Qualitative Explanation of Molecular Structures by Various Approaches

Abstract

Valence bond (VB) picture and valence shell electron pair repulsion principle (VSEPR) [1,2,3] are localized bond approaches.  VB picture is the historic landmark for modern chemistry, which is based on the valence bond. It invests each atom with the valence according to the number of electron, which is the basic concept for chemical bonds of the molecule; however, it has the fatal fault in which hybridization of the bonding orbital is determined after empirically knowing the molecular structure. VSEPR is more applicable to elucidate various molecular structures but is still an empirical picture. Both have no proper way to explain molecular structure in the excited state. On the other hand, molecular orbital (MO) theory is the approach based on delocalized electrons.  It starts from the molecular Hamiltonian, and is a more systematic and rational way to understand molecular structures. Many textbooks have given explanations for the molecular deformation [4]. In this chapter, several well-known issues are discussed, the bond elongation of ethylene under the anionization, the bent structure of water, the nonplanar structure of NH$_3$, the tautomerism between benzenoid and quinoid forms of organic molecule, the triangle structure of C$_3$H$_3$, and the chemical process of Diels–Alder reactions.