Steric and electrostatic contributions

Steric and Electrostatic Contributions to the Colloidal Properties of Nonaqueous Dispersions... 

Polyelectrolytes provide excellent stabilisation of colloidal dispersions when attached to particle surfaces as there is both a steric and electrostatic contribution, i.e. the particles are electrosterically stabilised. In addition the origin of the electrostatic interactions is displaced away from the particle surface and the origin of the van der Waals attraction, reinforcing the stability. Kaolinite stabilised by poly(acrylic acid) is a combination that would be typical of a paper-coating clay system. Acrylic acid or methacrylic acid is often copolymerised into the latex particles used in cement sytems giving particles which swell considerably in water. Figure 3.23 illustrates a viscosity curve for a copoly(styrene-... 

Comparative Molecular Field Analysis The basic idea of Comparative Molecular Field Analysis developed by R.D. Cramer et al. (Tripos CoMFA ) is that a suitable sampling of the steric and electrostatic field around a ligand molecule may provide all the information necessary for explaining its biological property in a 3D-QSAR. The steric and electrostatic contributions to the total interaction energy between the ligand and a chosen probe are calculated... 

According to second-order perturbative estimates, each och— o cf interaction of the cis isomer (lower left panel of Fig. 3.61) contributes 6.84 kcal mol-1 stabilization, about twice that of the ctch— o ch (3.75 kcal mol-1) 84 or (2.29 kcal mol-1) interactions of the trans isomer. Even though the remaining ocf och interaction of the cis isomer contributes only 1.17 kcal mol-1 stabilization, the sum of the four anti donor-acceptor interactions favors the cis isomer by almost 4 kcal mol-1. This hyperconjugative advantage of the cis isomer is sufficient to overcome the inherent steric and electrostatic advantage of the trans isomer. 

Reactions of nitrile oxides with 1,1-disubstituted alkenes afford products in which the oxygen atom of the nitrile oxide gets attached to the most crowded carbon atom of the dipolarophile. This high regioselectivity does not seem to depend on the type of substituent present on the alkene (142-152). Some of the results cannot be satisfactorily interpreted on the basis of FMO theory (149,151). Both steric and electrostatic effects often counteract each other and contribute to the regioselectivity actually observed. With trisubstituted alkenes, the orientation of cycloaddition is apparently dominated by this phenomenon. The preference is for the more substituted carbon atom to end up at the 5-position of the heterocyclic product (153,154). However, cases of opposite regiodifferentiation are also found, in particular with donor-substituted alkenes (42,155-157) (Scheme 6.21). 

The polyd-proline) /- 11 transition is known to exhibit positive cooperativity. In the present study, the contributions of intramolecular steric and electrostatic interactions to this phenomenon are computed. The calculations suggest that the total end effects are 48 kJ mol-1 for form 1 but only 14 kJ mol-1 for form II. 

Substituted norsnoutanes (61) have been introduced as substrates with sterically unbiased re-faces, which allow electronic effects in re-facial selectivity of nucleophilic additions to be evaluated.96 Examples indicate how this system allows separation of long-range electronic effects into orbital and electrostatic contributions. 

This is the dependence expected for purely electrostatic stabilization of polyelectrolyte layers. Electro-steric interactions involve steric contributions [Eq. (3)] and electrostatic contributions [Eq. (4)]. For high ionic strength as in the interior of the polyelectrolyte shell the measured shear modulus should exhibit a characteristic G (r) e Kr-dependence which is apparent in a semi-logarithmic presentation of the data as in Fig. 13. The measured shear moduli are plotted as a function of the reduced distance r/2Rm which is... 

The next step was made by Klebe et al. [50]. Two 3D-QSAR methods were applied to get three-dimensional quantitative structure-activity relationships using a training set of 72 inhibitors of the benzamidine type with respect to their binding affinities toward Factor Xa to yield statistically reliable models of good predictive power [51-54] the widely used CoMFA method (for steric and electrostatic properties) and the comparative molecular similarity index analysis (CoMSlA) method (for steric, electrostatic, hydrophobic, hydrogen bond donor, and hydrogen bond acceptor properties). These methods allowed the consideration of various physicochemical properties, and the resulting contribution maps could be intuitively interpreted. 

The statistics and the isocontour maps of our preliminary CoMFA model are shown in Figure 4. It can be seen that both steric and electrostatic fields contribute to the model and, within the limits of the physicochemical domain explored, the former is prevalent. 

Since the calculated change in the electrostatic interaction between the chromophore and the protein cavity reveals a contribution of other ca. +6 kcal mol , the change in steric and electrostatic interaction must account for only half of the stored energy. So the photon energy has to be mainly stored in terms of the strain energy of the chromophore. In particular, it is due to the distorted tt system of bathoRh (bond lengths and angles of the other moiety of the bathoRh are close to Rh) with respect to Rh. 

Afterwards, the relative activation energies of the diastereoisomeric transition structures are split into contributions able to represent the factors the organic chemist is used to invoke in the attempt to "understand" the variety of the reaction outcome, i.e. steric and electrostatic repulsions, deformation energy of reactants, incipient bond energies, trough-space delocalisations and their subsets often called secondary interactions. The an ysis of delocalisation energies rests on the use of hybrid basis sets derived from Del Re s Maximum Localisation Criterion. ... 

Fig 2). This is shown by the more compact sediments. Alumina with an amphoteric surface was stabilized by dispersants and strongly acid or strongly basic solvents by a combined steric and electrostatic mechanism. Hexane, which is a nonpolar solvent with no hydrogen bonding capability, is shown to be a poor solvent for AHAS, which is a highly polar dispersant. Hexane would introduce a low potential as well hence the stability had to rely solely on the steric stabilization contribution. When the adsorbate AHAS is not highly soluble in the medium, the dispersant is ineffective as a steric stabilizer. Despite the high dielectric constant of EtOH, it is shown to be an ineffective medium with LNA. EtOH as a weakly acid solvent introduced nearly a zero potential... 

A possible computational strategy to get G is to write it as a sum of various contributions of different physical origin, related to dispersion, repulsion, steric and electrostatic forces between solute and solvent molecules ... 

The cyclopropane ring in both epimers occupies the same position with respect to the departing group both epimers form a stable bisected cyclopropylcarbinyl cation. The exo endo rate ratio can reflect the contribution ratio of steric factors on the formation of the unsubstituted 2-benzonorbomenyl cation out of the two epimers. The hydrolysis of compounds 269 and 270 has resulted in an exo endo rate ratio of 12 and agrees fairly well with the previous value. A comparison of these data with the value of 15000 for the unsubstituted compounds clearly shows the steric and electrostatic effects to have a low value for secondary systems and in the absence of the ring 7i-participation the exo-isomer solvolyzes faster than its endo epimer only 2- to 10-fold. 

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