index of the medium though deviations may occur as a result of the nature of the excited states involved. Such deviations are more prominent when polar groups are attached to the tt backbone and become quite large for the dipolar structure of NATSB. It was shown by Frediani et al. [117] that to enhance the solvent effect it is more important to have a solvent with a high refractive index and that the static polarity of the solvent plays a minor role for the nondipolar structures which are known as the most promising ones. Another source of solvent dependence can also be found in how the electronic structure of the... 

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