Waals radii of several atoms and groups

The rotational profile of n-butane can be understood as a superimposition of van der Waals forces on the ethane potential energy diagram. The two gauche 

This is the value most usually quoted (actually 0.77 kcal/mol) obtained by Raman spectroscopy G.J. Szasz, N. Sheppard, and D. H. Rank, J. Chem. Phys. 16,704 (1948) a value of 0.68 kcal/mol has been obtained by NMR P. B. Woller and E. W. Garbisch, Jr., J. Am. Chem. Soc. 94, 5310 (1972). 

The populations of the various conformers are related to the energy differences between them by the equation 

The van der Waals forces are apparently of the attractive type in some halogenated hydrocarbons. This is illustrated for the case of n-propyl chloride, where the gauche conformation is slightly preferred over the anti conformation at equilibrium. The gauche is favored not only on entropy grounds, but on enthalpy grounds as well. For the equilibrium  

From E. L. Eliel, Stereochemistry of Carbon Compounds, McGraw-Hill Book Co. New York, 1962. 

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Table 3.1. Van der Waals Radii of Several Atoms and Groups (A)"...

The stack of BEDT is of the so-called a-phase. The sulphur-sulphur contacts between the stacks (S- S distances s= 3.47 A, shorter than the sum of the van der Waals radii of 3.6 A) and within the stack (S S distances w4.l-4.2A) give the layers a decidedly two-dimensional character. The BEDT donors are related by a two-fold screw axis, and therefore have no real chiral nature (the molecules have pseudo-centre symmetry). There are several hydrogen bonds between the hydrogen atoms of the ethylene groups of BEDT and the carboxylate groups of the counter-ion. The material is a semiconductor, where the conductivity falls as the temperature is lowered, from about 1 S cm 1 at room temperature. 

Covalent and van der Waals radii are other fundamental properties of atoms in molecules that are influenced by nuclear charge and electron distribution. A glance at a molecular model or graphic suggests that most atoms have several different dimensions. There is the distance between each bound atom and also a dimension in any direction in which the atom in not bonded to another atom. The former distance, divided between the two bonded atoms, is called the covalent radius. The nonbonded dimension of an atom or group in a molecule is called the van der Waals radius. This is the distance at which nonbonded atoms begin to experience mutual repulsion. Just short of this distance, the interatomic forces are weakly attractive and are referred to as dispersion or London forces and are attributed to mutual polarization of atoms. 

Figure 19.2 shows the noble gases superimposed on the network of interconnected ideas. Table 19.1 is a slightly amended version of the usual table of periodic properties. Note that these properties are exactly as expected on the basis of effective nuclear charge and the distance of the valence electrons from that charge. Consistent with the noble nature of these elements, the usual entries for atomic and ionic radii have been replaced by van der Waals radii. Only two entries, for xenon and krypton, have been made in the table under covalent radii. (Several radon compounds are known, but the covalent radius has not been well-established.) As expected, these radii increase regularly down the group. 

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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