Steric volume

Another contribution to AjF was observed to arise in some reactions involving high steric hindrance (see references cited in Ref. 275) this is indicated as A V, the steric volume of activation. According to observation, high pressure promotes the most hindered process. This effect has been ascribed to a displacement of the most hindered transition states toward the more compact products in the reaction profiles [285]. 

It was also mentioned by McLean et al. that ions and polar solvent molecules must necessarily diffuse across thin amorphous fluorocarbon or interfacial regions between swollen polar domains. However, all of this does not require the need for channels with diameters of 10 A that are coated with SOs groups for long-range transport. In any case, a simple consideration of the steric volume of SOs groups in relation to the size of these channels leads to the conclusion that more than one group would have difficulty fitting into this very small volume. Related... 

Another QSAR study utilizing 14 flavonoid derivatives in the training set and 5 flavonoid derivatives in the test set was performed by Moon et al. (211) using both multiple linear regression analysis and neural networks. Both statistical methods identified that the Hammett constant a, the HOMO energy, the non-overlap steric volume, the partial charge of C3 carbon atom, and the HOMO -coefficient of C3, C3, and C4 carbon atoms of flavonoids play an important role in inhibitory activity (Eqs. 3-5, Table 5). 

A general theory of quantitatively comparing molecular shapes using common overlap steric volume(33-36) and, more recently, descriptors derived from superimposed molecular potential energy fields of pairs of molecules(37) has been derived and tested. This theory allows a "marriage between Hansch analysis and conformational analysis. 

It has been shown previously that the catalytic activity for epoxidation reactions depends strongly on the adsorption properties. Indeed, the most hydrophobic catalysts, the highest catalytic activity is obtained [15,16,17]. These observations have moved us to carry out the systematic study of the effect of silylation on the catalytic activity of Ti-MCM-41 materials. Here, we have varied the loading of alkylsilane groups and the steric volume of the organosilane moieties bounded on the surface of the Ti-MCM-41 walls. 

Effect of the steric volume of the anchored silyl groups... 

The influence of the steric volume of the silyl groups anchored in the surface of Ti-MCM-41 was studied by grafting a suite of alkyldimethyldisilazanes (see table 1) along this study the number and volume of the silyl groups was modified. 

A combination of two effects of F-alkyl groups - a large sterical volume and the strong electron-withdrawing effect - are probably the factors responsible for the extremely low reactivity of the C=C bond in cyclic and internal F-olefins, although these olefins still have the ability to interact with very strong electrophiles such as chlorine and bromine fluorosulfates [8] ... 

The absence of electronwithdrawing properties and small steric volume of hydrogen substituents are the reasons for the higher stability of cation 36. For 1-substituted analogs of F-allyl and F-methallyl cations (existing as a mixture of cis- and trans-isomers, except 43, X=H for which only czs-isomer was found in solution), stability correlates well with electronegativity of the substituent [63] ... 

The main-chain mobility depends slightly on the ease of the lateral group motion, so the respective positions of the methyl groups influence this mobility because of their important steric volume [157],... 

The large steric volume of some of the bulky Cp s is, of course, the source of slightly different reactivity patterns in complex formation when compared with normal or pentamethylcyclopentadienyl reaction schemes. For example, many metathetical reactions between the pentaphenylcyclo-pentadienyl anion and transition metal halides fail in etheral solvents [such as tetrahydrofuran (thf) and ether] (40,95), although the same reaction with the tetraphenylcyclopentadienyl anion proceeds without problems (48,96,97). Aside from the high steric volume, a lower base strength hypothesis is invoked for the pentaphenyl-Cp anion to account for the failures (40). The desired decaphenyl- or pentaphenylmetallocenes were eventually obtained using much less basic solvents, such as xylenes (40). 

Although the exact mechanism of the reaction is difficult to resolve, from our accumulated data we can propose a mechanism which explains the experimental results obtained and enables the prediction of the stereochemistry of the sulfinate formed, based on the steric volume of the amine. The experimental data clearly demonstrate that the diastereoselective formation of a sulfinate is base-dependent. Therefore, the proposed mechanism has to include the effect of the base on the stereocourse of the reaction. Some additional circumstances have to be taken into account (1) the reaction of sulfiny 1 chlorides with alcohols in the presence of bases does not proceed via a sulfine intermediate117 (2) Mislow has proved that the reaction of chiral alcohols with sulfinyl chlorides in the presence of a base is kinetically controlled814,83 and (3) on first inspection, one could imagine the extreme case where DAG, under the influence of the base, reacts with only one of the enantiomeric sulfinyl chlorides. However, this is not the case, because the same yield and ee are obtained when 1.2 or 2.0 equiv of MeSOCl are used. Accordingly, the first step of the process could be an equilibrium reaction involving the sulfinyl chloride and the base. The racemic sulfinamide formed would be the active sulfur species that interacts with the sugar derivative. 

Two arbitrary functions of the common overlap steric volume were also introduced as alternative molecular shape descriptors ... 

The nonoverlap steric volume V on is another MSA descriptor defined as [Tokarski... 

W [(Vy n Vy) - Mo i,j)] + (1 - w) Mp i,j) where Mo(i, /) and Mp(i, j) are the common overlap steric volume and the integrated spatial difference in field potential, and w is a weighting factor between zero and one. The two descriptors are considered complementary in the sense that the overlap volume measures the shape within the van der Waals surface formed by superimposition of i and j, while ISDFP measures the shape outside the van der Waals surface. 

The most common steric descriptors are - molar refractivity, -> surface areas and several - volume descriptors such as molecular volume. Other steric descriptors are -> steric interaction fields, MTD descriptors, -> common overlap steric volume, and several topological descriptors accounting for both size and - molecular branching. Other popular molecular steric descriptors are listed below. 

The van der Waals radius cf N- bond gives an estimate of the steric volume and, therefore, steric hindrance for the interaction of the cationic head at the muscarinic receptor. 

When starting a QSAR study it is important to decide which physicochemical parameters are going to be studied and to plan the analogues such that the parameters under study are suitably varied. For example, it would be pointless to synthesize analogues where the hydrophobicity and steric volume of the substituents are correlated, if these two parameters are to go into the equation. 

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