Sizes and Shapes of Molecules

Space filling van der Waals models (A3) are useful for illustrating the actual shape and size of molecules. These models represent atoms as truncated balls. Their effective extent is determined by what is known as the van der Waals radius. This is calculated from the energetically most favorable distance between atoms that are not chemically bonded to one another. 

A knowledge of molecular shapes and sizes is important to an understanding of chemical reactions. The shape of a molecule (and the bond types it possesses) has important implications for the manner in which it enters into chemical reactions. The shape and size of molecules also influence their packing in the crystalline state. 

Thus, the structure of nucleic acid molecules determines the structure of protein molecules. The structure of protein molecules, we have seen, determines the way in which they control living processes. Biology is becoming more and more a matter of shapes and sizes of molecules. 

Molecules consist of atoms held together, bonded, by interatomic forces. These interatomic forces are electromagnetic interactions arising from the motion of electrons and determine the shape and size of molecules. At absolute zero, in the classical interpretation, the molecules have an equilibrium configuration that is determined by the energy minimum of these interatomic forces. We shall analyse how the motions of atoms in a molecule can be calculated and describe how the interactions between atoms in molecules are calculated. 

When two molecules approach each other, the electron clouds surrounding each molecule would be expected to interact and create a repulsive force tending to push them apart. This repulsive force would arise from simple coulombic repulsion of charges. However, because of the differences in shape and size of molecules, no definite theoretical treatment of this short-range repulsion force has been established. This repulsive interaction has been described by the following general formula ... 

WE SAW IN CHAPTER 8 THAT LEWIS STRUCTURES help US Understand the compositions of molecules and their covalent bonds. However, Lewis structures do not show one of the most important aspects of molecules—their overall shapes. The shape and size of molecules—sometimes referred to as... 

The separation mechanism of inorganic membranes is even more complex than that of polymeric separating layers. Compared with polymeric pervapora-tion membranes these hydrophiHc inorganic ones are not dense, but porous. Molecular-sieving effects, caused by shape and size of molecules, and shape and size of the pores determine the separation. The surface of the membrane and the inside of the pore walls are highly hydrophiHc, so preferential sorption of water on the membrane and inside the pores and surface diffusion in the adsorbed layer play an additional very important role. 

Sdectivity - a very important plus for many heterogeneous catalysts whereby the pore structure Hmits the diffusion in for reactants or out for products effectively restricting the chemistry that occurs and the shape and size of molecules that can react or be formed shape selectivity can also affect the stereochemistry through the control of reaction pathways (e.g., use of zeoHtes to hmit the alkylation of benzene to mono-substituted products). 

The three-dimensional shapes and sizes of molecules are determined by their bond angles and bond lengths. Molecules with a central atom A surrounded by n atoms B, denoted AB , adopt a number of different... 

Fig. 3. Shape and size of molecules according to the rigid convex-body model determined from diffusion coefficients at 0°C.

It is the electrochemical reactions at various potentials that should be considered as an ideal homologous series, i.e. the same reaction with different energies. The introduction of substituents into the series of chemical reactions leads to some secondary effects, like the change in the shape and size of molecules, resulting in a scattering of points on the AG vs. AG graph. 

---