Erich Huckel – proposed Huckel method

Erich Huckel – proposed Huckel method

Erich Huckel

Biography & Contributions

Erich Huckel [Erich Armand Arthur Joseph Huckel] was a German physicist, physical chemist was born on August 09, 1896 – died on February 16, 1980. Huckel is famous for his greatest contributions towards Debye-Huckel theory and Huckel method.

In the year of 1923 along with Debye developed their theory of electrolytic solutions, elucidating the behavior of strong electrolytes by considering interionic forces, in order to account for their electrical conductivity and their thermodynamic activity coefficients.

Huckel is most famous for developing simplified quantum mechanics methods to deal with planar unsaturated organic molecules.

In 1930 he proposed an σ/π separation theory to explain the restricted rotation of alkenes (compounds containing a C=C double bond). According to Huckel, only the Huckel σ bond is axially symmetric about the C-C axis, but the π bond is not; this restricts rotation.

In 1931 generalized his analysis by formulating both valence bond (VB) and molecular orbital (MO) descriptions of benzene and other cyclo-conjugated hydrocarbons.

In 1936, Huckel developed the theory of π-conjugated biradicals. In 1937 Huckel refined his MO theory of pi electrons in unsaturated organic molecules.

Huckel’s 4n+2 rule for determining whether ring molecules composed of C=C bonds would show aromatic properties was first stated clearly by Doering in a 1951.

Huckel Method

Huckel method also termed as Huckel molecular orbital method. It is a very simple linear combination of atomic orbitals molecular orbitals (LCAO MO) method for the determination of energies of molecular orbitals of pi electrons in conjugated hydrocarbon systems, such as ethene, benzene and butadiene.

It is the theoretical basis for the Huckel’s rule. It was later extended to conjugated molecules such as pyridine, pyrrole and furan that contain atoms other than carbon, known in this context as heteroatoms.

According to Huckel’s rule only pi electron molecular orbitals (MO's) are included because these determine the general properties of these molecules and the sigma electrons are ignored. This is referred to as sigma-pi separability. It is justified by the orthogonality of sigma and pi orbitals in planar molecules.

This method predicts how many energy levels exist for a given molecule, which levels are degenerate and it expresses the MO energies as the sum of two other energy terms, called alpha, the energy of an electron in a 2p-orbital, and beta, an interaction energy between two p orbitals which are still unknown but importantly have become independent of the molecule.

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