Taft sigma

Taft sigma values cr perform a similar function with respect to aliphatic and alicyclic systems. Values of cr are listed in Table 9.1. 

Hammett-Taft sigma constants Electron density TT-Bond reactivity Electron polarizability Dielectric constant Dipole moments Ionization potential Electron affinity... 

R.W. Taft, Sigma Values from Reactivities, J. Phys. Chem., 1960, 64, 1805. 

Table and discussion Electronic effects in molecules (Hammett and Taft sigma values). 644... 

TAFT SIGMA AND SIGMA, CONSTANTS IMPROVE LOG OCTANOL/WATER PARTITION COEFFICIENT BASED OSAR FOR FATHEAD MINNOW TOXICITY... 

Quantitative structure-activity relationships for fathead minnow Pimephales promelas) LC50 data were developed. As others have observed, log toxicity and log octanol/water partition coefficent (log P) are linearly related for alcohols (n = 20), alkyl ketones (n = 14), alkyl esters (n = 16), and alkyl nitriles (n = 8). When the sums of Taft sigma constants for the substituents of the most anionic carbon in each chemical structure were included in the regression of log LC50 versus log P, the coefficient of determination (r ) improved from 0.84 (for log P only) to 0.92 for alcohols, from 0.82 to 0.92 for ketones, from 0.77 to 0.91 for esters, and from 0.15 to 0.94 for nitriles. The r for the sum of Taft sigma constants versus LC50 for each class was less than 0.17. When the sum of a, was used instead of Taft sigma constants similar results were obtained. 

Quantitative structure-activity relationships, fathead minnow, Pimephales promelas, Taft sigma constants, octanol/water partition coefficients, ketones, nitriles, alcohols, esters. 

The Taft sigma constant and a, constants are inductive or polar substituent constants (Hansch and Leo 1979) derived from hydrolysis rates of substituted esters (Taft 1956) and ionization of substituted acetic acids (Charton 1981), respectively. Either constant could be useful in describing anionic potential. Therefore, they were evaluated as descriptors for the above mentioned data sets to determine if the outliers could be predicted. The following data indicate that either constant is a good descriptor. 

TABLES AND DISCUSSION ELECTRONIC EFFECTS IN MOLECULES (HAMMETT AND TAFT SIGMA VALUES)... 

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