Separation Steric

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... 

After separating steric effect by the STERIMOL term, the slope of the ct° terms in Eq. 40 and 41 is close to those observed for the same type of acylation reactions in a number of physical organic systems37>. Thus, STERIMOL values can well be used as free-energy-related parameters in combination with ct and parameters and provide informations about three-dimensional aspects of the interactions between drugs and biomolecules. 

The natural conclusion to be drawn from Table 12 is that steric effects play a very big part indeed in determining the rate and orientation of radical addition. We believe this conclusion to be correct, but, just as it is impossible to separate bond strength effects from polar effects, so it is impossible to separate steric effects in radical addition from polar effects. Any steric hindrance in the addition of a radical to an olefin will depend to a major extent on the shape of the radical. However, the shape of the radical is directly connected to the electron density at the trivalent carbon atom. 

The conformational behavior of these compounds involves several steric and 7r-barriers separating twisted or folded intermediates. The similarity in steric and Tt-barriers makes it difficult to separate steric from electron effects. 

Probably substituents on the aryl part do not have contact with the enzyme surface but remain outside the gorge exposed to the solvent. As a consequence, given that changes in the log P values are mostly due to the different aryl substituents, if they are all or in part exposed to the solvent, their desolvation is not required and lower log P can be more favourable.In the cases where no role for lipophihcity is foimd, there is a considerable colhnearity problem, an overlap between substituent bulk and hydrophobicity (a most common problem through out drug development). This overlap makes it difficult to separate steric and hydrophobic effects. 

In this paper an approach has been presented that will facilitate selection and evaluation of possible membrane materials. Having selected potential membrane materials for desired separations, steric considerations are taken Into account In membrane preparation. This approach avoids starting with and subsequently modifying aqueous-separation membranes, allows a vast spectrum of polymers to be considered as potential membrane materials and focuses the selection process. A brief review of membrane material evaluation procedures have been discussed with emphasis on those techniques which do not require the fabrication of membranes. Finally, the survey of materials evaluated for possible membrane use Indicates both the Interest In this field and the need for appropriate material selection and evaluation procedures. The ideas presented here will continue to grow In value In the future as membranes are called upon to achieve more difficult separations in an energy efficient fashion. 

NBO analysis has been used to dissect the contributions of the three principal contributors to ethane s structural preferences and separate steric and hyperconjugative interactions. It was found that removal of vicinal hyperconjugation interactions yields the eclipsed structure as the preferred conformation, whereas Pauli exchange (steric) and electrostatic (Coulombic) repulsions have no decisive influence on the preference for a staggered conformation. The hyperconjugative preference for the staggered conformation is attributed to the antiperiplanar stereoelectronic requirement sununarized in Figure 6.5. 

Hancock, Meyers and Yager [22] have pointed out that a-alkyl groups have two effects, a steric effect and a reduction in hyperconjugation. They derived Equation (21) to separate steric effect from conjugation effect, n represents the... 

Using the analytes triphenylene and o-terphenyl, two molecules with the same molecular weight and nearly the same size but different spatial arrangements (planar/helically deformed), several columns were investigated for their ability to separate steric complex molecules. It was found that besides phases with a polymer layer (e.g., SMT OD, Nucleosil AB/HD, Gromsil CP), polar phases show a good steric selectivity. 

Attempts have been made to separate steric and electronic factors governing ligand substitution in tetranuclear clusters of cobalt and iridium. For a series of isostructural complexes [Co4(CO)nL], the rate of dissociative loss of CO has been determined by measuring CO exchange rates. The ligand L is a phosphine or phosphite, and occupies an axial position as shown in structure 21. The relative rates of CO dissociation in heptane... 

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