Solute polarization

Complex tautomerism for azoles with heteroatoms in the 1,2-positions occurs for pyrazoles which are not substituted on nitrogen. Scheme 10 shows the four important tautomeric structures (148)-(151) for 3-methylpyrazolin-5-one, and (152) and (153) as examples of other possible structures. A detailed investigation of this system disclosed that in aqueous solution (polar medium) the importance of the tautomers is (149) > (151) (150) or (148), whereas in cyclohexane solution (non-polar medium) (151) > (148) (149) or (150). 

The interactions between solute and the pha.ses are exactly the same as those present in LC separations, namely, dispersive, polar and ionic interactions. At one extreme, the plate coating might be silica gel, which would offer predominately polar and induced polar interactions with the solute and, con.sequently, the separation order would follow that of the solute polarity. To confine the polar selectivity to the stationai y phase, the mobile phase might be -hexane which would offer only dispersive interactions to the solute. The separation of aromatic hydrocarbons by induced polar selectivity could be achieved, for example, with such a system. 

Normal-phase LC tends to separate according to solute polarity since the stationary phase is polar and retention is often dominated by hydrogen bonding. Thus, normal-phase LC is useful in sorting out classes of materials according to the polarity of the solutes. Fatty acids are easily separated from monoglycerides, but the separation of individual saturated fatty acids from each other on the basis of their carbon... 

Table 4.3. Influence of solute polarity on water stmcture...

In this application we examine the influence of solute polarity on its partitioning into, and diffusion through the membrane. The concentration of solute is held constant at [S] = 30 cells in the W2 compartment. The dynamics are run 10 times for each value of Pb(WS), and the results averaged. 

H-bonding is an important, but not the sole, interatomic interaction. Thus, total energy is usually calculated as the sum of steric, electrostatic, H-bonding and other components of interatomic interactions. A similar situation holds with QSAR studies of any property (activity) where H-bond parameters are used in combination with other descriptors. For example, five molecular descriptors are applied in the solvation equation of Kamlet-Taft-Abraham excess of molecular refraction (Rj), which models dispersion force interactions arising from the polarizability of n- and n-electrons the solute polarity/polarizability (ir ) due to solute-solvent interactions between bond dipoles and induced dipoles overall or summation H-bond acidity (2a ) overall or summation H-bond basicity (2(3 ) and McGowan volume (VJ [53] ... 

Contribution of solvent and solute polarization to overall polarization effects showing that polarization of the solvent is the main effect but the solute itself is polarized by the aqueous environment. 

In most cases the order of elution for a series of isomers on liquid crystalline stationary phases is generally in accord with the solute length-to-breadth ratios with differences in vapor pressure and solute polarity also being of Importance in some cases, leading to an inversion of elution order to that predicted from length-to-breadth ratios [828,829,838]. Long and planar molecules fit better into the ordered structure of the liquid crystal phase whereas nonlinear and nonplanar molecules do not permeate so easily between the liquid crystal molecules of the stationary phase and are more easily eluted from the column. 

Nonpolar extraction Ci8, C8, C2, phenyl, Nonpolar functional Polar solutions Polar solvents such as... 

Wu JH, Winn PJ, Ferenczy GG, Reynolds CA (1999) Solute polarization and the design of cobalt complexes as redox-active therapeutic agents. Int J Quant Chem 73(2) 229-236... 

The same expression was also deduced by Lippert [57] and Mataga [58], which neglected by a change of the angle y however, the function / (To. no) has another form than in (19) as the effect of polarizability of solvent molecules was not accounted. Mac Ray also accounted the solute polarization and obtained [59] the same equation as (20) with similar function/(fio, oj as in (19). 

The physical interpretation of these equations is that when the solute polarizes the solvent to lower the solute-solvent interaction energy by an amount Gss,half the gain in free energy is canceled by the work in polarizing the solvent, which raises its own internal energy. 

It is clear in both of these studies that the small cavity size (which fails to entirely contain all of the atoms given standard van der Waals radii) causes electrostatic solvation free energies to be seriously overestimated — the difference in the 4-nitroimidazole system seems much too large to be physically reasonable. This overestimation would be still more severe were a correct DO model to have been used (i.e., one which accounted self-consistently for the full solute polarization using eq 30). Nevertheless, the D02 results may be considered qualitatively useful, to the extent that they identify trends in tautomer electrostatic solvation free energies. 

On the other hand, some limitations are present in the proposed formalism. For instance, all the solute polarization is assumed to be electronic in nature. Orientational (temperature dependent) effects are not introduced in the present formulation of solvation effects. However, this limitation allowed us to adopt a simple linear response model for the representation of the induced electronic polarization through the polarization of an electron... 

The values of P and selectivity factors are calculated from the experimentally derived solute polarity distribution coefficient for the test solutes ethanol, dioxane, and nitromethane. The solute distribution... 

Case 1 appears to accurately predict the observed dependence on persulfate concentration. Furthermore, as [Q]+otal approaches [KX], the polymerization rate tends to become independent of quat salt concentration, thus qualitatively explaining the relative insensitivity to [Aliquat 336]. The major problem lies in explaining the observed dependency on [MMA]. There are a number of circumstances in free radical polymerizations under which the order in monomer concentration becomes >1 (18). This may occur, for example, if the rate of initiation is dependent upon monomer concentration. A particular case of this type occurs when the initiator efficiency varies directly with [M], leading to Rp a [M]. Such a situation may exist under our polymerization conditions. In earlier studies on the decomposition of aqueous solutions of potassium persulfate in the presence of 18-crown-6 we showed (19) that the crown entered into redox reactions with persulfate (Scheme 3). Crematy (16) has postulated similar reactions with quat salts. Competition between MMA and the quat salt thus could influence the initiation rate. In addition, increases in solution polarity with increasing [MMA] are expected to exert some, although perhaps minor, effect on Rp. Further studies are obviously necessary to fully understand these polymerization systems. 

Torres-Lapasio, J.R., Garcia-Alvarez-Coque, M.C., Roses, M., Bosch, E Zissimos, A.M. and Abraham, M.H. (2004) Analysis of a solute polarity parameter in reversed-phase liquid chromatography on a linear solvation relationship basis. Analytica Chimica Acta, 515, 209-227. 

Because the dissolution of a solid is the result of molecular interaction between a solvent and the solid (which, once dissolved, becomes a solute), polar compounds capable of forming hydrogen bonds are water soluble, whereas nonpolar compounds dissolve only in organic solvents as the result of van der Waals and hydrophobic bonds. Compounds... 

Compounds that are more polar, and which can better hydrogen-bond with water, require less drastic alterations to the solvent environment to cause dissolution to occur. On the right side of Figure 3, we associate solute polarity with each formulation concept. Drugs that are good hydrogen donors, in the extreme sense, have acidic properties. Likewise, those that are very good acceptors have basic properties. For these compounds, formation of a salt by protonation or deprotonation is a feasible route. 

Of course, the presence of an electric field means dial a term accounting for the interactions of charged particles with this lield should be included in the solute Hamiltonian. When it is included, the effect is to increase the solute polarity in a fashion proportional to the solute polarizability and the strength of the external lield. Thus, die dipole moment of A increases. The solvent, seeing this increase, itself polarizes and moreover increases its own orientation to oppose A s dipole, and so on. 

The conventional viewpoint, which assumes that the ionic atmosphere is spherically symmetric, does not take account of the inevitable effects of ionic polarization. From an analysis of the general solution (19), however, it is evident that the ionic atmosphere must be spherically symmetric for nonpolarizable ions, and the DH model is therefore adequate. (Moreover, in very dilute solution polarization effects are negligibly small, and it does not matter whether we choose a polarizable or unpolarizable sphere for our model.) But once we have made the realistic step of conferring a real size on an ion, the ion becomes to some extent polarizable, and the ionic cloud is expected to be nonspherical in any solution of appreciable concentration. Accordingly, we base our treatment on this central hypothesis, that the time-average picture of the ionic solution is best represented with a polarizable ion surrounded by a nonspherical atmosphere. In order to obtain a value for the potential from the general solution of the LPBE we must first consider the boundary conditions at the surface of the central ion. 

Light microscopy has been used in a number of contexts to characterize block copolymer morphology. For crystalline block copolymers, spherulitic structures that result from organization of crystalline lamellae can be examined using microscopy. In solutions, polarized light microscopy can reveal the presence of lamellar and hexagonal-packed cylindrical micellar phases. Cubic micellar phases are optically isotropic, and consequently cannot be distinguished from sols only on the basis of microscopy. 

Carbonic anhydrase is a zinc(II) metalloenzyme which catalyzes the hydration and dehydration of carbon dioxide, C02+H20 H+ + HC03. 25 As a result there has been considerable interest in the metal ion-promoted hydration of carbonyl substrates as potential model systems for the enzyme. For example, Pocker and Meany519 studied the reversible hydration of 2- and 4-pyridinecarbaldehyde by carbonic anhydrase, zinc(II), cobalt(II), H20 and OH. The catalytic efficiency of bovine carbonic anhydrase is ca. 108 times greater than that of water for hydration of both 2- and 4-pyridinecarbaldehydes. Zinc(II) and cobalt(II) are ca. 107 times more effective than water for the hydration of 2-pyridinecarbaldehyde, but are much less effective with 4-pyridinecarbaldehyde. Presumably in the case of 2-pyridinecarbaldehyde complexes of type (166) are formed in solution. Polarization of the carbonyl group by the metal ion assists nucleophilic attack by water or hydroxide ion. Further studies of this reaction have been made,520,521 but the mechanistic details of the catalysis are unclear. Metal-bound nucleophiles (M—OH or M—OH2) could, for example, be involved in the catalysis. 

Tarantino, R., Bishop, E., Chen, F.-C., Iqbal, K., and Malick, AW Methyl-2-pyrrolidone as a cosolvent relationship of cosolvent effect with solute polarity and the presence of proton-donating groups on model drug compoundspharm. Sci., 83,1213-1216, 1994. 

As far as the overall potential drop (E) is concerned, the contribution of solute polarization, namely the electric resistance (R ) and junction potential difference (TTj) across any boundary layer, may be neglected. On the contrary, the Donnan potential difference (Eu) in any cell pair, which behaves as a DC generator with inverted polarities with respect to those of the external DC generator (Figure 9), has to be accounted for as the solute concentration difference at both sides of the anionic and cationic membranes increases ... 

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