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In organic chemistry, Hückel's rule predicts that a planar ring molecule will have aromatic properties if it has 4n + 2 π-electrons, where n is a non-negative integer. The quantum mechanical basis for its formulation was first worked out by physical chemist Erich Hückel in 1931.
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 .
Inspection of the Hückel one on the right, plus–minus overlaps are seen between orbital pairs 2-3, 3-4, 4-5, and 6-1, corresponding to an even number (4), as required by a Hückel system. The plus–minus orientation of each orbital is arbitrary since these are just basis set orbitals and do not correspond to any molecular orbital.
Extended Hückel method, considers also sigma orbitals (whereas the original Hückel method only considers pi orbitals) Hückel's rule (named after Erich Hückel), a method of determining aromaticity in organic molecules; Walter Hückel (1895-1973), German chemist; Wolfgang Hückel (born 1936), German diplomat, Ambassador of the GDR in Chad
Erich Armand Arthur Joseph Hückel ForMemRS [1] (August 9, 1896, Berlin – February 16, 1980, Marburg) was a German physicist and physical chemist. [2] He is mainly known for the Debye–Hückel theory of electrolytic solutions and the Hückel method of approximate molecular orbital (MO) calculations on π electron systems.
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Another example of a non-benzylic monocyclic arene is the cyclopropenyl (cyclopropenium cation), which satisfies Hückel's rule with an n equal to 0. [12] Note, only the cationic form of this cyclic propenyl is aromatic, given that neutrality in this compound would violate either the octet rule or Hückel's rule. [12]
This polarity can be explained by regarding azulene as the fusion of a 6 π-electron cyclopentadienyl anion and a 6 π-electron tropylium cation: one electron from the seven-membered ring is transferred to the five-membered ring to give each ring aromatic stability by Hückel's rule.