Welcome
Content
Introduction
Linear Combination of Atomic Orbitals
H2 and He2 - The Simplest Examples
The Effect of Electronegativity
p-Orbitals and p-Overlap
Combining AOs to Build MOs
The AH2 Walsh Diagram
Larger Molecules
Reactions
Elementary Symmetry
Glossary
Molecules in VRML
General Information - Installation - Use
| 2. Linear Combination of Atomic Orbitals The Linear Combination of Atomic Orbitals (LCAO) approximation is fundamental to many of our current models of chemistry. Both the vast majority of the calculational programs that we use, be they ab initio, density functional, semiempirical molecular orbital, or even some sophisticated force-fields, and our qualitative understanding of chemistry are based on the concept that the orbitals of a given molecule can be built from the orbitals of the constituent atoms. We feel comfortable with the p-HOMO (highest occupied molecular orbital) of ethylene depicted as a combination of two carbon p-orbitals, as shown in Figure 2.1, although this is not a very accurate description of the electron density of this molecular orbital (MO). The use of the p-atomic orbitals (AO s), however, makes it easier to understand both the characteristics of the MO itself and the transformations that it can undergo during reactions. Figure 2.1 p-HOMO of ethylene. In principle, we could build up MOs from many sorts of function that can describe an electron density probablility distribution, but we have learnt to understand combinations of AOs and to use them in our models of chemical bonding and reactivity. Indeed, if confronted with an MO that was calculated, for instance, using plane waves, most chemists would immediately translate it into a combination of AOs. In the following, we will describe the LCAO approximation and demonstrate some of the effects that are important when AOs interact with each other to form MOs. Next: H2 and He2 -The Simplest Examples The Effect of Electronegativity p-Orbitals and p-Overlap Combining AOs to Build MOs The AH2 Walsh Diagram