Steric contribution

The stereochemical course of the first reaction of 99 can be rationahzed in terms of the relative steric contributions of the flanking bridge, in particular of methylene... 

As long as there are no important steric contributions to the transition-state energies, the elementary rate constant of Eq. (1.22) does not sensitively depend on the detailed shape of the zeolite cavity. Then the dominant contribution is due to the coverage dependent term 9. 

Estimated Electronic and Steric Contributions to Intrinsic Activation Barriers and Reaction Energies for Oxidative Coupling via the Most Feasible Pathway along... 

Estimated Electronic and Steric Contributions to the Thermodynamic Stabilities of the Favorable Isomers of 2a and 4a of the [Nih(octadienediyi, )LJ Complex3... 

In several cases, the experimental plots were indeed linear. According to a molecular theory,11 39 40 the slope Hq and the intercept Sq are related to the chiral dispersion forces and to the entropic gain obtained by a twisted relationship between the polymer molecules (Figure 7.4), respectively. The value of Sq reflects the fact that the helices take up less space when packed in a helical arrangement and should be related to the hard-core steric contribution to the twist. Positive or negative values of Sq should be related to a right- or left-handed twist between the polymer molecules, respectively, and should be determined solely by steric effects. 

To summarize, in the field of lyotropic cholesterics formed by helical polymers of known geometry, it is possible to predict qualitatively only the entropic steric contribution to the cholesteric twist The prediction relies on the model of Figure 7.4 and should be compared to the experimental Sq data obtained by extrapolation of the twist to l/T = 0. A practical model for calculating the often dominant dispersion contribution is not yet available. 

In addition to the electronic difference between PR3 and PH3, bulkier ligands on the phosphine can change the reaction through their steric effect. Using the R = Bu on the anthraphos system, Haenel et al. calculated the available molecular surface (AMS) around the metal center as a measure of the space available to the alkane (13b). They correlated the AMS to the relative reactivities of the catalysts and the results show that two bulky tert-butyl groups on each P certainly limit the access to the metal center, and thus, may reduce the reactivity. Other theoretical studies on the pincer complexes showed that this steric contribution/ limitation plays a less important role than the activation barriers introduced by the catalyst itself (22), where the increase in energy barrier induced by the bulky 4Bu is smaller than the original barriers calculated... 

Before continuing, it has to be noted that the energy difference between the secondary and primary propene insertion, AEK 0, can be considered composed by two main contributions, electronic and steric. The steric contribution to AE po, due to steric interaction between the monomer, the growing chain and the ligand skeleton, was modeled successfully through simple molecular mechanics calculations [78-80], and was reviewed recently [11,24], For this reason in the following we will focus only on the electronic contribution to AEKgio. 

Geminal substituents in multiply substituted compounds present the situation of closest mutual proximity. For that reason it is often difficult, if not impossible, to separate steric from electronic contributions to nonadditivity effects (NAEs, ICSs). A predominance of steric contributions to the ICS is expected when at... 

The values for % and 0 are as defined above and used as dimensionless numbers. Property as in Hammett relations should be the logarithm of a rate constant, equilibrium constant, etc. An important refinement for the steric contribution was made after the recognition that below a certain threshold of cone angles the property no longer changed and one should use the value 0 for c [29], Above the threshold a linear relation was found. Thus, for each property a steric threshold 0 was defined and the equation now reads ... 

In order to be able to include a steric contribution in the interparticle energy calculation, an estimate of the adsorbed layer thickness is required. This is very difficult to access experimentally probably the only technique which might be able to provide an estimate is small-angle neutron scattering which was beyond the scope of this work. As a result, a theoretical estimation of the thickness was made, based on a few key observations. This is described below. 

Using this data for the layer thickness, the total energy of interaction was calculated by summing the electrostatic and steric contributions, the latter of which was calculated according to the method of Ottewill. Total interaction energies at three values of interparticle separation are shown in Figure 11 as a function of polymer dose. 

There are a number of assumptions made in the model that are questionable, and these are probably responsible for the discrepancy between the predicted and observed behaviour of the slurries. For example, the steric contribution has been calculated assuming that the adsorbed layer has a well-defined thickness. For adsorbed polymers this is unlikely to be the case, as the volume fraction profile of the polymer will decay gradually as a function of distance from the surface. Furthermore, it was assumed that the effective ionic strength in the adsorbed polymer layer is the same as in solution. However, this also is unlikely since one of the main components of the solution ionic strength is the polymer itself, and unadsorbed polymer will be excluded from the adsorbed layer. Finally, the connectivity of the charged groups on the polymer was not considered, so its contribution to ionic strength may have been overestimated. 

The last two free-energy contributions involve only the surfactant heads. The steric contribution, gst, accounts for steric interactions between the surfactant heads at the micellar interface. This contribution can be responsible for synergism in mixed micelle formation if the heads of the two-surfactant types have different sizes, as illustrated by the large grey heads and small black heads ii Figure 12.14. The steric contribution depends only on the size of the surfactant heads. 

Both the thermodynamics and kinetics of electron transfer reactions (redox potentials and electron transfer rates) have steric contributions, and molecular mechanics calculations have been used to identity them. A large amount of data have been assembled on Co3+/Co2+ couples, and the majority of the molecular mechanics calculations reported so far have dealt with hexaaminecobalt (III/II) complexes. 

Rijnaarts, H. H. M., W. Norde, J. Lyklema, and A. J. B. Zehnder. 1999. DLVO and steric contributions to bacterial deposition in media of different ionic strengths. Colloids and Surfaces B — Biointerfaces 14 179—199. 

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