Spatial demand

Nonpolar side chains Glycine is the smallest anino acid because it has no side chain. The main service it offers is to the polypeptide chain itself. It can add length and flexibility to a polypeptide without sacrificing strength or making spatial demands of its own. 

The interfacial activity is determined by the sterical properties of the molecule. At the interface the spatial demand A0 of the hydrophobic part of the molecule is higher because of the second chain of the internal sulfonate compared with the terminal sulfonate. Thus, the surface concentration of the surfactant molecules is lower. That means that the hydrocarbon chains are laterally oriented and therefore cover the interface between the solution surface and air more completely. Because the ratio of the spatial demand of the head group to the volume of the alkyl chain governs the radius of the micellar surface, it... 

An interesting observation reported in Table XLIX is the increase in the hydroquinone/catechol ratio from 1.44 to 1.99 when the dielectric constant of the medium is decreased from 58.9 to 39.2 by addition of methanol to water. A similar increase in the hydroquinone/catechol ratios was also observed in phenol hydroxylation catalyzed by TS-1 (266) in dioxane-water and tert-butyl alcohol-water mixtures. The para/ortho ratio increased nearly 10-fold when 10% dioxane was added to water. Similarly, the para/ortho ratio more than doubled (1.3-3.0) when 10% tert-butyl alcohol was added to water. An opposite trend, namely, a decrease in the para/ortho ratio from 1.4 to 0.6, was observed when 10% formamide (s = 108) was added to water. Because of geometric constraints in the MFI pores, catechol is expected to be formed more easily on the external surface of TS-1 crystallites than in the pores (91). Hydroquinone, less spatially demanding, can form in the TS-1 channels. A greater coverage of the hydrophobic... 

The interior surface area of a pigment powder may be assessed through a series of determinations, using a range of gaseous, liquid, or dissolved substances of various molecular sizes with different spatial demands, and measuring the amount of adsorbed material gravimetrically as well as volumetrically [9],... 

Epoxidations of chiral allenylsilanes are also highly stereoselective, especially if the silyl group is spatially demanding (Eq. 9.54) [60]. A bis-epoxide intermediate is formed which rearranges to an a,/8-unsaturated a -hydroxy ketone. Such products are of interest as branched carbohydrate analogues. 

C. Jackel, M. Salwiczek, B. Koksch, Fluorine in a native protein environment—How the spatial demand and polarity of fluoroalkyl groups affect protein folding, Angew. Chem. Int. Ed. 45 (2006) 4198-4203. 

Fig. 10. The contrary effects of spatial demand and polarization on side chain fluorination on hydrophobic interactions. (See Colour Plate Section at the end of this book.)...

The molecular geometry of the parent cis-1,2-di-9-anthrylethylene 38a has not been established by X-ray diffraction, but crystal structure analyses of several 1,2-substituted cis-dianthrylethylenes 38 are available. Depending on the spatial demand of the substituents R and R, the planes of the anthracene moieties are twisted out of the plane of the ethylene by 59-84° [80],... 

Contrary to expectations, the catalytic activity was found to decrease with increasing generation number. This was explained in terms of the increasing spatial demands of the nickel complex units bound in increasing numbers to the dendrimer periphery and increasingly limiting access to their active sites (see Fig. 6.28). 

Fig. 25. ORTEP drawing of Sr(OC6H2- -Bu1)2(THF) 0.5 THF showing the trigonal bipyramidal ligand arrangement around the metal center. The more spatially demanding aryl oxide ligands are located at the equatorial sites. (Redrawn from Ref. I32.)...

Space-filling substituents induce increased chain lengths also with Ni-based polymerization catalysts. Here a spatially demanding transition state, required for p-H transfer to a monomer molecule and subsequent release of the unsaturated polymer chain end, appears to be suppressed by ligand substituents which block the coordination positions above and below the tetragonal coordination plane (Figure 21). 

Although Diels-Alder reactions of disilenes to 1,3-dienes rather represent an exception, [2+4]-cycloaddition is the preferred reaction route for the photolysis of 1 in the presence of 1,4-heterodienes. Thus, for example, 1,4-diaza-1,3-butadienes react smoothly to yield the six-membered ring products 15 when the spatial demands of the substituents at nitrogen are not too great. 

At the time of this work, many chemists considered that a metallic ion of a particular valency had a stable coordination number and definite coordination configuration, these being presumably dependent primarily on the electronic distribution within the compound. The stability of the above bis-triaminopropane compounds made chemists admit (some very reluctantly) that an additional factor is involved—namely, that a metallic atom may adjust its coordination number and its coordination configuration to the spatial demands of a suitable multi-bridging ligand. The dimensions of the triamine molecule clearly allow two molecules to fit firmly around the 6-coordination octahedron, and the metal (e.g., zinc(II), plati-num(II)) adapts itself to this requirement to obtain maximum stability. These considerations are now of course the accepted commonplace factors of coordination chemistry. 

Summary Methoxy-bis[tris(trimethylsilyl)silyl]methane (4), the first geminal di(hyper-silyl) compound with a central carbon atom, was prepared by the reaction of tris(tri-methylsilyl)silyl lithium with dichloromethyl methyl ether. The structure of 4, which is characterized by considerable distortions due to the spatial demand of the two (MesS aSi groups, is discussed on the basis of an X-ray crystal structure analysis. 

Hydrophobicity, Spatial Demand and Polarity - Fluorine in a Hydrophobic and a Polar Polypeptide Environment... 

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