Acetonitrile permittivity

On the assumption that = 2, the theoretical values of the ion solvation energy were shown to agree well with the experimental values for univalent cations and anions in various solvents (e.g., 1,1- and 1,2-dichloroethane, tetrahydrofuran, 1,2-dimethoxyethane, ammonia, acetone, acetonitrile, nitromethane, 1-propanol, ethanol, methanol, and water). Abraham et al. [16,17] proposed an extended model in which the local solvent layer was further divided into two layers of different dielectric constants. The nonlocal electrostatic theory [9,11,12] was also presented, in which the permittivity of a medium was assumed to change continuously with the electric field around an ion. Combined with the above-mentioned Uhlig formula, it was successfully employed to elucidate the ion transfer energy at the nitrobenzene-water and 1,2-dichloroethane-water interfaces. 

The dielectric constant (or relative permittivity) is usually expressed using the symbol c. The dielectric e is defined as the ratio of electric fields EJE for a vacuum and a substance placed between the plates of a capacitor. The dielectric constant of a vacuum is 1 and substances that can orient to greater or lesser extents in the applied field will have higher dielectric constants. The dielectric constant of heptane at 20°C is 1.9. Acetonitrile, CH3C=N , has a dielectric constant at 20°C of 37.5. The dielectric constant for water is near 80. 

However, for certain applications non-aqueous solvents have their advantages. Uni-univalent electrolytes dissolved at low to moderate concentrations in solvents with a relative permittivity larger than, approximately, 30 are completely dissociated into ions. Of the solvents on the List, methanol, glycols, glycerol, formic acid, ethylene and propylene carbonate, 4-butyrolactone, ethanolamine, 2-cyanopyridine, acetonitrile, nitromethane and -benzene, the amides, whether N-substituted or not, dimethyl sulfoxide, sulfolane, dimethyl sulfate, and hexamethyl phosphoramide have s > 30 at ambient conditions (Table 3.5). Most of these solvents have, indeed, been used in electrochemical processes. 

FIGURE 9.5 Polarity-dependent polarization patterns in photosensitized hydrogen abstractions from triethylamine DH (sensitizer A, 9,10-anthraquinone). For the formulas, see Chart 9.3. Shown are the signals of the olehnic a and P protons of the product N, A-di ethyl vinylamine, V-a (6.05ppm) and V-P (3.45ppm), as functions of the relative permittivity e (given at the right). Top, pure acetonitrile-fi 3 bottom, pure chloroform-fi 3 other traces, mixtures of these two solvents. All spectra were normalized with respect of the absolute amplitude of V-a. Further explanation, see text. 

FIGURE 9.6 Photosensitized hydrogen abstraction from triethylamine DH by 9,10-anthra-quinone A (for the formulas, see Chart 9.3). The rate constant of in-cage deprotonation as obtained from the polarity pattern, is shown as a function of the relativity permittivity e of the reaction medium (mixtures of acetonitrile and chloroform). The timescale of the CIDNP effect provides a kinetic window, within which such a quantitative treatment is apphcable. Further explanation, see text. 

When dissolved in nonpolar solvents such as benzene or diethyl ether, the colourless (2a) forms an equally colourless solution. However, in more polar solvents [e.g. acetone, acetonitrile), the deep-red colour of the resonance-stabilized carbanion of (3a) appears (1 = 475... 490 nm), and its intensity increases with increasing solvent polarity. The carbon-carbon bond in (2a) can be broken merely by changing from a less polar to a more polar solvent. Cation and anion solvation provides the driving force for this heterolysis reaction, whereas solvent displacement is required for the reverse coordination reaction. The Gibbs energy for the heterolysis of (2a) correlates well with the reciprocal solvent relative permittivity in accordance with the Born electrostatic equation [285], except for EPD solvents such as dimethyl sulfoxide, which give larger values than would be expected for a purely electrostatic solvation [284]. 

In contrast, dipolar aprotic solvents possess large relative permittivities (sr > 15), sizeable dipole moments p > 8.3 10 ° Cm = 2.5 D), and average C.f values of 0.3 to 0.5. These solvents do not act as hydrogen-bond donors since their C—H bonds are not sufficiently polarized. However, they are usually good EPD solvents and hence cation sol-vators due to the presence of lone electron pairs. Among the most important dipolar aprotic solvents are acetone, acetonitrile [75], benzonitrile, A,A-dimethylacetamide [76, 77], A,A-dimethylformamide [76-78], dimethylsulfone [79], dimethyl sulfoxide [80-84], hex-amethylphosphoric triamide [85], 1-methylpyrrolidin-2-one [86], nitrobenzene, nitro-methane [87], cyclic carbonates such as propylene carbonate (4-methyl-l,3-dioxol-2-one) [88], sulfolane (tetrahydrothiophene-1,1-dioxide) [89, 90, 90a], 1,1,3,3-tetramethylurea [91, 91a] and tetrasubstituted cyclic ureas such as 3,4,5,6-tetrahydro-l,3-dimethyl-pyr-imidin-2-(l//)-one (dimethyl propylene urea, DMPU) [133]. The latter is a suitable substitute for the carcinogenic hexamethylphosphoric triamide cf. Table A-14) [134]. 

Inspection of Table 4-9 reveals that the axial cis isomer (34a), which is the con-former with the higher dipole moment, becomes more favoured as the solvent polarity increases. In the most polar solvent studied, acetonitrile, AG° is nearly zero. Benzene, toluene, trichloromethane, dichloromethane, and methanol are seen to behave as more polar solvents than their relative permittivities would lead one to predict. The deviation for trichloromethane was particularly dilficult to explain (for a full discussion, see reference [89]). In general, good correlations between values and other solvent-... 

Fig. 4.105. Dielectric absorption spectrum (imaginary part of the complex permittivity, e") of LiBr solutions in acetonitrile at 25 °C. 1, Pure solvent 2, 0.107 M 3,0.194 M 4,0.303 M 5,0.479 M 6, 0.657 M. S and IP indicate the frequency regions of the relaxation processes of solvent and solute. For the sake of clarity, experimental data ( ) are added only for curves 1, 4, and 6 (J. Barthel, H. Hetzenauer, and R. Buchner, Ber. Bunsenges. Phys. Chem. 96 988, 1992).

Acetonitrile is a polar solvent with a relative permittivity of 35.9. It may be represented as a hard sphere with a diameter of 427 pm. Estimate the Gibbs energy of solvation of Na in acetonitrile according to the Born and MSA models. Compare the theoretical estimates with the experimental estimate given that the Gibbs energy of transfer for Na" " from water to acetonitrile is 15.1 kJmoP ... 

Dielectric permittivity data for water and acetonitrile in the temperature range from 0 to 50°C are plotted according to the Kirkwood equation in fig. 4.3. The straight lines shown are based on one-parameter least-squares fits, the slopes giving the value of 7VLgKf /3kB o- On the basis of this analysis for water, assum-... 

Here Y denotes a general bulk property, Tw that of pure water and Ys that of the pure co-solvent, and the y, are listed coefficients, generally up to i=3 being required. Annotated data are provided in (Marcus 2002) for the viscosity rj, relative permittivity r, refractive index (at the sodium D-line) d. excess molar Gibbs energy G, excess molar enthalpy excess molar isobaric heat capacity Cp, excess molar volume V, isobaric expansibility ap, adiabatic compressibility ks, and surface tension Y of aqueous mixtures with many co-solvents. These include methanol, ethanol, 1-propanol, 2-propanol, 2-methyl-2-propanol (tert-butanol), 1,2-ethanediol, tetrahydrofuran, 1,4-dioxane, pyridine, acetone, acetonitrile, N, N-dimethylformamide, and dimethylsulfoxide and a few others. 

In aprotic solvents, such as acetonitrile (H3CCN), dimethyl sulfoxide (H3CSOCH3), or methylisobutyl ketone (H3CCOCH(CH3)2), the potential electrolytes can be dissolved, but not ionized. These solvents have moderate permittivity, and they support the dissociation of true electrolytes. Dissolved acids (e.g., C6H5OH or H2O) may act as proton donors if a certain proton acceptor is created in the electrode reaction. 

According to equation [9.51], the permittivity increase leads to decreasing absolute value of electrostatic components of conformer transformations free energies in universal solvents. For instance, conformer transformation free energy of a-bromocyclohexanone in cyclohexane (8=2) is 5.2 kJ/mol, but in acetonitrile ( 36) it is -0.3 kJ/mol. 

Aprotic solvents with high relative permittivity, which are neutral (e.g. acetonitrile) ... 

The Mossbauer parameters of the tin halides show that in these systems the correlation between the isomer shifts and the donicity values is by no means as clear as for antimony pentachloride. The isomer shifts of tin tetrachloride agree, within experimental error, in frozen dimethyl sulphoxide, dimethylformamide and tributyl phosphate solutions, and those of tin tetraiodide agree in dimethyl sulphoxide, dimethylformamide and ethanol solutions. The Gutmann donicities of these solvents lie in the range 20-30. For both tin compounds, a further decrease in the donicity of the solvent causes an increase in the isomer shift, which indicates an increase in the electron density at the tin nucleus. This effect appeared in acetonitrile solution with tin tetrachloride, and in tributyl phosphate and carbon tetrachloride solutions with tin tetraiodide. (It must be emphasized that the last two solvents have low relative permittivities, whereas that of acetonitrile is high.)... 

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