Solvent effects basicities

A bathochromic shift of about 5 nm results for the 320-nm band when a methyl substituent is introduced either in the 4- or 5-posiiion, The reverse is observed when the methyl is attached to nitrogen (56). Solvent effects on this 320-nm band suggest that in the first excited state A-4-thiazoline-2-thione is less basic than in the ground state (61). Ultraviolet spectra of a large series of A-4-thiazoline-2-thiones have been reported (60. 73). 

In this section, we review three studies on the coupled substimtion and solvent effects on basicity and acidity. 

For a broad review of substituent and solvent effects on acidity and basicity, see R. W. Taft, Prog. Phys. Org. Chem. 14 247 (1983). 

The structure of a molecule can affect its acidity or basicity in a number of ways. Unfortunately, in most molecules two or more of these effects (as well as solvent effects) are operating, and it is usually very difficult or impossible to say how much each effect contributes to the acid or base strength. Small differences in acidity or basicity between similar molecules are particularly difficult to interpret. It is well to be cautious when attributing them to any particular effect. 

In the general case, an incoming nucleophile would be expected to be favoured by (i) a high basicity consistent with ( ) a high polarizability, and the metal complex to favour its approach if (in) it contains electron-acceptive, or B class ligands. An interpretation of the available data may be essayed on these lines. The infrared data upon Ni(CO)4 are consistent with a weakening of the C-O bond , and it would be of interest to examine the solvent effect upon the Ni-C bond. 

The least problematic issues are UV spectral changes as a function of different solvents between the reference and the test sample. Solvent effects on UV spectra in solvents of decreased dielectric constant compared with water parallel solvent effects on apparent pKa. The changes are most marked for acids, for example, leading to a numerical increase of up to two pKa units - an apparent decrease in the acidity of the carboxylic acid. Effects on bases are considerably less. The apparent pKa of a base in a reduced dielectric constant solvent might be up to about half a pKa unit numerically lower (less basic). The UV spectra of neutral compounds... 

Polar solvents have no effect on the rate constant of the reaction R02 + RH [56], This means that the solvation energies of the peroxyl radical R02 and TS R02 HR are very close. A different situation was observed for the reaction of cumylperoxyl radical with benzyl alcohol (see Table 7.10). The rate constant of this reaction is twice in polar dimethylsulfoxide (s = 33.6) than that in cumene (a 2.25). It was observed that the very important property of the solvent is basicity (B), that is, affinity to proton. A linear correlation... 

Local density functional theory may be introduced within the RF model of solvent effects thorugh the induced electron density. The basic quantity for such a development is the linear density response function [39] ... 

Carboxylic acids were introduced by the reaction of the polysilane(II) with carboxylic acid anhydrides in the presence of an amine. Table 2 shows the results. In order to examine the effect of unsaturated bonds and halides in the side chain on photolysis, double bonds or chloride groups were introduced with the carboxylic acid. Polysilanes(III) were also soluble in polar solvents and basic aqueous solution. The solubility in tetramethyl ammonium hydroxide aqueous solution(TMAH) depended strongly... 

Basic Itait" - Colloidal Particle Secondary Forces ( ) - Solvent Effects... 

From the data available so far it is clear that basic patterns in the solvent effect start to become recognizable, but that further studies are needed in order to arrive at a model that has any predictive value. 

The multiparametric equation 14 has been also applied to estimate solvent effects on the relative basicities (SAG) of propylamines97 ... 

Grob K (1987) On-column injection in capillary gas chromatography basic techniques, retention times and solvent effects. Springer, Berlin Heidelberg New York, p 67... 

In 1976, Kamlet and Taft introduced their solvatochromic comparison method [25, 26], The hydrogen-bond donor acidity a and basicity /3 together with the solvent polarity and polarizability jv were employed to correlate the solvent effects on reaction rates, equilibria, and spectroscopic properties XYZ according to equations of the form... 

First, we remove the solvent and consider only the system of adsorbent and ligand molecules. We make this simplification not because solvent effects are unimportant or negligible. On the contrary, they are very important and sometimes can dominate the behavior of the systems. We do so because the development of the theory of cooperativity of a binding system in a solvent is extremely complex. One could quickly lose insight into the molecular mechanism of cooperativity simply because of notational complexity. On the other hand, as we shall demonstrate in subsequent chapters, one can study most of the aspects of the theory of cooperativity in unsolvated systems. What makes this study so useful, in spite of its irrelevance to real systems, is that the basic formalism is unchanged by introducing the solvent. The theoretical results obtained for the unsolvated system can be used almost unchanged, except for reinterpretation of the various parameters. We shall discuss solvated systems in Chapter 9. 

Palm s group has continued to develop statistical procedures for treating solvent effects. In a previous paper, a set of nine basic solvent parameter scales was proposed. Six of them were then purifled via subtraction of contributions dependent on other scales. This set of solvent parameters has now been applied to an extended compilation of experimental data for solvent effects on individual processes. Overall, the new procedure gives a signiflcantly better flt than the well-known equations of Kamlet, Abboud, and Taft, or Koppel and Palm. 

In contrast to ISEs with neutral ion carriers in the membrane, not even qualitative rules have been formulated for the solvent effect on the behaviour of ISEs with ion-exchanger ions in a liquid membrane. A basic condition for the ion-exchanger ions is that they be strongly hydrophobic. It must hold for the standard Gibbs energy of transfer of the ion-exchanger ion X and the deter-minand Y that... 

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