Molecular shape valence-bond method

Unfortunately, the Schrodinger equation can be solved exactly only for one-electron systems such as the hydrogen atom. If it could be solved exactly for molecules containing two or more electrons,3 we would have a precise picture of the shape of the orbitals available to each electron (especially for the important ground state) and the energy for each orbital. Since exact solutions are not available, drastic approximations must be made. There are two chief general methods of approximation the molecular-orbital method and the valence-bond method. 

Using the valence-bond method, describe the bonding and molecular shape of... 

Although the discussions of the preceding molecules have been couched in valence bond terms (Lewis structures, hybridization, etc.), recall that the criterion for molecular shape (rule 2 above) was that the cr bonds of the central atom should be allowed to gel as far from each other as possible 2 at 180°. 3 at 120°, 4 at 109.5°, etc. This is (he heart of the VSEPR method of predicting molecular structures, and is, indeed, independent of valence bond hybridization schemes, although it is most readily applied in a VB context. 

X-Ray crystal-structure analysis is one of the most precise techniques at present available for determining the conformation of a sugar molecule, as it provides extremely accurate information on bond lengths, conformational (dihedral) and valency angles, and interatomic distances. However, it is to be noted that molecular shapes, as determined by crystallographic methods, refer to molecules held rigidly in the crystal lattice, and do not necessarily correspond to the favored conformation in solution, although such a correspondence is frequently observed. 

The fact that the molecule has such a distinct geometric form can be explained by the branch of physics known as quantum mechanics. In other words, the tetrahedral configuration of C-H bonds is the consequence of the repulsion of electron pairs which tend to be as far apart as possible from each other. This method of prediction of the molecular shape by considering the optimal distribution of bonds (bonding electron pairs) in which the electron repulsion is minimal, is called VSEPR (valence shell electron pair repulsion). Although this method is widely used, in practice we must point out that this procedure is a simplified approach that can afford only an approximate picture of the molecule. 

Like so many other molecular properties, shape is determined by the electronic structure of the bonded atoms. The approximate shape of a molecule can often be predicted by using what is called the valence-shell electron-pair repulsion (VSEPR) model. Electrons in bonds and in lone pairs can be thought of as "charge clouds" that repel one another and stay as far apart as possible, thus causing molecules to assume specific shapes. There are only two steps to remember in applying the VSEPR method ... 

We now turn from the use of quantum mechanics and its description of the atom to an elementary description of molecules. Although most of the discussion of bonding in this book uses the molecular orbital approach to chemical bonding, simpler methods that provide approximate pictures of the overall shapes and polarities of molecules are also very useful. This chapter provides an overview of Lewis dot structures, valence shell electron pair repulsion (VSEPR), and related topics. The molecular orbital descriptions of some of the same molecules are presented in Chapter 5 and later chapters, but the ideas of this chapter provide a starting point for that more modem treatment. General chemistry texts include discussions of most of these topics this chapter provides a review for those who have not used them recently. 

Valence shell electron pair repulsion theory (VSEPR) provides a method for predicting the shape of molecules, based on the electron pair electrostatic repulsion. It was described by Sidgwick and Powell" in 1940 and further developed by Gillespie and Nyholm in 1957. In spite of this method s very simple approach, based on Lewis electron-dot structures, the VSEPR method predicts shapes that compare favorably with those determined experimentally. However, this approach at best provides approximate shapes for molecules, not a complete picture of bonding. The most common method of determining the actual stmctures is X-ray diffraction, although electron diffraction, neutron diffraction, and many types of spectroscopy are also used. In Chapter 5, we will provide some of the molecular orbital arguments for the shapes of simple molecules. 

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