Hammet-Taft constant

Figure 4. Dependence of Hammet-Taft s type for the adsorption equilibria constants (Kx and K2) on the nature of the surface oxides, according to Eq. 2.

The Hammet substituent constant o Steric effects, described by the Taft values... 

Regression analysis is currently the most widely used correlative method in drug design. This is because it simplifies problems within a set of compounds by using a limited number of descriptors, notably the Hansch hydrophobic constant n, Hammet constants, or other electronic characteristics of substituents, and the Taft steric constant Eg. 

The electrochemical behaviour of the ribbed-functionalized iron(II) [65, 68] and ruthenium(II) [78] clathrochelates with alkylamine, thioaryl, thioalkyl, phenoxyl and crown ether substituents in a-dioximate fragments was characterized by E1/2 values for Fe3+/2+ and Ru3+/2+ couples (Table 37). The Tomes criterion values of most complexes exhibited reversible or quasi-reversible anodic processes. Moreover, the quasi-reversible oxidation processes are accompanied by the formation of insoluble products followed by passivation of the working electrode. The Ev2 values depend on the electron-donating properties of the substituents in the ribbed fragments. The correlations of E1/2 values for Ru3" 2+ and Fe + 2+ couples with these substituents Hammet s Opara constants were observed in Refs. 65, 68, and 78. These correlations are rather qualitative, but they enable one to conclude that ruthenium complexes are less sensitive to the change of substituents in dioximate fragments. There was no correlation between the Em values and the inductive Taft s (cr,) constants for substituents in dioximate fragments. 

Taft s (o ) and Hammet s (of,ara) constants for substituents in a-dioximate fragments n... 

XPS spectra of adsorbed PdCl2 contain no lines that could be attributed to PddD complexes with Pd-0 bonds. Although it points out that PdCl2 does not interact directly with the surface oxides, the adsorption eqtiilibria constants still increase after e carbon surface has been covered by oxygen groups [7]. Ki and K2 were found to depend on the type and number of the surface oxides in accordance with an equation derived with some assumptions from the equation of Hammet and Taft (fig. 4). 

A ] between the cation radical of la and the anion radical of m-DCB were proposed. In the absence of quencher, the quantum yield (cationic species at a nearly diffusion-controlled rate (1 x 10 dm mol s" ). In the presence of Q, the quantum yield (rate constants (kf ) for the nucleophilic addition of RNHj were obtained, as shown in Table 6.8. Hammet plots of Infc vs Taft o gave a linear correlation with a large negative slope (-2.1), showing that a substantial positive charge should be populated on the nitrogen atom of RNHj in the transition state (Figure 6.1). 

This behavior of X., or (E = hc/A , with Planck s constant h and velocity of light c) and the dependence on varying substituents or different solvents can be correlated with the Hammet Op parameter and the Kamlet-Taft tt parameter, respectively. With protic solvents and in aqueous solutions, the dependence of the 7T—band maxima on polarity and water content is less clear. The effect of water on the position of the low-energy band in two 4-(donor)-4 -nitroazobenzenes was studied in polyethyleneglycol/water mixtures. Although the polarity of the mixture obviously increased with increasing water content, the values showed a maximum. This could be due to an increased probability of... 

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