MOTheorie

MO-Theorie, also known as molecular orbital theory, is a framework in quantum chemistry that describes electrons in molecules as occupying molecular orbitals—wavefunctions that extend over the entire molecule. MOs are formed by linear combinations of atomic orbitals (LCAO), reflecting the delocalized nature of electrons in many chemical systems.

In MO theory, bonding MOs are typically lower in energy than the contributing atomic orbitals, while antibonding MOs are higher. The occupation of these orbitals by electrons determines bond formation and strength. The bond order is calculated as (number of electrons in bonding orbitals minus number in antibonding orbitals) divided by two, providing a simple measure of bond strength and stability.

The theory applies to both homonuclear and heteronuclear molecules. For diatomic species, MO diagrams illustrate the energy ordering of sigma and pi orbitals derived from s and p atomic orbitals, though s–p mixing can modify the ordering in some cases. For conjugated and pi-electron systems, simplified approaches such as Hückel MO theory offer insights into the delocalization of electrons and properties like aromaticity.

Historically, MO theory developed in the 1920s and 1930s with contributions from Mulliken, Hund, and others, building on the linear combination of atomic orbitals concept. In practice, modern applications rely on computational methods such as Hartree–Fock and density functional theory to obtain accurate molecular orbitals and energies.

MO theory provides a natural picture of delocalized bonding and is widely used to predict molecular structure, bond orders, spectra, and reactivity. It is often complemented by valence bond theory, which emphasizes more localized bonding descriptions and electron pairing.