Proton primary medium effect

If we consider the studied solution to be infinitely diluted we can neglect the term connected with the activity coefficients of the charged particles in the solution and obtain finally that the Hammett function is dependent only on the proton activity and the primary medium effects for the conjugate indicator pair ... 

Now let us return to the approaches connected with the estimation of the primary medium effect for protons, log y0 n+, that are used for obtaining quantitative information on the acidity of pure protolytic or aprotic solvents relative to the standard solution of a strong acid in water. From the thermodynamics, these are known to be a measure of the Gibbs free energy of proton transfer from the standard solution in water to the one in a non-aqueous solvent (M). This parameter is connected with the energy of proton resolvation in the following way ... 

This expression means that the values of pAM and the H0 values obtained for the standard solutions should coincide in the case when the primary medium effects for acid and base are the same, i.e. log y0 BH+ - log 7o,b = 0. Practical investigations of proton acidity in different solvents gives the evidence that this is not usually the case [50]. 

Izmailov et al. developed methods for the estimation of the primary medium effect of protons [51], which permit determination of the relative acidic properties of different solvents and solutions. The essence of their proposition consisted in the division of the primary medium effect of a separate ion into two terms... 

The kinetic solvent-isotope effects on these reactions are made up of primary and secondary kinetic isotope effects as well as a medium effect, and for either scheme it is difficult to estimate the size of these individual contributions. This means that the value of the isotope effect does not provide evidence for a choice between the two schemes (Kresge, 1973). The effect of gradual changes in solvent from an aqueous medium to 80% (v/v) Me2SO—H20 on the rate coefficient for hydroxide ion catalysed proton removal from the monoanions of several dicarboxylic acids was interpreted in terms of Scheme 6 (Jensen et al., 1966) but an equally reasonable explanation is provided by Scheme 5. 

Some of the most important evidence for the two-step mechanism comes from studies of base catalysis, in this regard, reactions involving primary and secondary amines have played a central role1-5. The initially formed cx-adduct, 1, is zwitterionic and contains an acidic proton, which can be removed by a base which may be the nucleophile itself. Conversion of 1 to products can then occur via the uncatalysed k2 pathway or via the base-catalysed hl pathway. The influence of Brpnsted base catalysis, the experimental observation of 1,1- and 1,3-cr-adducts, the sensitivity of the system to medium effects, are some experimental evidence of the mechanism depicted in equation 1. 

Microstructures of CLs vary depending on applicable solvenf, particle sizes of primary carbon powders, ionomer cluster size, temperafure, wetting properties of carbon materials, and composition of the CL ink. These factors determine the complex interactions between Pt/carbon particles, ionomer molecules, and solvent molecules, which control the catalyst layer formation process. The choice of a dispersion medium determines whefher fhe ionomer is to be found in solubilized, colloidal, or precipitated forms. This influences fhe microsfrucfure and fhe pore size disfribution of the CL. i It is vital to understand the conditions under which the ionomer is able to penetrate into primary pores inside agglomerates. Another challenge is to characterize the structure of the ionomer phase in the secondary void spaces between agglomerates and obtain the effective proton conductivity of the layer. 

Thus, the chemical shift of a proton attached to an organic compound is known to be affected by primary structure, i.e., the identity of the atom to which it is directly attached, the identities of those one atom removed, etc., by hybridization changes on these atoms, by van der Waals interactions with nearby nonbonded atoms, by electric Helds associated with permanent moments in the molecule, and by neighboring magnetic anisotropy Chemical shifts of substances in the liquid state may also be affected sizably by the medium. The magnitudes of these effects are comparable, and each is not readily calculated. Thus only a bold NMR spectroscopist will be conHdent in predicting the proton chemical shift to within 0.5 ppm. 

In view of the complications imposed on interpretation of kinetic isotope effects by quantum mechanical tunnelling and a variable profile of isotope effect with proton transfer to different bases, a more certain prediction would seem most probable if comparisons are restricted to reactions of a series of similar substrates within a given reaction medium. Within this framework it is possible to make reasonable predictions of the effect of substrate structure on the nature of the transition state for elimination using only primary kinetic hydrogen isotope effects. 

The effect of pH of the medium on transfection can be illustrated using chitosan. The transfection efficiency of chitosan largely depends upon the pH of the culture medium. The transfection efficiency at pH 6.9 was higher than that at pH 7.6. The dependence of transfection efficiency on the pH of the culture medium is considered to be due to the protonation of amines in chitosan. The p a of the primary amines in chitosan has been reported to be around 6.3-6.4. ... 

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