Chemical bonding molecular shape

The shape of a molecule deteraiines many of its physical and chemical properties. Molecular shape, in turn, is determined by the overlap of orbitals that share electrons. Theories have been developed to explain the overlap of bonding orbitals and are used to predict the shape of the molecule. 

Clusters are intennediates bridging the properties of the atoms and the bulk. They can be viewed as novel molecules, but different from ordinary molecules, in that they can have various compositions and multiple shapes. Bare clusters are usually quite reactive and unstable against aggregation and have to be studied in vacuum or inert matrices. Interest in clusters comes from a wide range of fields. Clusters are used as models to investigate surface and bulk properties [2]. Since most catalysts are dispersed metal particles [3], isolated clusters provide ideal systems to understand catalytic mechanisms. The versatility of their shapes and compositions make clusters novel molecular systems to extend our concept of chemical bonding, stmcture and dynamics. Stable clusters or passivated clusters can be used as building blocks for new materials or new electronic devices [4] and this aspect has now led to a whole new direction of research into nanoparticles and quantum dots (see chapter C2.17). As the size of electronic devices approaches ever smaller dimensions [5], the new chemical and physical properties of clusters will be relevant to the future of the electronics industry. 

What Are the Key Ideas The central ideas of this chapter are, first, that electrostatic repulsions between electron pairs determine molecular shapes and, second, that chemical bonds can be discussed in terms of two quantum mechanical theories that describe the distribution of electrons in molecules. 

We have to refine our atomic and molecular model of matter to see how bulk properties can be interpreted in terms of the properties of individual molecules, such as their size, shape, and polarity. We begin by exploring intermolecular forces, the forces between molecules, as distinct from the forces responsible for the formation of chemical bonds between atoms. Then we consider how intermolecular forces determine the physical properties of liquids and the structures and physical properties of solids. 

The Lewis stmcture of a molecule shows how the valence electrons are distributed among the atoms. This gives a useful qualitative picture, but a more thorough understanding of chemistry requires more detailed descriptions of molecular bonding and molecular shapes. In particular, the three-dimensional structure of a molecule, which plays an essential role in determining chemical reactivity, is not shown directly by a Lewis structure. 

The doubling or amplification that is inherent in dendrimer chemistry is the dominant process that controls the dendrimer shape [1], With each generation, the number of terminal units usually doubles. Each shell (generation) enhances at approximately a constant value, whereas the total molecular mass approximately doubles with each generation as does the number of branch points. While the dendrimer mass doubles with generation, the space to fit the units increases at a much slower rate. The contour length of any chain from the core to the terminal units is proportional to the number of chemical bonds and hence the number of... 

In this chapter, you will review and extend your understanding of chemical bonding. You will discover how and why each molecule has a characteristic shape, and how molecular shape is linked to the properties of substances. You will also consider the importance of molecular shape to the development of materials with specific applications in the world around you. 

Tbis explanation of enzyme action is belpful, but far from complete. For one thing, enzymes differ significantly in the ways that they interact with other compounds. Some enzymes bond and react with only specific compounds, while others bond and react with an array of compounds in a chemical family that have the same or similar functional groups. Some enzymes fit neatly into an opening in a substrate, while others actually change the shape of the substrate on which they operate. The fact that enzyme actions are so diverse simply conhrms that the chemical structures of enzymes and substrates differ signihcantly, and the chemical mechanisms by which they interact can be very complex indeed. In fact, the tools needed to understand the precise molecular shapes of enzymes and substrates have become available only recently. Once these shapes have become known, scientists are able to unravel the exact steps that take place when enzyme and substrate interact with each other. 

This system has its merits and uses - indeed, we shall employ the line notation almost exclusively - but to understand how bonding occurs, and to explain molecular shape and chemical reactivity, we need to use orbital concepts. 

Graphical representations of the electron density will be provided in Chapter 4, and connections drawn between electron density and both chemical bonding and overall molecular size and shape. 

The concept of conformational isomerism is central to any consideration of molecular shape. Molecules that are flexible may exist in many different shapes or conformers. Conformational isomerism is the process whereby a single molecule undergoes transitions from one shape to another the physical properties of the molecule have not changed, merely the shape. Conformational isomerism is demonstrated by compounds in which the free rotation of atoms around chemical bonds is not significantly hindered. The energy barrier to the transition between different conformations is usually very low... 

Throughout the book, theoretical concepts and experimental evidence are integrated An introductory chapter summarizes the principles on which the Periodic Table is established and describes the periodicity of various atomic properties which are relevant to chemical bonding. Symmetry and group theory are introduced to serve as the basis of all molecular orbital treatments of molecules. This basis is then applied to a variety of covalent molecules with discussions of bond lengths and angles and hence molecular shapes. Extensive comparisons of valence bond theory and VSEPR theory with molecular orbital theory are included Metallic bonding is related to electrical conduction and semi-conduction. 

---