Steric components

The ortho effect may consist of several components. The normal electronic effect may receive contributions from inductive and resonance factors, just as with tneta and para substituents. There may also be a proximity or field electronic effect that operates directly between the substituent and the reaction site. In addition there may exist a true steric effect, as a result of the space-filling nature of the substituent (itself ultimately an electronic effect). Finally it is possible that non-covalent interactions, such as hydrogen bonding or charge transfer, may take place. The role of the solvent in both the initial state and the transition state may be different in the presence of ortho substitution. Many attempts have been made to separate these several effects. For example. Farthing and Nam defined an ortho substituent constant in the usual way by = log (K/K ) for the ionization of benzoic acids, postulating that includes both electronic and steric components. They assumed that the electronic portion of the ortho effect is identical to the para effect, writing CTe = o-p, and that the steric component is equal to the difference between the total effect and the electronic effect, or cts = cr — cte- They then used a multiple LFER to correlate data for orrAo-substituted reactants. 

A Gst is the difference in free energy due to steric constants in reactant and transition state, k is the rate constant of the nonsterically constrained reaction. The contribution of the steric component to the transition-state energy cannot be deduced accurately from DFT calculations because van der Waals energies are poorly computed. Force field methods have to be used to properly account for such interactions. 

The stress curve sharply increases when the steric component appears upon compression. The initial thickness of a deformed layer is equal to be half the distance Dq obtained by extrapolating the sharpest initial increase to stress zero. The value Do is 21 1 nm, which is close the thickness of two molecular layers (19.2 nm) of the a-helix brush, calculated using the CPK model and the orientation angles obtained by FTIR analysis. We have calculated the elastic compressibility modulus Y,... 

Fundamental studies of the effects of sodium polyacrylate have shown it to be an electrosteric dispersant, for which the steric component of interparticle repulsion is the dominant one at the typical dose rates used commercially. In such dispersions the main contributor to solution ionic strength is unadsorbed polyacrylate. As well as representing a waste of valuable dispersant polymer, this unadsorbed material also reduces the effectiveness of the adsorbed material as a steric dispersant by compressing the conformation of the adsorbed layer. 

The special, and somewhat complex, role played by a substituent in the 4-position was first observed in the quinazoline series. The carbon atom that received the —OH group during hydration was located by preparing all six C-methylquinazolines. Only when inserted in the 4-position did a methyl group prevent the addition of water.41,23 This inhibition has both an inductive (+1) and a steric component. A kinetic study of quinazoline-4-carboxamide showed that the carbamoyl substituent, because of its steric effect, slowed the attainment of hydration from less than a second to 2-4 hours. However, this substituent has the... 

The inductive effect of the donating C—D bonds to the observed large inverse secondary deuterium isotope effect has not been given proper consideration but treated as a rather minor component superimposed on the important steric component caused by larger amplitudes of vibrations of C—H bonds than those of the C—D bonds. 14C KIE have not been studied in this reaction. The C2, C V. C3 , C3, endo hydrogens are separated only by 2.11 A, substantially less than van der Waals radii (2 x 1.2 A)404. 

Examples for compounds are given in Figure 8.1, and the regression analysis equation is provided below for the QSAR of triazine derivatives in photosynthesis (Draber, 1992). The inhibitory potency expressed as a pl50 value is equal to a lipophilicity parameter tt (log of the partition coefficient P), an electronic substitution parameter a (the Hammett constant) and to a lesser degree to a steric component Es (the Taft constant). 

In AlPOz, molecular sieve synthesis the steric component appears to dominate. 

It can be anticipated that further studies of the isoelectric state at the PEO polymer solution/air interface would indicate that the steric component of disjoining pressure (see Section 3.3.3) appearing in foam films from long chain PEO polymers would prevent the foam films from rupturing. 

The preceding analysis gives the following rate constants (M s ) for Eq. (20) at 20°C Cr (1 x 10 ), Mo (5 x 10 ), W (1 x 10 ). This translates into the relative reactivity order Mo > W > Cr (5 x 10 10 1). Because of the ethyl groups in 7, this series has a steric component, but it is likely that the same qualitative order will always obtain because studies with [(mesitylene)M(CO)3] indicate that W > Cr by ca. 10 1. In dissociative reactions of 18-electron chromium triad complexes, the normal reactivity order is Mo > Cr > W. Steric effects only serve to increase the rate for Cr compared to W. The much greater rate of W compared to Cr for associative reactions at 17-electron centers is likely due to the greater size of W (and Mo). In any event, it is apparent that the reactivity of 17-electron complexes can be very dependent on the metal within a triad. 

The steric component is achieved by subtracting the energy E(strained) of a constrained or sterically hindered molecule conformation from the energy values E(g/o-bal) of the sterically most favourable conformation (AEx)- The steric influences of all substituents in the compound is determined by referencing AEx to AEh-... 

Charton made use of the dissociation constants of the readily available substituted acetic acids (Equation 22) stable for a wide range of X-substituents. There are negligible resonance and steric components to the transmission of the polar effect to the reaction centre in the equilibrium of Equation (22) and the pXg values fit Equation (23). The Gj value may be determined according to Equation (23) obtained from the dissociation which is very accurately documented for a very large number and variety of X-substituents and pj = 3.95. Other equilibria and rates (such as in Equations 17, 19 and 21) may be used as secondary standards to define Gj on the basis of Equation (23). 

Fernandez JM, Emerson K, Larsen RD, Gladysz JA (1988) J Chem Soc Chem Common 37 There is also a steric component to this turnover in binding mode Shambayati S, Crowe WE, Schreiber SL (1990) Angew Chem Int Ed Eng 29 256 Reetz MT, Hullmann M, Massa W, Berger S, Rademacher P, Heymanns P (1986) J Am Chem Soc 108 2405... 

---