Steric effects polarizability

Figure 3-6. a) The charge distribution, b) the inductive effect, and c) the resonance effect, d) the polarizability effect, e) the steric effect, and f) the stereoelectronic effect,... 

CMR represents the overall calculated molar refractivity. Its negative sign bring out a steric effect. It is interesting to note here that there is a high mutual correlation between ClogF and CMR (r= 0.966). Thus, it is very hard to predict for this data set if it is a negative hydrophobic or a polarizability effect. 

From topological methods. This method is best restricted to steric effect and polarizability parameters. 

MR, the molar refractivity, which parameterizes polarizability and steric effects and Verloop s parameters, which are steric substituent values calculated from bond angles and distances. 

The origin of stereofacial selectivity in electrophilic additions to methylene-cyclohexanes (2) and 5-methylene-l,3-dioxane (3) has been elucidated experimentally (Table 2) and theoretically. Ab initio calculations suggest that two electronic factors contribute to the experimentally observed axial stereoselectivity for polarizable electrophiles (in epoxidation and diimide reduction) the spatial anisotropy of the HOMO (common to both molecules) and the anisotropy in the electrostatic potential field (in the case of methylenedioxane). The anisotropy of the HOMO arises from the important topological difference between the contributions made to the HOMO by the periplanar p C-H a-bonds and opposing p C—O or C—C cr-bonds. In contrast, catalytic reduction proceeds with equatorial face selectivity for both the cyclohexane and the dioxane systems and appears to be governed largely by steric effects. ... 

Due to the small size of this data set no reliance can be placed on these results. We have carried out the correlation only to illustrate the method. In an earlier paper Arnett and Moe reported AH prin for the deprotonation of i-PrOH by lithium A-alkyl and N,N-diaUcyl amides, and by lithium alkyls (sets CR3 and CR4, Table 9). As the electrical effects of alkyl groups are constant within experimental error, only the number of alkyl groups, their steric effects and their polarizabilities can be variables. Values of AH pm for the amides were correlated with equation 28 ... 

The sulfur compounds depend only on polarizability the oxygen compounds show a small dependence on steric effects as well. 

From topological algorithms . This method is best restricted to steric effects and polarizability parameters. The nature of topological parameters has been described. They are composite parameters and result from a count of structural features. ... 

Steric effects are not directly related to bulk, polarizability is. 

Set 0X14, l-oximino-3-X-5-methyl-l,2-benzoquinones, is of interest because the substituents in this data set do not differ significantly from each other in electrical effects. As chemical reactivities in water normally do not depend on polarizability, it would seem that the variations in pXa in this data set should be a function of steric effects. Correlation with the two parameter segmental steric effect equation gave best fit with the v/ parameters. The correlation is significant at the 99.0% confidence level and is good. It seems probable that the pXa values of these compounds are dependent on steric effects. 

Product or reactant stabilizing factors that have been studied thus far include resonance/charge delocalization, solvation, hyperconjugation, intramolecular hydrogen bonding, aromaticity, inductive, jr-donor, polarizability, steric, anomeric, and electrostatic effects, as well as ring strain and soft-soft interactions. Product or reactant destabilization factors are mainly represented by anti-aromaticity, steric effects in some types of reactions, and, occasionally, electrostatic effects. What makes the PNS particularly useful is that it is completely general, mathematically provable,4 and knows no exception. 

As expected, the bioactivities of alkylstannanes are a function only of steric effects and polarizability. 

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