Molecular shape molecules with single bonds

The shape of a molecular particle plays a major role in determining the macroscopic properties of a substance. We examine this role in other chapters in this book. To better understand and predict the shape-property relationship, you should know what is responsible for molecular shape, and this is discussed in this section and the next. Discussion in these sections is limited to molecules having only single bonds. Molecules with multiple bonds are considered in Section 13.4. 

Figure 2.2 Selected families (DDj(aj)) of density domains of the water molecule, as calculated with the GAUSSIAN 90 ab initio program [253] and the GSHAPE 90 molecular shape analysis program [254], using a 6-3IG basis set. There are only two topologically different types of families of density domains either a single density domain, or a family of three density domains. The sequence of topologically distinct cases provides a topological description of chemical bonding.

All immunoglobulins have a number of structural features in common.2 They possess two light polypeptide chains, each with an approximate molecular weight of 25 kDa, and two heavy polypeptide chains of 50 kDa each. These four chains are bound together in a single antibody molecule by disulfide bonds, and form a Y-shape with a central axis of symmetry (Fig. 5.2). The two halves of a natural immunoglobulin are identical. 

There is no quantum-mechanical evidence for spatially directed bonds between the atoms in a molecule. Directed valency is an assumption, made in analogy with the classical definition of molecular frameworks, stabilized by rigid links between atoms. Attempts to rationalize the occurrence of these presumed covalent bonds resulted in the notion of orbital hybridization, probably the single most misleading concept of theoretical chemistry. As chemistry is traditionally introduced at the elementary level by medium of atomic orbitals, chemists are conditioned to equate molecular shape with orbital hybridization, and reluctant to consider alternative models. Here is another attempt to reconsider the issue in balanced perspective. 

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