Van der Waals’ repulsion

The urigin uf van der Waals repulsive forces is mutual interaction of electrons in atttrn 1 and those in atom 4. 

Planar geometry required for aromaticity destabilized by van der Waals repulsions between indicated hydrogens... 

Steric strain (Section 3 2) Destabilization of a molecule as a result of van der Waals repulsion distorted bond distances bond angles or torsion angles... 

Van der Waals radius (Section 2 17) A measure of the effec tive size of an atom or a group The repulsive force between two atoms increases rapidly when they approach each other at distances less than the sum of their van der Waals radii Van der Waals strain (Section 3 2) Destabilization that results when two atoms or groups approach each other too closely Also known as van der Waals repulsion Vicinal (Section 6 14) Describing two atoms or groups at tached to adjacent atoms... 

Entries 11 and 13 in Table 3.4 present data relating the efiect of methyl substitution on methanol and methylamine. The data show an increased response to methyl substitution. While the propane barrier is 3.4 kcal/mol (compared to 2.88 in ethane), the dimethylamine barrier is 3.6kcal/mol (compared to 1.98 in methylamine) and in dimethyl ether it is 2.7 kcal/mol (compared to 1.07 in methanol). Thus, while methyl-hydrogen eclipsing raised the propane barrier by 0.5 kcal/mol, the increase for both dimethylamine and dimethyl ether is 1.6 kcal/mol. This increase in the barrier is attributed to greater van der Waals repulsions resulting from the shorter C—N and C—O bonds, relative to the C—C bond. 

Dienes would be expected to adopt conformations in which the double bonds are coplanar, so as to permit effective orbital overlap and electron delocalization. The two alternative planar eonformations for 1,3-butadiene are referred to as s-trans and s-cis. In addition to the two planar conformations, there is a third conformation, referred to as the skew conformation, which is cisoid but not planar. Various types of studies have shown that the s-trans conformation is the most stable one for 1,3-butadiene. A small amount of one of the skew conformations is also present in equilibrium with the major conformer. The planar s-cis conformation incorporates a van der Waals repulsion between the hydrogens on C—1 and C—4. This is relieved in the skew conformation. 

The case of a, -unsaturated caAonyl compounds is analogous to that of 1,3-dienes, in that stereoelectronic factors favor coplanaiity of the C=C—C=0 system. The rotamers that are important are the s-trans and s-cis conformations. Microwave data indicate that the s-trans form is the only conformation present in detectable amounts in acrolein (2-propenal). The equilibrium distribution of s-trans and s-cis conformations of a,fi-unsatuiated ketones depends on the extent of van der Waals interaction between substituents. Methyl vinyl ketone has minimal unfavorable van der Waals repulsions between substituents and exists predominantly as the s-trans conformer ... 

Conformations in which there is a 1,3-diaxial interaction between substituent groups larger than Iqidrogen are destabilized by van der Waals repulsion. Equilibration of mixtures of cis- and /ran5-l,l,3,5-tetramethylcyclohexane reveals that the cis isomer is favored by 3.7 kcal/mol. This provides a value for a 1,3-diaxial methyl interaction that is 1.9 kcal/mol higher than that for the l,3-methyl-4iydrogen interaction. 

The preferred conformation is D because it maximizes the number of antiperiplanar relationships between nonbonded electron pairs and C—O bonds while avoiding the R -R van der Waals repulsions in conformations E and F. 

The collision tlieory for bimolecular reactions assumes that a chemical reaction occurs when two molecules collide with enough energy to penetrate the molecular van der Waals repulsive forces, thus combining together. For the bimolecular collisions of unlike molecules A, the collision number is ... 

Steric hindrance (Sections 3.2, 6.3, and 8.6) An effect on structure or reactivity that depends on van der Waals repulsive forces. 

Van der Waals strain (Section 3.2) Destabilization that results when two atoms or groups approach each other too closely. Also known as van der Waals repulsion. 

The parameter redundancy is also the reason that care should be exercised when trying to decompose energy differences into individual terms. Although it may be possible to rationalize the preference of one conformation over another by for example increased steric repulsion between certain atom pairs, this is intimately related to the chosen functional form for the non-bonded energy, and the balance between this and the angle bend/torsional terms. The rotational banier in ethane, for example, may be reproduced solely by an HCCH torsional energy term, solely by an H-H van der Waals repulsion or solely by H-H electrostatic repulsion. Different force fields will have (slightly) different balances of these terms, and while one force field may contribute a conformational difference primarily to steric interactions, another may have the... 

These equations show that hydrophobic and steric (van der Waals) interactions are of prime importance in the inclusion processes of cyclodextrin-alcohol systems. The coefficient of Es was positive in sign for an a-cyclodextrin system and negative for a P-cyclodextrin system. These clear-cut differences in sign reflect the fact that a bulky alcohol is subject to van der Waals repulsion by the a-cyclodextrin cavity and to van der Waals attraction by the p-cyclodextrin cavity. 

Inspection of models show that the interaction between the non-bonded atoms is different in an isotactic and a syndiotactic sequence. A somewhat naive calculation based on van der Waals repulsions between the nonbonded atoms, suggests that for nonpolar vinyl monomers the syndiotactic sequence may be favored by a few tenths of a kcal/mole, and this difference might increase to... 

Similarly, examples of barriers arising largely from simple steric hindrance can be found, as for instance in the hindered diphenyls.35 On the other hand there are many arguments suggesting that this is not the important force in ethane and similar molecules. It would be difficult to understand the relatively slow fall in barrier from ethane to methyl silane to methyl germane on a van der Waals repulsion basis. Furthermore, the small effect of substituting F, Cl, or Br on one end would also seem mysterious. The equilibrium orientation in propylene is opposite to the predictions of one of the quantitative van der Waals theories. Finally, the apparently small effect of bending back the C—H bonds is not in accord with either the electrostatic or van der Waals pictures. 

Let us now turn our attention to an interpretation of Ycr particularly to the question of what numerical values might be appropriate under certain conditions. Based on the discussion above, we would expect to find that values in excess of 10 kg/mol-ns would be appropriate for materials below Tg. What we seek is a method of checking this prediction by calculating an approximate value from molecular parameters. To do this we will consider the repulsive interaction as largely a steric one due to the van der Waals repulsions of a pair of chain elements. To the extent that this picture applies, we can calculate an approximate 7c by expanding the van der Waals pair interaction, energy,... 

Torsional strain and van der Waals repulsions between hydrogen atoms across rings (transannular strain) cause the small instabilities of these higher cycloalkanes. 

Why there is energy difference between the staggered and the eclipsed form is not completely understood. It is not simply due to the van der Waals repulsive forces, because such forces are too small to account for the observed differences in potential energy. Now it is known that the differences are in some way due to interaction of electron clouds in C—H or C—C bonds and various causes for such interactions are known. 

The calculated radii with all interactions included are somewhat smaller than the radii measured with SANS, whereas the radii obtained with only the van der Waals repulsions taken into account are somewhat larger. As could be anticipated, the sizes of the dendrimers are dependent on the pH of the solution. Since both the primary and the tertiary amine groups may be protonated, repulsions begin when the pH of the solution is decreased. 

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