Steric hindrance model

Since regular helices with the inner layer matching the catalyst particle size have been observed[4,5], we propose a steric hindrance model to explain the possible formation of regular and tightly wound helices. 

Wang, X. L., et al. (1997). Electrostatic and Steric-Hindrance Model for the transport of charged solutes through nanofiltration membranes. J. Membrane Science. 135, 1, 19-32. 

Model 1 (Fig. 4-20) illustrates classical competitive inhibition in which an inhibitor competes with a substrate for a single binding site. Models 2-4 represent other ways in which an inhibitor and substrate would be mutually exclusive steric hindrance (Model 2) steric hindrance or competition fora common binding group (Model 3) and overlapping binding sites (Mode ... 

Recently, the peak splitting observed in the CP/MAS NMR spectrum (Fig. 11.1(a)) for the methyl and methylene carbons of a-form i-PP were interpreted tentatively in terms of the steric shielding effect proposed by Grant and Cheney [5]. They developed a steric-hindrance model that predicts the effect on the chemical shifts of the additional crowding of the C—H bonds in the solid state. In this model, a C—H bond is compressed, or... 

The solid-state spectrum of diglycidyl ether of bisphenol-A (DGEBA) has been compared with the liquid-state spectrum [9], and the chemical shifts were examined using the steric-hindrance model. This model has also given consistent results in the analysis of chemical shifts of the a and conformers of tran5-l,4-polyisoprene in the solid state [10]. The sterie-hindrance model prediets shifts that are the same order of magnitude as those observed. 

The simplest and most quickly computed models are those based solely on steric hindrance. Unfortunately, these are often too inaccurate to be trusted. Molecular mechanics methods are often the method of choice due to the large amount of computation time necessary. Semiempirical methods are sometimes used when molecular mechanics does not properly represent the molecule. Ah initio methods are only viable for the very smallest molecules. These are discussed in more detail in the applicable chapters and the sources mentioned in the bibliography. 

When viscometric measurements of ECH homopolymer fractions were obtained in benzene, the nonperturbed dimensions and the steric hindrance parameter were calculated (24). Erom experimental data collected on polymer solubiUty in 39 solvents and intrinsic viscosity measurements in 19 solvents, Hansen (30) model parameters, 5 and 5 could be deterrnined (24). The notation 5 symbolizes the dispersion forces or nonpolar interactions 5 a representation of the sum of 8 (polar interactions) and 8 (hydrogen bonding interactions). The homopolymer is soluble in solvents that have solubility parameters 6 > 7.9, 6 > 5.5, and 0.2 < <5.0 (31). SolubiUty was also determined using a method (32) in which 8 represents the solubiUty parameter... 

A model for the allosteric behavior of hemoglobin is based on recent observations that oxygen is accessible only to the heme groups of the a-chains when hemoglobin is in the T conformational state. Perutz has pointed out that the heme environment of /3-chains in the T state is virtually inaccessible because of steric hindrance by amino acid residues in the E helix. This hindrance dis-... 

Examine space-filling models and electron density surfaces for alkene A and alkene B. For each, which face of the double bond is less hindered Which atoms cause steric hindrance of the alkene Is this reaction controlled by steric hindrance If so, explain which step(s) in the catal3 ic mechanism would be most affected. 

The reactivity of aldehydes and ketones toward cyanide may be influenced by the steric and/or electronic properties of the carbonyl substituents, X. Examine spacefilling models of formaldehyde (X=H), acetone (X=Me), and benzophenone (X=Ph). Which compound offers the least steric hindrance to nucleophilic attack The most ... 

Dimerization is markedly subject to steric hindrance, thus, whereas 3-n-propylindole dimerizes readily, neither 3-isopropyl- nor Z-tert-butyl-indole dimerizes. This failure is most probably the result of steric hindrance of approach of the electrophilic reagent to position 2 by the bulky 3-substituent in the unprotonated molecule. On the other hand, models show that approach of a nucleophilic reagent to position 2 of a 3-protonated molecule is quite open, it should, there-... 

The three water ligands located at meridional positions of the J ,J -DBFOX/Ph aqua complexes may be replaced by another molecule of DBFOX/Ph ligand if steric hindrance is negligible. Based on molecular model inspection, the hetero-chiral enantiomer S,S-DBFOX/Ph looks like a candidate to replace the water ligands to form the heterochiral meso-2 l complex J ,J -DBFOX/Ph-S,S-DBFOX/... 

Figure 11.6 Steric hindrance to the Sn2 reaction. As the computer-generated models indicate, the carbon atom in (a) bromomethane is readily accessible, resulting in a fast Sfyj2 reaction. The carbon atoms in...

Spin orbitals, 258, 277, 279 Square well potential, in calculation of thermodynamic quantities of clathrates, 33 Stability of clathrates, 18 Stark effect, 378 Stark patterns, 377 Statistical mechanics base, clathrates, 5 Statistical model of solutions, 134 Statistical theory for clathrates, 10 Steam + quartz system, 99 Stereoregular polymers, 165 Stereospecificity, 166, 169 Steric hindrance, 376, 391 Steric repulsion, 75, 389, 390 Styrene methyl methacrylate polymer, 150... 

Starting from ( )-enolates, the corresponding argument predicts that transition state 10a is lower in energy than the alternative model which suffers from mutual steric hindrance of the substituent R1 and the methyl group in a gauche orientation thus, stereoconvergence results. 

Although the chiral propanoates I and 5 are similar and the reaction conditions are almost identical, the stereochemical outcomes arc explained by completely different transition state models. Predominant attack of the ketene acetal 2 to the Ai-face of 2-methylpropanal is interpreted by assuming a Zimmerman-Traxler like model, which minimizes steric hindrance in a plausible way65. 

The stereochemical course of the reaction may be rationalized by assuming a six-mem-bered chelate which presumably exists in a chair-like conformation. Molecular models support the assumption that the nucleophile preferentially attacks from the bottom face of the complex due to the steric hindrance of the axial A-methyl group. 

Model Si-H head-group derivatization demonstrated the potential usefulness of this group. While the Si-H group can be easily hydrosilylated with high yield24,35) steric hindrance may reduce the yield. 

We have modelled the [CDopen - methyl pyruvate] complex. The result is shown in Figure 2. In this complex there is no steric hindrance to prevent the free rotation of the substrate around the quinuclidine nitrogen. Thus, in complex shown in Figure 2. there is no preferential stabilization of the substrate. In earlier computer modeling it was suggested that Pt is involved in the stabilization of the [CDopew-a-lfeto ester] complex, i.e. the Pt surface prevent the free rotation of the substrate, however the driving force for enantio-differentiation, i.e. for preferential adsorption of the substrate, was not discussed [14]. 

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