CHAPTER 2 - A Unified View of Stereochemistry and Stereochemical Changes

Abstract

This chapter presents a unified view of stereochemistry and stereochemical changes. The variety of structures observed in chemical systems leads to confusion when one studies molecular geometry. One solution, which the organic chemist is likely to adopt, is to limit consideration essentially to the first and second rows of the periodic table where the geometries are fairly well predicted by a molecular model kit and a little knowledge. The chapter describes the identification of symmetry operations of molecules. They are collected together in point symmetry groups. A symmetry operation may be defined to be some action that when performed on the molecule makes no observable or measurable difference. Point symmetry operations are separated into two classes: (1) proper operations or rotations and (2) improper operations or rotations. A proper operation may be performed without destroying the integrity of the molecule; it simply turns the molecule around. The product of any two proper operations must be another proper operation. An improper operation may not be physically performed on a molecule. Analogously to negative and positive numbers, the product of two improper operations must be proper as two inversions of the handedness of a system have no net effect. Conversely, the product of an improper and a proper operation is always improper. The chapter also illustrates the use of projection operators to determine molecular orbitals' (MO) vibrations and wave functions. It also reviews the relationships among different molecular geometries, both statically and kinetically.