Extended Huckel methods

TheHiickel constant (k) scales the interaction energy between two atomic orbitals (see Extended Huckel Method on page 125). HyperChem uses the default value of 1.75 (see the second part of this book. Theory and Methods). You should use this value for most cases. A suggested range for experimental adjustment of this constant is 1.6-2.0. ... 

The conformational properties of various 1,1 -diheteroferrocenes (7-10) have been the subject of three computational studies using extended Huckel methods.19,46 471,1 -Diphosphaferrocene has also been studied using the Fenske-Hall approach.48 and an MS Xa method.46 Where they overlap, the four treatments are in reasonable qualitative agreement. 

Table 5. Average values of bonds order and effective charges (Zeff) in pertechnetates and binuclear technetium acetates calculated by the extended Huckel method [58]...

The extended Huckel method [13] is an extension of the traditional Huckel method [14] expanding its range of applicability beyond planar conjugate systems. From a mathematical point of view, it consists simply in solving the matricial equation 1, where H is the hamiltonian matrix, C are... 

Ligand Li forms infinite chain by the coordination to Cu(I). Ligand L2 coordinates to Cu(I) as pendant The t and t values were calculated by the extended Huckel method... 

The photocyclodehydrogenation of thienyl ethylenes is well-defined when both thiophene rings are bound via a C(2) atom to the ethylenic bond as in (70). In other cases, however, more cyclization products are possible. To predict the photocyclization mode for heterohelicenes the F s rule fails in many cases, because correction factors for the hetero atoms in the Huckel MO calculation have to be introduced and the systems are not well comparable with carbocyclic diaryl ethylenes. A better reaction parameter in these cases is the Mulliken overlap population (nrs)51), introduced by Muszkat52) for these cases. The overlap populations of the atoms r and s in ground and excited state (nIiS and n s), are calculated using the extended Huckel method. Cyclizations should not occur when nr>s and An s (= nr>s — n s) have negative values. (This method can also be used for diaryl olefins, but in these cases calculation of F s is more simple.). 

The distances found between platinum centers in these molecules have been correlated with the resonating valence bond theory of metals introduced by Pauling. The experimentally characterized partially oxidized one-dimensional platinum complexes fit a correlation of bond number vs. metal-metal distances, and evidence is presented that Pt—Pt bond formation in the one-dimensional chains is resonance stabilized to produce equivalent Pt—Pt distances.297 The band structure of the Pt(CN)2- chain has also been studied by the extended Huckel method. From the band structure and the density of states it is possible to derive an expression for the total energy per unit cell as a function of partial oxidation of the polymer. The equilibrium Pt-Pt separation estimated from this calculation decreases to less than 3 A for a loss of 0.3 electrons per platinum.298... 

Hiickel and extended Huckel methods are termed semi-empirical because they rely on experimental data for the quantification of parameters. There are other semi-empirical methods, such as CNDO, MINDO, INDO, in which experimental data are still used, but more care is taken in evaluating the Htj. These methods are self-consistent field procedures based on 3 SCF. They are discussed in various works on molecular orbital theory.4... 

The simple Hiickel method basis set for ethene. The extended Huckel method basis set for ethene. 

Fig. 4.26 The simple Huckel method normally uses only one basis function per heavy atom only one 2p orbital on each carbon, oxygen, nitrogen, etc., ignoring the hydrogens. The extended Huckel method uses for each carbon, oxygen, nitrogen, etc., a 2s and three 2p orbitals, and for each hydrogen a Is orbital. This is called a minimal valence basis set...

The tight-binding band structure calculations were based upon the effective one-electron Hamiltonian of the extended Huckel method. [5] The off-diagonal matrix elements of the Hamiltonian were calculated acording to the modified Wolfsberg-Helmholtz formula. All valence electrons were explicitly taken into account in the calculations and the basis set consisted of double- Slater-type orbitals for C, O and S and a single- Slater-type orbitals for H. The exponents, contraction coefficients and atomic parameters were taken from previous work [6],... 

Methods. Initial calculations were done using the semi-empirical Extended Huckel method (5). The purpose of these quantum chemical calculations was to explore the main electronic characteristics of chemical bonding in silica tetrahedral networks. The calculations showed that bonding can be considered covalent and can be considered to be due to localized SiOSi units. 

Very small differences in bond strength between different silica polymorphs were found. Since the Extended Huckel Method is too approximate to calculate reliably the small diffferences in energy between low-density material, containing micropores, and high-density material without micropores, work was initiated to study the same problem but now with two rigorous techniques that are currently considered to be state of the art. 

In these calculations averaged charges on the intra-tetrahedral lattice cation positions were used. The difference between the two heats of formation due to ionic bonding is added to the heat of formation due to covalent bonding resulting from the simple Extended Huckel Method for zeolitic silicas in order to arrive at the total heat of formation of the zeolite structure as a function of the amount of aluminum. 

The semi-empirical extended Huckel method, which takes into account all valence electrons in a molecule, was introduced by Wolfsberg and Helmholz [27] it has been used over the years extensively by Lipscomb and co-workers, especially Hoffmann, [28-31] and has been applied by a large number of investigators. 

The extended Huckel method was devised in the exciting atmosphere of the Lipscomb laboratory L. L. Lohr played an essential part in its formulation. The first Cornell implementation to an extended material was made by M.-H. Whangbo [Whangbo, M.-H. Hoffmann, R. J. Am. Chem. Soc. 100, 6093... 

A possibility which we have pursued unsuccessfully is the preparation of a monomeric MSO2 complex which, like the more extensively studied partially bent MNO cases, should have a geometry intermediate between / -pyramidal and ly -planar. The angle between the M-S bond and the sum of the S-O vectors has been predicted to be 131° by extended Huckel methods compared to 120° for / -pyramidal complexes. 

The Extended Huckel method, as it is known today, was first implemented by Lohr and Lipscomb in studies of boron hydrides (23)... 

The molecular structure entry is performed by sketching the compounds on a graphics terminal, and then storing them as connection tables. The geometries are optimized using adapt s molecular mechanics routine, MM2 [4], the semiempirical molecular orbital program (MOPAC) [5], or a newly developed extended Huckel method [6]. 

Dissociation constants for a series of substituted nitrobenzene anion radicals have been successfully correlated with the electronic energy changes calculated by the Extended Huckel method (Carsky et al., 1972). The linear plot was found to have a slope similar to that observed for the parent compounds. 

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