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The Hückel method or Hückel molecular orbital method (HMO), proposed by Erich Hückel in 1930, 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.〔E. Hückel, ''Zeitschrift für Physik'', 70, 204 (1931); 72, 310 (1931); 76, 628 (1932); 83, 632 (1933).〕〔''Hückel Theory for Organic Chemists'', C. A. Coulson, B. O'Leary and R. B. Mallion, Academic Press, 1978.〕 It is the theoretical basis for the Hückel'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.〔Andrew Streitwieser, ''Molecular Orbital Theory for Organic Chemists'', Wiley, New York (1961).〕 The extended Hückel method developed by Roald Hoffmann is computational and three-dimensional and was used to test the Woodward–Hoffmann rules.〔"Stereochemistry of Electrocyclic Reactions", R. B. Woodward, Roald Hoffmann, ''J. Am. Chem. Soc.'', 1965; 87(2); 395–397. .〕 It is a very powerful educational tool, and details appear in many chemistry textbooks. ==Hückel characteristics== The method has several characteristics: * It limits itself to conjugated hydrocarbons. * 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. For this reason, the Hückel method is limited to planar systems. * The method takes as inputs the LCAO MO Method, the Schrödinger equation and simplifications based on orbital symmetry considerations. Interestingly the method does not take in any physical constants. * The 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. In addition it enables calculation of charge density for each atom in the pi framework, the bond order between any two atoms, and the overall molecular dipole moment. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Hückel method」の詳細全文を読む スポンサード リンク
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