Chemistry Reference
In-Depth Information
9
Valence Bond Theory
and the Chemical Bond
In this chapter we shall consider, first, elements of the Born-Oppenheimer
approximation, concerning the separation in molecules of the motion of the
electrons from that of the nuclei. It will be seen that, by neglecting small
vibronic terms, the nuclei move in the field provided by the nuclei themselves
and the molecular charge distribution of the electrons, determining what is
called a potential energy surface, a function of the nuclear configuration.
Next, we shall introduce the study of the chemical bond by considering
the simplest two-electron molecular example, the H
2
molecule. It will be
seen that the single configurationMOapproach fails to describe the correct
dissociation of the molecule in ground-state H atoms because of the
correlation error. A qualitatively correct description of the bond dissocia-
tion in H
2
is instead provided by the Heitler-London (HL) theory, where
different electrons are allotted to different atomic orbitals, the resulting
wavefunction for the ground state then being symmetrized with respect to
electron interchange in order to satisfy Pauli's antisymmetry principle.
HL theory may be considered as introductory to the so-called valence
bond (VB) theory of molecular electronic structure, where localized
chemical bonds in molecules are described in terms of covalent and ionic
structures. The theory is considered at an elementary level for giving
qualitative help in studying the electronic structure of simple molecules,
in a strict correspondence between quantum mechanical VB structures
and chemical formulae. The importance of hybridization is stressed in
