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The Hückel method or Hückel molecular orbital theory, proposed by Erich Hückel in 1930, is a simple method for calculating molecular orbitals as linear combinations of atomic orbitals. The theory predicts the molecular orbitals for π-electrons in π-delocalized molecules , such as ethylene , benzene , butadiene , and pyridine .
In the extended Hückel method, only valence electrons are considered; the core electron energies and functions are supposed to be more or less constant between atoms of the same type. The method uses a series of parametrized energies calculated from atomic ionization potentials or theoretical methods to fill the diagonal of the Fock matrix.
Until 1969 there was no obvious relationship except that the two methods lead to the same predictions. As noted earlier, the Woodward–Hoffmann method requires symmetry. But in 1969 and 1970 a general formulation was published, [ 9 ] [ 10 ] namely, A ground-state pericyclic change is symmetry-allowed when the total number of (4 q + 2) s and (4 ...
This is still used occasionally as an approximation, though the more precise PPP Pariser–Parr–Pople method succeeded it in 1953. "Extended Hückel MO theory" applies to both sigma and pi electrons, and has its origins in work by William Lipscomb and Roald Hoffmann for nonplanar molecules in 1962.
For table salt in 0.01 M solution at 25 °C, a typical value of () is 0.0005636, while a typical value of is 7.017, highlighting the fact that, in low concentrations, () is a target for a zero order of magnitude approximation such as perturbation analysis. Unfortunately, because of the boundary condition at infinity, regular perturbation does ...
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The method has wide applicability in precise prediction of electronic transitions, particularly lower singlet transitions, and found wide application in theoretical and applied quantum chemistry. The two basic papers on this subject were among the top five chemistry and physics citations reported in ISI, Current Contents 1977 for the period of ...
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