O parameter

How does the Hiickel ff-electron model deal with pyridine Nitrogen is more electronegative than carbon, so the N atom ought to have a higher (more negative) T-electron charge than a carbon atom in benzene. It is conventional to write the heteroatom (X and Y are used to denote heteroatoms such as N and O) parameters in terms of the standard ac and yScc... 

Correlation between o parameters and k (CO) for substituted arene Cr(CO)3 complexes... 

In this and later regression equations, the 95% confidence limits are given in parentheses. Eqn. 8.1 quantitates the facilitation of hydrolysis by electron-withdrawing substituents. Because such substituents decrease the electron density on the carbonyl C-atom and render it more susceptible to nucleophilic attack, Eqn. 8.1 is compatible with a base-catalyzed reaction, as indeed shown. Eqn. 8.1, thus, leads to mechanistic insights, but its predictive power is narrow since the o parameter is available for only a few substituents. 

Fig. 2.10 Plots of logAr for the reactions of CH (CHj)3 with IrCl (A) and Fe(CN)6 (O) vs the Taft o parameter. " Reprinted with permission from S. Steenken and P. Neta, J. Amer. Chem. Soc. 104, 1244 (1982). 1982 American Chemical Society.

Fig. 48. The o-parameter for PVAc obtained by emulsion polymerization. The transition indicates gel-formation in the latex particle. (Compare also Figs. 41 and 42188 16,>)...

In summary, the data for a wide range of activating and deactivating substituents in aromatic substitution reactions involving the replacement of hydrogen or other metalloid groups are correlated with truly remarkable accuracy by the o-+-parameters. 

With respect to a-substituents bearing p- or 7r-electrons which are directly attached to the C—Cl bond (Table 6, Z = CH2=CH to CH3CH20), these may delocalize their electrons through resonance or mesomeric effects with the positively charged carbon atom in the transition state. Because of this, they were not plotted in the Taft figure for a-substituted ethyl chlorides. Furthermore, the rates for these substituents also could not be correlated with the electrophilic substituent constants a+. The o+ parameters have been defined for substituents on the benzene ring which are far from the reaction site. Even though steric effects may interfere with the coplanarity and hence with delocalization, the effect of these substituents was believed to be polar in nature. 

The Dewar group of programs (MINDO/3, MNDO, AMI, PM3) use thermochemical data wherever available for parameterization, so as to yield reasonably correct conformations, gas-phase heats of formation, and spectra. They work well and provide geometry-minimized ground states for compounds of C, H, N, and O parameters were added for B, P, Cl, Br, Se, and so on, but not all combinations of all these elements in a new compound are available (because of lack of thermochemistry data). 

A straightforward determirtation of Kl, Ng, k, and Cl,o parameters can be done in the following way. k and Cl.o niay be easily determined by considering two asymptotic situations of Langmuir adsorption isotherm, that is, very high or very low substrate concentration. At very high concentrations of substrate the following inequality may be assumed to hold ... 

The first conclusion is that the integrals I 2, i are negligible in comparison with the integrals for A = 0, 1, and so the static parameters Si, (Eq. (6-46)-(6-47) are close in absolute value to the integrals /oo, That the o-parameter is likely to be much greater than the jr-parameter can be seen from the following argument. Suppose we... 

There are very few examples in our data bank where only an electronic term (o-, pKa or M.O. parameters) suffices to correlate structure with activity. This accounts for the fact that up to this point molecular orbital calculations (28, 29) of charge densities have yielded so few equations correlating chemical structure and biological activity in quantitative terms. However, the combination of electron distribution obtained via... 

Table Ic lists equations in which an electronic term is significant. Since in all of these examples the same electronic term (a) has been used, the intercepts are comparable. Again the intercept contains information on the stereoelectronic character of the essential pharmacophoric function. The o- parameter simply accounts for the electronic perturbation of various substituents on this intrinsic activity. Some of these equations contain a (log F)2 term, and some do not. Those equations lacking this term indicate sets comprised of molecules with suboptimal lipophilic character. More active members could be made by adding more lipophilic substituents. In almost all of these sets it should be possible to make more active derivatives by the proper combination of substituents, bearing in mind that log F0 must first be found and then not exceeded while the electron-releasing or withdrawing (as the sign of p calls for) effect of the substituents is maximized.

Tazi, B. and Savadogo, O., Parameters of PEM fuel-cells based on new membranes fabricated from Nafion-silicotungstic acid and thiophene, Electrochim. Acta, 45,4329, 2000. 

Table 4. D and O parameters of various acids and bases, taken from Ref. ...

The appearance of the o-" parameter in a large number of reactions and interactions involving X-phenols indicates that the phenoxy radical can be a potent, reactive intermediate in myriad reactions. The availability of a fast, easily retrievable computerized database to corroborate this phenomenon was useful. This approach of lateral validation was crucial in establishing a QSAR model that was not only statistically significant but also mechanistically interpretable. 

Substance Type of structure Lattice O parameters (A) Spacing of, cleavage planes (A)... 

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