Van der Waal radii, table

When two atoms approach each other so closely that their electron clouds interpenetrate, strong repulsion occurs. Such repulsive van der Waals forces follow an inverse 12th-power dependence on r (1/r ), as shown in Figure 1.13. Between the repulsive and attractive domains lies a low point in the potential curve. This low point defines the distance known as the van der Waals contact distance, which is the interatomic distance that results if only van der Waals forces hold two atoms together. The limit of approach of two atoms is determined by the sum of their van der Waals radii (Table 1.4). 

In a similar manner, the diffusion of hexane into dichloromethane solutions containing mixtures of the alkylammonium salts of bromide and the olefinic acceptors o-CA and TCNE result in the formation of brown-red crystals [23]. X-ray analysis reveals the (1 1) complex of bromide with o-CA, in which the anion is located over the center of the C - C bond of the acceptor moiety (Fig. 15b) and Br - C contacts are shortened by as much as 0.6 A relative to the sum of van der Waals radii (Table 3). In bromide complexes with TCNE, the location of the anion relative to the acceptor is variable. In fact, a 2 1 complex [(Br )2,TCNE] is isolated in which both anions reside over the olefinic bond when the tetraethylammonium salt of bromide is used. In comparison, if the tetrapropyl- or tetrabutylammonium salts of the same anion are employed, the (1 1) complexes [Br ,TCNE] are formed in which the bromide donors are shifted toward the cyano substituents (Fig. 15a). In both cases however, the short intermolecular separations that are characteris-... 

It is characteristic that the interatomic E14 E15 distances in model betaines is by 0.6-0.8 A shorter than the sums of the corresponding van der Waals radii (Table XI). This also increases the steric strains in the gauche-coniormers. 

The Ge atom in germocanes is shifted from the equatorial plane toward the X substituent. Both axial bonds are elongated as predicted by the concept of the 3c-4e hypervalent bond. The D — Ge distance is larger than the sum of covalent radii of these elements but is still significantly shorter than the sum of their van der Waals radii (Table 1). 

Bondi developed a method based on covalent bond distances and van der Waals radii (Table V3) to calculate van der Waals volume [Bondi, 1964]. The volume calculated in this way is sometimes called Bondi volume. It is obtained easily by summing up appropriate volume contributions of atoms and functional groups, as proposed by Bondi note that the Bondi volume does not account for the overlaps which are possible whenever three or more atomic spheres intersect, it is roughly 60-70% of molecular volume. 

When two or more atoms are forced together, they repel each other, and experience van der Waals repulsion as the electrons associated with each atom start to occupy a common space. The effective size of atoms is given by the van der Waals radii (Table 4.3), which are related to how close atoms or groups can come without severe repulsion. Van der Waals repulsion is also called steric hindrance, and the energy of that interaction is steric strain. In the eclipsed conformation of propane, the hydrogen atoms at C-1 and C-3 are not close enough to produce a large van der Waals repulsion. 

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