Dielectric constants of solvents

K = electrical conductivity of solution Vf = flow rate volume/time of solution through ES capillary y = surface tension of solvent e = permitivity of solvent Co = permitivity of vacuum e/ o = dielectric constant of solvent... 

Figure 3.11 Catalytic efficiency, (fcca,/K ,)app ( ), of salt-activated subtilisin Carlsberg in hexane, THF, and acetone in comparison with T2 (transverse relaxation constant) (O) of mobile deuterons as a function of dielectric constant of solvent [103].

Eluting power roughly parallels the dielectric constants of solvents. The series also reflects the extent to which the solvent binds to the column material, thereby displacing the substances that are already adsorbed. This preference of alumina and silica gel for polar molecules explains, for example, the use of percolation through a column of silica gel for the following purposes-drying of ethylbenzene, removal of aromatics from 2,4-dimethylpentane and of ultraviolet absorbing substances from cyclohexane. 

Figure 4.9 Effect of refractive index and dielectric constant of solvent on O—O transitions of a polar molecule.

In the homogeneous mechanism, the reaction is assumed to start by protonation of one of the reactants, either ester (mechanisms denoted as Aac1 and Aac2 [397,398]) or, less frequently, alcohol (mechanism Aal1). It seems likely that protonation of reactants is an important step in esterification catalysed by ion exchangers, too. This follows from all that has been said above about the effect of the acidic properties of ion exchangers on their catalytic activity and is further supported by the effect of the dielectric constant of solvents (Fig. 18), which indicates that the reaction mechanism involves a positive ion and a dipolar molecule [454]. 

It is interesting to note that this relation was verified for polymerizations of styrene, p-methoxystyrene, and isobutylvinylether with iodine by Kanoh and Higashimura (19). They have demonstrated that the apparent rate constants of termination, propagation, and monomer transfer increased with increasing dielectric constant of solvent, and the rates of the increase for these three rate constants were of the order of Eq. (21). 

Fuoss (40) has improved Bjerrum s original treatment (37) of this situation and, although a number of other sophistications have been introduced, his formulation (41) is the one most used today. In fact rather fortuitously the relatively low dielectric constants of solvents employed in organic chemical reactions, particularly ionic polymerisations, are ideal media for the application of these theories. The analysis carried out by Fuoss leads not surprisingly to an equation... 

Table 4. Reaction rates and dielectric constants of solvents and reaction mixtures for the copolymerization of ethylene glycol carbonate (0.1 mol) with phthalic anhydride (0.1 mol) in, 100 ml of solvent initiated with KC1 (0.001 mol-%) at 200 °C. 541 (Reproduced by courtesy of Hiithig and...

The medium in which a species is dissolved or on which it is adsorbed may exert considerable influence on the intensity and wavelength of the fluorescence. Polar materials such as alcohols or esters frequently increase the intensity of the fluorescence relative to non-polar hydrocarbon solvents. The solvent environment often prevents or inhibits intersystem crossing to a triplet state in favour of excitation to a singlet state and fluorescence, while in many cases the opposite is true. The dielectric constant of solvents has been shown to influence the fluorescence intensity and wavelength maxima of some compounds [33,34]. Fig.2.9 shows the effect of solvent dielectric constant on the fluorescence intensity of DNS-phenol, while Table 2.4 shows the corresponding effect on the fluorescence wavelength [34]. For DNS-phenol, solvents of low dielectric constants result in the most intense fluorescence and shift the wavelength maxima to lower values. 

Figure 20 Relation between diastereomer excess (De%) and dielectric constant of solvent.

Where e is the elementary electric charge, r is the radius of ions, and e is the dielectric constant of solvent. 

By plotting log d[M]/dt( vs. log[I]o it could be shown that the order with respect to initiator is indeed one. However, the order with respect to monomer depends on the polymerization solvent being above two in solvents with dielectric constant below the monomers and only equal to two when dielectric constants of solvent and monomer are equal. 

Table 5. Dielectric constants of solvents suitable for cationic polymerisation (taken from Ref. The temperature is 20 °C unless otherwise stated...

Samulski and Samulski have presented a discussion on the solvent effect in terms of relative magnitudes of the dielectric constants of PBLG and solvent molecules. This theory predicts that compensations occurs for a critical value of the dielectric constant of solvent e = (EjE ), where Ej and 2 are the principal values of the dielectric constant perpendicular to the long molecular axis of PBLG. This... 

Fig. 21. Plots of the helical twisting power vs. dielectric constants of solvents at 25 °C...

Fig. 22. Plots of the parameters S vs. dielectric constants of solvents at 25 °C. a dichlorometha-ne, b chloroform c 1,2-dichloroethane, d 1,2-dichloropropane, e 1,1,1,1,2-tetrachloroethane, f 1,2,3-trichIoropropane, g pentachloroethane, h tetrahydrofuran, i N,N-dimethylformamide, j 1,4-dioxane, k m-cresol...

The analysis of the experimental results by the theory of Kimura et al. give interesting clues for understanding the polypeptide liquid crystal. In Fig. 22 the parameters fi, which are related to the attractive dispersion force proposed by Maier and Saupe and can be obtained from the slopes in the 1/S vs. 1/T plots or the values of Tj, are plotted against the dielectric constants of solvents. B is... 

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