Steric parameters for

It has been suggested that the discrepancies between the value of k ikK observed and that predicted on the basis of simple statistics may reflect the greater sensitivity of combination to steric factors. Beckhaus and Rtichardt164 reported a correlation between log(A )/ ,<.,) (after statistical correction) and Taft steric parameters for a scries of alkyl radicals. 

The rates of radical-forming thermal decomposition of four families of free radical initiators can be predicted from a sum of transition state and reactant state effects. The four families of initiators are trarw-symmetric bisalkyl diazenes,trans-phenyl, alkyl diazenes, peresters and hydrocarbons (carbon-carbon bond homolysis). Transition state effects are calculated by the HMD pi- delocalization energies of the alkyl radicals formed in the reactions. Reactant state effects are estimated from standard steric parameters. For each family of initiators, linear energy relationships have been created for calculating the rates at which members of the family decompose at given temperatures. These numerical relationships should be useful for predicting rates of decomposition for potential new initiators for the free radical polymerization of vinyl monomers under extraordinary conditions. 

The steric parameters for the estimation of reactant state effects were chosen to be the conformational free energy differences for cyclohexane axial-equatorial equilibria (A-values) (8). In order to establish the methyl group as the standard size group, modified A-values (A ) for the various groups were used, by simply subtracting the A value for the methyl group (1.70) from the A values of the various substituents ... 

The steric parameter for NCD groups can be obtained directly from van der Waals radii or calculated from them. The values for SCD groups are often obtainable from van der Waals radii, although in some cases they must be derived as secondary values from regression equations obtained by correlating rate constants with known values of... 

Taft steric parameter for para substituent on benzamide side chains. fTaft steric parameter for para substituent on benzyloxy side chains. gTaft steric parameter for para substituent on benzoyloxy side chains. hIndicator variable for polycyclic aromatics I = 1 if present, I = 0 if absent. 

Figure 8. Experimental data on the effect of (a) Taft s steric parameter for ethers and (b) modified Small s number for hydrocarbons on their reverse osmosis separations in systems involving dilute aqueous solutions and cellulose acetate membranes...

Topological Parameterization of Sterle Effects We have noted that different rv based steric parameters will be required for conformatlonally dependent groups in order to account for phenomena with different ster-ic requirements This must result in a multiplicity of steric parameters for different types of phenomena ... 

Methods should be available for the estimation of steric parameters for those groups for which experimental data is unavailable. 

In his calculation of Es values, Taft made use of average values of log (krx/kr0). Evidence has been presented which suggests that this procedure is unjustified, as the dependence of log kvx/kvo0) on X is not independent of medium. Effective values, oef, of the steric parameter for some ICD groups have been obtained by means of a two step procedure, 24, 25 ... 

Eq. 62 and 63 show the dependence of the steric parameter for an group upon the dihedral angle, 0. The ctd constant of an group must also depend upon the dihedral angle since the extent of delocalization is a function of 0. 

We examined whether the steric effect of substituents R of type CR1R2R3 can be expressed in terms of the steric effect of component substituents R1, R2 and R3 using E, parameters. With increasing substitution at the a-carbon, the total steric effect of substituents R has been observed to increase telescopically in such series as Me, Et, i-Pr and t-Bu4). Thus, the simple addition of steric parameters for a-substituents is inadequate to represent the situation. We have found that the E° parameter for 24 primary, secondary and tertiary alkyl groups can be formulated as Eq. 21 by a linear combination of E parameters of component a-substituents 20). 

The hydroxide anion catalyzed rate constants for the scries of alkyl tris-(2-methoxyethoxy)silanes obtained by Fohl were used to define a modified Taft equation, log(/cHO//cHO ) = 2.48cr +1.67 Es [42], A good correlation was obtained, except for vinyl and phenyl substituents. The anomalous behavior observed for phenyl and vinyl tris-(2-methoxyethoxy)silanes may have resulted because the steric parameter or the polar parameter may be influenced by the carbon-carbon double bond. The steric parameter for a,yS-unsaturated substituents may include an appreciable resonance effect. The polar parameter values may be influenced by the ability of silicon to back-bond through d orbitals with the a,/J-unsaturated system [37,49]. 

Taft s o has nearly the same meaning as Hammett s c (with different numerical values), Es is the steric parameter for substituent X, and 8 is a measure of the sensitivity of the reaction to changes in steric properties near the reaction center (similar to p). Extensive compilations of values of o, o and Es appear in Perrin et al. (1981) and McPhee et al. (1978), respectively. 

For each ortho substituent, the deviation from the reference line is far greater than the error in the slope of this line. A steric parameter for each... 

A geometric steric parameter for the X substituent based on the non-hydrogen atoms of the substituent and their topological distance from the atom to which the substituent is linked [Austel et al, 1979] it is calculated as ... 

Austel, V., Kutter, E. and Kalbfleisch, W. (1979). A New EasUy Accessible Steric Parameter for Structure-Activity Relationships. Arzneim.Forsch., 29,585-587. 

Muresan, S., Bologa, C, Mracec, M., Chiriac, A., Jastorft B., Simon, Z. and Naray-Szabo, G. (1995). Comparative QSAR Study with Electronic and Steric Parameters for cAMP Derivatives with Large Substituents in Position 2, Position 6 and Position 8.1.MolStruct(Theochem), 342,161-171. 

---