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Molecular orbital theory

Valence bond (VB) theory gave us a qualitative picture of chemical bonding, which was useful for predicting the shapes of molecules, bond strengths, etc. It fails to describe some bonding situations accurately because it ignores the wave nature of the electrons.

Molecular orbital (MO) theory has the potential to be more quantitative.

Usually we settle for simplified models here too. These simple models do not give very accurate orbital and bond energies, but they do explain concepts such as resonance (e.g., in the ferrocene molecule) that are hard to represent otherwise. We can get accurate energies from MO theory by computational "number crunching."

While MO theory is more correct than VB theory and can be very accurate in predicting the properties of molecules, it is also rather complicated even for fairly simple molecules. For example, you should have no trouble drawing the VB pictures for CO, NH3, and benzene, but we will find that these are increasingly challenging with MO theory.

Constructing the molecular orbitals for a molecule:

We use atomic orbitals (AO) as a basis for constructing MO's.

LCAO-MO = linear combination of atomic orbitals. In physics, this is called this the tight binding approximation.

The molecular orbitals, also called wavefunctions (ψ), are obtained by adding and subtracting atomic orbitals (φ). The φ's are multiplied by scalar coefficients (c) to give normalized linear combinations.

For example, to make MO's (ψ1 and ψ2) from two AO's φ1 and φ1, we write:

ψ1 = c1φ1 + c2φ2 and

ψ2 = c1φ1 - c2φ2