Delocalized electrical effects

The quantity 7ig is therefore a measure of the transmission of localized electrical substituent effects through the group G relative to the transmission of localized electrical substituent effects through the group Go. Similarly, for the transmission of the delocalized electrical effect, we may write... 

Taft and his coworkers14-16 developed a diparametric model which separated the electrical effect into contributions from the inductive (actually the field) and resonance effects. This separation depends on the difference in the extent of electron delocalization when a substituent is bonded to an sp3 -hybridized carbon atom in one reference system and to an sp2-hybridized carbon atom in another. As the first case represents minimal delocalization and the second extensive delocalization, we have referred to the two effects as the localized and delocalized electrical effects. This diparametric electrical effect model can be written in the form ... 

These Xpjr groups represent an especially difficult problem because their delocalized electrical effect depends on the steric effect when they are bonded to planar jr-bonded skeletal groups, Gp7r. An approach to the problem has been developed45,46. The electrical effect parameters are a function of the dihedral angle formed by Xp3r and Gpn. The relationship generally used has the form ... 

Xp groups when attached to planarttbonded skeletal groups, dpT, will exhibit a variable delocalized electrical effect i elated to their variable sterlc effect (5) ... 

Ui is the intrinsic delocalized (resonance) electrical effect parameter it represents the delocalized electrical effect in a system with no electronic demand. 

Og A composite delocalized electrical effect parameter of the ag, type with r] equal... 

We now consider the problem of the calculation of delocalized electrical effect substituent constants, crD, which correspond to a particular value of the dihedral angle 0, of the group. From the geometry previously determined for this type of system (Fig. 13) we may define the effective Van der Waals radius of the Xptt group MZ Z2 as... 

In order to include an group in a data set in which the substituents exhibit an steric effect it is necessary to know 0 so that the appropriate values of the delocalized electrical effect parameter, ctd and the steric parameter can be used. Values of 0 are usually unavailable. A method has been developed to circumvent this difficulty for chemical reactivities. The procedure is 1) A basis set, which contains only those members of the data set that show minimal dependence of their steric effect on conformation, is correlated with the LDS equation,... 

When an group is bonded to an sp3 hybridized carbon atom there is no delocalized electrical effect contribution and only the steric effect of the group need be considered. 

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