Extended Huckel method
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The extended Hückel method is a way of determining the molecular orbitals, but it is not very successful in determining the structural geometry of an organic molecule. It can however determine the relative energy of different geometrical configurations. It involves calculations of the electronic interactions in a rather simple way where the electron-electron repulsions are not explicitly included and the total energy is just a sum of terms for each electron in the molecule.
It is common in many theoretical studies to use the extended Hückel molecular orbitals as a preliminary step to determining the molecular orbitals by a more sophisticated method such as the CNDO/2 method and ab initio quantum chemical methods. This leads to the determination of more accurate structures and electronic properties.
The method was first used by Roald Hoffmann who developed, with Robert Burns Woodward, rules for elucidating reaction mechanisms (the Woodward-Hoffmann rules). He used pictures of the molecular orbitals from extended Hückel theory to work out the orbital interactions in these cycloaddition reactions.
A closely similar method was used earlier by William Lipscomb for studies of boron hydrides and a related method has been used for studies of other inorganic molecules.
The method works poorly for molcules that contain atoms of very different electronegativity. To overcome this weakness, several groups have suggested iterative schemes that depend on the atomic charge. One such method, that is still widely used in inorganic and organometallic chemistry is the Fenske-Hall method.
[edit] Reference
Hoffmann, R., Journal of Chemical Physics, 39, 1397 (1963).
Hall, M. B. and Fenske, R. F., Inorganic Chemistry, 11, 768 (1962)
