Steric rejection

Use SpartanView to examine transition states for the nucleophilic addition of CN" to formaldehyde, acetone, and benzophenone (PhCOPh). Assuming that the length of the developing C-C bond rejects steric repulsion between the nucleophile and the electrophile, which transition state is least strained and which is most strained ... 

Brown and his coworkers had rejected steric hindrance to hydration as a main cause of these ortho effects, because they considered that a more gradual increase in the anomalies should be found . Wepster points out that substitution of a hydrogen atom by a methyl group removes at least one water molecule from the solvation shell, so that for instance in the ion of 2-f-butylaniline the solvation energy will be decreased considerably as compared with the 2-isopropylammonium... 

Now that we have a model, we must check its consistency with various experiments. Sometimes such inconsistencies result in the complete rejection of a model. More often, they indicate that we need to refine the model. In the present case, the results of careful experiments show that the collision model of reactions is not complete, because the experimental rate constant is normally smaller than predicted by collision theory. We can improve the model by realizing that the relative direction in which the molecules are moving when they collide also might matter. That is, they need to be oriented a certain way relative to each other. For example, the results of experiments of the kind described in Box 13.2 have shown that, in the gas-phase reaction of chlorine atoms with HI molecules, HI + Cl — HC1 I, the Cl atom reacts with the HI molecule only if it approaches from a favorable direction (Fig. 13.28). A dependence on direction is called the steric requirement of the reaction. It is normally taken into account by introducing an empirical factor, P, called the steric factor, and changing Eq. 17 to... 

The mechanism described in Scheme 2 was rejected on the grounds that the steric requirement for the abstraction of a hydrogen atom from -CHD-of species (IV) could not be met. Assuming an atomically flat surface, and sp3 hybridization of the carbon atom bonded to the surface, the plane of the Ce-ring in (IV) is in such a configuration that the hydrogen atom of -CHD- is directed away from the surface, and the deuterium atom toward the surface. Thus, unless the species is adsorbed near a step in the metal lattice, the loss of this hydrogen and the formation of a second carbon-metal bond would require a very considerable distortion of adsorbed species. 

Finally, a few words about previous explanations of torsional isomerism in 1-substituted propene. The most common rationale for the observed trends has been steric hindrance. However, steric effects alone cannot account for the greater stability of the cis isomer. Hence, the steric interpretation should be rejected. 

Rejection of protein adsorption to the outermost grafted surface is attributed to a steric hinderance effect due to the tethered chains. A grafted surface in contact with an aqueous medium, a good solvent of the chains, has been identified to have a diffuse structure [57,151,152]. Reversible deformation of the tethered... 

Model III has furanose rings fixed at the values given by Arnott and Huklns (46,47). As a result the other (varied) conformation angles are somewhat less standard (Table 1) although the fit to the X-ray amplitudes is trivially better. Although the X-ray amplitudes detect no difference between Models I and III, the addition of steric considerations permits Model III to be rejected (Table 2) with better than 99.5% confidence. 

While the steric polarization model may be called into question by the considerations presented above, it would be hard to reject the obvious parallels of steric interaction and the y-gauche shielding effect. A case in point is the highly strained molecule [14] (compound 480). The effects... 

The Sn2 reaction involves the attack of a nucleophile from the side opposite the leaving group and proceeds with exclusive inversion of configuration in a concerted manner. In contrast to the popular bimolecular nucleophilic substitution at the aliphatic carbon atom, the SN2 reaction at the vinylic carbon atom has been considered to be a high-energy pathway. Textbooks of organic chemistry reject this mechanism on steric grounds [175]. 

The permeability Ps is a measure of the transport of a molecule by diffusion. The reflection coefficient a of a given component is the maximal possible rejection for that component (at infinite solvent flux). Various models have been proposed for the reflection coefficient [75-77]. In the lognormal model [78], a lognormal distribution is assumed for the pore size. No steric hindrance in the pores or hydrodynamic lag is taken into account, but it is assumed that a molecule permeates through every pore that is larger than the diameter of the molecule. Moreover, the diffusion contribution to the transport through the membrane is considered to be negligible. Therefore, the reflection curve can be expressed as ... 

Let us examine separately the case of the cis and trans isomers of pentadiene. For the trans isomer the cis conformation is permissible so that one cannot assume a priori that this isomer won t coordinate to Ti by the two double bonds. This hypothesis, however, can be easily rejected by the following considerations. If the steric situation around Ti during the polymerization were to permit the coordination of the trans isomer of pentadiene by the two double bonds, in the cis conformation, butadiene or isoprene should also coordinate the same way. In this case, however, cis-1,4 units should be obtained both from butadiene and isoprene, and not 1,2 and 3,4, respectively, as observed. It seems reasonable to conclude, therefore, that the trans isomer of pentadiene coordinates to Ti by the vinyl group only, as butadiene or isoprene, before it is incorporated as a cis-1,4 unit. 

The relatively high conformational flexibility of many drug molecules requires that an effective conformational sampling is performed for a pharmacophore-based analysis. The method used in ChemDiverse is based on an explicit "on-the-fly" generation of conformers done at search time. A conformation is accepted or rejected based on a fast evaluation of steric... 

A comparison of the experimental data on quenching of triplet states of dyes in solutions in the absence and in the presence of DNA permits estimation of the steric complexation effect on the quenching process and conclusions about the structure of the dye-DNA complexes formed. In the case of dye K4, we may conclude that complexation with the biopolymer has relatively weak effect on the kq value. This is probably due to the fact that the quenching process for K4 occurs in the kinetic regime (kc k a, see reaction (2)), and diffusion of the quencher to dye molecules boimd to DNA exerts no substantial effect on kq (another assumed reason for this phenomenon could be partial decomposition of the dye(T)-DNA complex and the presence of free triplet dye molecules in the solution however, the experiments on quenching of the K4 triplet state by iodide ion considered above reject this possibility). 

In aqueous solutions, NF membranes become charged, allowing separation of specific ionic species. It is believed that sieving (steric hindrance) is the dominant rejection mechanism in NF for colloids and large molecules, while the physicochemical interactions of solute and membrane become increasingly important for ions and low-molecular-weight organics [13]. 

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