Extended Htickel Theory

Hermann RB. Structure-activity correlations in the cephalosporin C series using extended Htickel theory and CNDO/2.1 Antibiot 1973 26 223-7. 

The reactivity and regioselectivity of 1,3-dipolar cycloadditions have been discussed in terms of the frontier orbitals [271]. Most of the features may be understood on the basis of simple Htickel MO theory. The HOMO and LUMO n orbitals and n orbital energies for all 18 combinations of the parent dipoles are shown in Figure 12.8. The frontier orbitals of many of the 1,3-dipoles have previously been derived by CNDO/2 and extended Htickel theory [272]. The first six structures, all of 16-electron type, are shown in greater detail ... 

Hoffman, R. (1963). An extended Htickel theory. I. Hydrocarbons. J. Chem. Phys. 39, 1397-412. 

In this Appendix, Htickel theory calculations are used to demonstrate that the polynomials of Appendix 1 can be applied as basis functions for the irreducible subspaces of a Hamiltonian invariant under icosahedral point symmetry, while extended Htickel theory calculations on cubium cages of cubic point symmetry are used to demonstrate the same result for the kubic harmonics, since single bond-length regular orbits are not possible in all cases. 

Extended Htickel theory calculations are the maximum level of approximation that can be employed for the calculation of the eigenvalue spectra for the regular orbit cages of the cubic point groups, since different bond lengths have to be accommodated on the His orbital... 

The main advantage of the extended Htickel theory is that only atomic ionization potentials are required, and it is easily parameterized to the whole periodic table. Extended Htickel theory can be used for large systems involving transition metals, where it often is the only possible computational model. The very approximate method of extended Htickel theory makes it unsuitable for geometry optimizations without... 

Pure atomic orbitals Extended Htickel theory 40,50) Iterative extended Hiickel theories80 81 82) Quasi SCF diagonal element method85) Kinetic-energy Hiickel theory 58) Zero-differential overlap approximation 61,69) Standard SCF method73 74 75 76,77,78) SCF group function method 83) Random-phase... 

W. Htickel, also provided a basis for a rational description of molecular orbitals in these systems. An extended Htickel theory allowed a study of molecular orbitals throughout chemistry at a certain level of approximation. 

A formalized analysis of the EFG was also employed by Bagin et al. (1980) their model effectively explained the large quadrupole splitting that is associated with octahedral Fe, again using vacancies as part of their model. More recently, an elegant paper by Aldridge et al. (1986) used molecular orbital methods to model the electronic structure of biotite. They showed that iterative extended Htickel theory calculations of... 

For example, semiempirical extended Htickel theory is nonvariational due to its internal parametrization. 

The latest contributions of physicists and the theoretical chemists to the explanation of this phenomenon stem from the results of ab initio and density functional theory calculations., which refute the previous s-d hybridization proposed by the extended Htickel theory. They suggest that the metallophilic attraction can be considered a correlation effect strengthened by the relativistic effect, which is important in the case of gold. ... 

In analogy to extended Htickel theory, there are also charge iterative methods for simple Htickel theory. The equivalent of eq. (3.99) is given in eq. (3.104). 

R. Hoffmann, Extended Htickel theory, n. Sigma orbitals in the azines, J. Chem. Phys. 40 (1964) 2745. 

A much more drastic approximation involves the complete neglect of electronic repulsion, in the so-called Extended Htickel Theory (EHT) approach developed in... 

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