Molecular orbital theory

MO diagram dihydrogen

Molecular orbital diagram of He2

Benzene structure

Molecular Orbital Theory (MOT) is a fundamental theory in chemistry that describes the electronic structure of molecules using quantum mechanics. It extends the concepts of atomic orbitals and electron configuration to molecular levels, providing a framework for understanding chemical bonding, molecular structure, and the properties of molecules. Unlike the Valence Bond Theory, which emphasizes electron pairs in localized bonds, MOT focuses on the delocalization of electrons across the entire molecule.

Overview

MOT is based on the principle that atomic orbitals combine to form molecular orbitals (MOs) when atoms bond together. These MOs are spread over the entire molecule and can be occupied by electrons. The theory categorizes MOs as either bonding, non-bonding, or anti-bonding, depending on their energy levels and the constructive or destructive interference of the atomic orbitals' wave functions.

Bonding and Anti-bonding Orbitals

• Bonding Orbitals: Formed by the constructive interference of atomic orbitals, resulting in lower energy levels than the original atomic orbitals. Electrons in these orbitals stabilize the molecule.
• Anti-bonding Orbitals: Formed by the destructive interference of atomic orbitals, resulting in higher energy levels. Electrons in these orbitals can destabilize the molecule.

Molecular Orbital Diagrams

Molecular orbital diagrams are visual representations that show the relative energy levels of the MOs and the distribution of electrons in a molecule. These diagrams are crucial for predicting the magnetic properties and stability of molecules.

Applications of MOT

MOT is instrumental in explaining phenomena that cannot be adequately described by other theories, such as:

• The paramagnetic nature of oxygen molecule, O2
• The stability of diborane (B2H6)
• The color and photochemical behavior of certain compounds

Limitations

While powerful, MOT has limitations. It requires complex calculations, especially for large molecules, making it less practical for routine use. Furthermore, it may not accurately predict the properties of very large molecules or those with extensive electron delocalization.

Conclusion

Molecular Orbital Theory is a cornerstone of modern chemistry, providing deep insights into the behavior of molecules. It complements other theories, offering a more comprehensive understanding of molecular structure and properties.