Huckel's molecular orbital theory of conjugative system
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The Hückel method or Hückel molecular orbital theory, proposed by Erich Hückel in 1930, is a very simple linear combination of atomic orbitals molecular orbitals method for the determination of energies of molecular orbitals of π-electrons in π-delocalized molecules, such as ethylene, benzene, butadiene, and pyridine.It is the theoretical basis for Hückel's rule for the aromaticity of {\displaystyle 4n+2} π-electron cyclic, planar systems. It was later extended to conjugated molecules such as pyridine, pyrroleand furan that contain atoms other than carbon, known in this context as heteroatoms.A more dramatic extension of the method to include σ-electrons, known as the extended Hückel method, was developed by Roald Hoffmann. The extended Hückel method gives some degree of quantitative accuracy for organic molecules in general (not just planar systems) and was used to test the Woodward–Hoffmann rules.