SIMPLE HUCKEL MOLECULAR ORBITAL THEORY

In this chapter, simple Hiickel molecular orbital (SHMO) theory is developed. The reference energy, a, and the energy scale in units of ji are introduced. 

The SHMO theory was originally developed to describe planar hydrocarbons with conjugated n bonds. Each center is sp2 hybridized and has one unhybridized p orbital perpendicular to the trigonal sp2 hybrid orbitals. The sp2 hybrid orbitals form a rigid unpolarizable framework of equal C—C bonds. Hydrogen atoms are part of the framework and are not counted. The Hiickel equations (3.3) described in the first part of Chapter 3 apply, namely, 

Each MO is expanded in terms of the unhybridized p orbitals, one per center. The overlap integral between two parallel p orbitals is small and is approximated to be exactly zero. Thus, 

The condition that the Nn linear equations have a solution is that the determinant of coefficients (of the c s) be equal to zero  

CHARGE AND BOND ORDER IN SHMO THEORY (SAB = 0, ONE ORBITAL PER ATOM) 

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Rauk, A. Cannings, R., SUMO, A Simple Huckel Molecular Orbital Theory Calculator, http //www.chem.ucalgary.ca/SHMO/. 

THE HUCKEL PROBLEM, OR SIMPLE HUCKEL MOLECULAR ORBITAL THEORY (SHMO)... 

Physical organic chemistry, the study of the basic physical principles of organic reactions, is not a new field in 1940, Hammett had already written a book with this title. This ai ea has developed during the last 20 yeai s mainly because of the explosive growth of sophisticated analytical instrumentation and computational techniques, going from the simple Huckel molecular orbital theory to ab initio calculations of increasing accuracy enabled by the advent of fast supercomputers. 

Despite its obvious limitations, simple Huckel molecular orbital theory is still widely used as a... 

Simple Huckel molecular orbital theory, 86-97 alpha, 87... 

Desulfurization of polynuclear thiophenes was performed by continuous stirring with a Raney nickel catalyst in ethanol at 0-78.8 °C. The major products were biphenyl in the desulfurization of dibenzothiophene, a-phenylnaphthalene in the desulfurization of benzo[b]naphtho[l,2-d]thiophene, and -phenylnaphthalene in the desulfurization of benzo[b]naphtho[2,3-d]thiophene and benzo[b]naphtho[2,l-d]thiophene. Observation of these products shows that the main reaction pathway is the extrusion of a sulfur atom to give the corresponding hydrocarbon. The tt-electron densities of the sulfur atoms, which were calculated by using simple Huckel molecular orbital theory, are considered to be related to the adsorption of the sulfur compounds to the surface and consequently to the C-S bond-breaking rate. 

Table III. TT-Electron Densities of Thiophenes Determined by Using Simple Huckel Molecular Orbital Theory...

Self-consistent field (SCF), 230 anti-Sesquinorbornatriene, 248 SHMO, see also Simple Huckel molecular orbital theory allyl, 89... 

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