CHAPTER 1 - Definition and Determination of Molecular Geometry

Abstract

This chapter discusses methods for determination of molecular geometry. The shape of a molecule may seem like one of its simplest properties, yet it can convey a wealth of information to any chemist who knows how to interpret it. Molecular shape arises as a balance between various steric and electronic-structural effects; both of these types of effect also constrain the possibilities for intermolecular interactions, reactivity, and spectroscopy. The chapter reviews the principles that govern the shape or shapes adopted by any given molecule, and it also discusses the advantages and limitations of different approaches to molecular geometry. In many situations, it is convenient to describe the shape of a molecule by focusing on a subset of the atoms and associating an idealized high symmetry shape or template with these atoms; the selected atoms lie at or near to the vertices of this shape. The chapter provides an overview of non-quantum models of molecular geometry and describes the way they fit together in terms of the types of energy they consider or ignore. These models include valence shell electron pair repulsion theory, nonbonded radii, atom-atom repulsion, atom-atom interaction, and molecular modeling approaches. The molecular mechanics (MM) has the power to give accurate predictions of molecular geometry even in quite complicated systems. Typical MM force fields have been designed to reproduce a range of experimental properties for small and moderate sized molecules, including detailed structures, energy differences between different isomers, vibrational frequencies, and torsional energy barriers.