Hückel

Hückel is a surname associated with several scientists, most notably Erich Hückel (1896–1980), a German physicist whose work helped establish modern quantum chemistry. He is best known for two contributions to the theory of π-electron systems: Hückel's rule for aromaticity and the Hückel molecular orbital (HMO) theory.

Hückel's rule states that planar cyclic conjugated molecules with (4n+2) π electrons are aromatic and unusually stable, while those with 4n π electrons are antiaromatic and destabilized. Compounds that do not meet these criteria due to nonplanarity or interrupted conjugation are typically nonaromatic. The rule provides a simple criterion that correlates electron count with observed stability and preference for planar, uniform bonding in many ring systems such as benzene.

The Hückel molecular orbital theory introduced a simple, semi-empirical method to describe π electrons in conjugated systems. It uses a tight-binding approach with two parameters, α and β, to construct and solve a secular determinant for the π-system. The resulting molecular orbitals and their energies explain qualitative features of aromaticity, bond length equalization in benzene, and the relative stabilities of polyenes. The method is deliberately simplified, neglecting σ electrons and electron correlation, but it remains a standard teaching tool and forms the basis for later approximations such as the extended Hückel method.

Erich Hückel's contributions to quantum chemistry—particularly the development of the Hückel theory and the aromaticity rule—have had a lasting influence on the study of molecular orbitals and chemical bonding.