Dipole moment of solutes

To answer this question, let us first consider a neutral molecule that is usually said to be polar if it possesses a dipole moment (the term dipolar would be more appropriate)1 . In solution, the solute-solvent interactions result not only from the permanent dipole moments of solute or solvent molecules, but also from their polarizabilities. Let us recall that the polarizability a of a spherical molecule is defined by means of the dipole m = E induced by an external electric field E in its own direction. Figure 7.1 shows the four major dielectric interactions (dipole-dipole, solute dipole-solvent polarizability, solute polarizability-solvent dipole, polarizability-polarizability). Analytical expressions of the corresponding energy terms can be derived within the simple model of spherical-centered dipoles in isotropically polarizable spheres (Suppan, 1990). These four non-specific dielectric in-... 

Dipole moments of solutes are also involved in the so-called reaction-field theory1112 which predicts generally the Gibbs energy of solvation, and from it the stability of con-formers as dependent on solvents. Besides the dipole the quadrupole moment is also taken into the calculations. For instance, conformational equilibria of cyclic halo ketones were predicted from the dipole moments of the two conformers with fair success13. However, the whole theory was criticised1415 that there is too much arbitrariness in the... 

The usual computation of electric dipole moments of solute molecules is by means of formulae due to Debye ( Polar molecules . Chemical Catalog Co., 1929, 11). In our notation Debye s formulae are... 

Figure 12.1.9. Dispersion forces as aresult of dipolar interactions between the virtually excited dipole moments of solute and solvent.

The angles ot, p, and x relate to the orientation of the dipole nionient vectors. The geonieti y of interaction between two bonds is given in Fig. 4-16, where r is the distance between the centers of the bonds. It is noteworthy that only the bond moments need be read in for the calculation because all geometr ic features (angles, etc.) can be calculated from the atomic coordinates. A default value of 1.0 for dielectric constant of the medium would normally be expected for calculating str uctures of isolated molecules in a vacuum, but the actual default value has been increased 1.5 to account for some intramolecular dipole moment interaction. A dielectric constant other than the default value can be entered for calculations in which the presence of solvent molecules is assumed, but it is not a simple matter to know what the effective dipole moment of the solvent molecules actually is in the immediate vicinity of the solute molecule. It is probably wrong to assume that the effective dipole moment is the same as it is in the bulk pure solvent. The molecular dipole moment (File 4-3) is the vector sum of the individual dipole moments within the molecule. 

Experimental values are collected in the McClellan book (B-63MI40400) and in a review on dipole moments and structure of azoles (71KGS867). Some selected values are reported in Table 3. The old controversy about the dipole moment of pyrazole in solution has been settled by studying its permittivity over a large range of concentrations (75BSF1675). These measurements show that pyrazole forms non-polar cyclic dimers (39) when concentration increases and, in consequence, the permittivity value decreases. 

MO methods have been used to calculate dipole moments of each of the three ring systems (73MI50403, B-70MI50400). Calculated values for aziridine are somewhat higher (2.09-2.40 D) than the known experimental value (1.89 D). Dipole moment studies on a few simple aziridines have led to the determination of the preferred conformation of N-arylaziridines in solution and in the vapor state (71JCS(C)2104, 66DOK(169)839). For the 1-azirine system, no values have been determined experimentally, but values of 2.40-2.56 D for 1-azirine and 2.50-2.51 D for 2-azirine have been calculated (73MI50403). 

Many of the properties oj -hydroxypyridines are typical of phenols. It was long assumed that they existed exclusively in the hydroxy form, and early physical measurements seemed to confirm this. For example, the ultraviolet spectrum of a methanolic solution of 3-hydroxypyridine is very similar to that of the 3-methoxy analog, and the value of the dipole moment of 3-hydroxypyridine obtained in dioxane indicates little, if any, zwitterion formation. However, it has now become clear that the hydroxy form is greatly predominant only in solvents of low dielectric constant. Comparison of the pK values of 3-hydroxypyridine with those of the alternative methylated forms indicated that the two tautomeric forms are of comparable stability in aqueous solution (Table II), and this was confirmed using ultraviolet spectroscopy. The ratios calculated from the ultraviolet spectral data are in good agreement with those de-... 

Here a = Spafi is the average value of the polarization tensor of the molecule, / = a —la. being its anisotropy, and fi the dipole moment of the molecule. We assume that the concentration of active molecules in the gas mixture or liquid solution is so small that intermolecular coupling may be neglected. 

The quantitative solution to the problem is given in section 11.3. The effectiveness factor T)P (< 1) which expresses the extent to which the promoting ion is fully utilized (qP=l) depends on three dimensionless parameters n, J and P n is the dimensionless dipole moment of the promoting ion, J is a dimensionless current and P, a promotional Thiele modulus, is proportional to the film thickness, L. 

The physical meaning of the g (ion) potential depends on the accepted model of an ionic double layer. The proposed models correspond to the Gouy-Chapman diffuse layer, with or without allowance for the Stem modification and/or the penetration of small counter-ions above the plane of the ionic heads of the adsorbed large ions. " The experimental data obtained for the adsorption of dodecyl trimethylammonium bromide and sodium dodecyl sulfate strongly support the Haydon and Taylor mode According to this model, there is a considerable space between the ionic heads and the surface boundary between, for instance, water and heptane. The presence in this space of small inorganic ions forms an additional diffuse layer that partly compensates for the diffuse layer potential between the ionic heads and the bulk solution. Thus, the Eq. (31) may be considered as a linear combination of two linear functions, one of which [A% - g (dip)] crosses the zero point of the coordinates (A% and 1/A are equal to zero), and the other has an intercept on the potential axis. This, of course, implies that the orientation of the apparent dipole moments of the long-chain ions is independent of A. 

The measurement of change in the surface potentials of aqueous solutions of electrolytes caused hy adsorption of ionophore (e.g., crown ether) monolayers seems to he a convenient and promising method to ascertain selectivity and the effective dipole moments of the ionophore-ion complexes created at the water surface. 

It is important to know the influence of the physicochemical parameters of the mobile phase (dipole moment, dielectric constant, and refractive index) on solvent strength and selectivity. The main interactions in planar chromatography between the molecules of the mobile phases and those of solutes are caused by dispersion forces related to the refractive index, dipole-dipole forces related to the dipole moment, induction forces related to a permanent dipole and an induced one, hydrogen bonding, and dielectric interactions related to the dielectric constant. Solvent strength depends mainly on the dipole moment of the mobile phase, whereas the solvent selectivity depends on the dielectric constant of the mobile phase. 

Fig. 2.2 Self-Consistent Reaction Field (SCRF) model for the inclusion of solvent effects in semi-empirical calculations. The solvent is represented as an isotropic, polarizable continuum of macroscopic dielectric e. The solute occupies a spherical cavity of radius ru, and has a dipole moment of p,o. The molecular dipole induces an opposing dipole in the solvent medium, the magnitude of which is dependent on e.

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