Extended Hiickel-type calculations

A detailed interpretation of the magnetic parameters of the ligand nuclei of Co(aca-cen) using an extended Hiickel-type calculation has been given by Rudin et al.12 219. ... 

The magnetic parameters of aquo-Mb obtained from Fig. 44 are collected in Table 15.1. The theoretical A values for the heme and histidine nitrogen, which are about 50% smaller than the observed values, have been determined by Mun et al.242), using an extended Hiickel-type calculation. According to these authors, the agreement between theoretical and experimental values could perhaps be improved further by considering electron core polarization effects. 

Extended Hiickel-type calculation on N, F compounds have led to a number of interesting and fruitful observations. 

The iterative extended Hiickel type of calculation has been widely used, especially by Gray and his co-workers (13) to interpret the visible and ultraviolet spectra of transition metal complexes, but from a quantitative point of view it has not proved to be very reliable. This difficulty arises primarily because the theory is based on a 1-electron Hamiltonian and, therefore, neglects important electron repulsion effects. 

CH)5+-Type Cations. The close relationship between carbocations and boranes led Williams1077 to suggest the square-pyramidal structure 615 for the (CH)5+ cation based on the square pyramidal structure of pentaborane. Stohrer and Hoffmann1078 subsequently came to the same conclusion concerning the preferred square-pyramidal structure for the (CH)5+ cation using extended Hiickel MO calculations. 

A theoretical study of a variety of tricyclic polymers with different types of bridging groups has been performed for the fused bithiophene system (Figure 9). Geometrical structures of the polymers were obtained from semi-empirical SCF band calculations and the electronic properties from the modified extended Hiickel band calculations <1997PCB10248, 1997JCP(107)10607>. 

Due to the shallow wells involved in some off-centre instabilities, the three parameter hybrid semiempirical B3LYP functional [246] implemented in Gaussian 98 [247] has also been used in shallow well cases. These calculations use the double zeta LANL2DZ basis, which employ Gaussian type orbitals and pseudopotentials to simulate the core electrons. Some semiempirical Self Consistent Charge Extended Hiickel (SCCEH) calculations have also been performed in order to study the effect of the removal of some orbitals from the basis set on the off-center instabilities. More details about this method can be found in [248]. 

A number of theoretical studies of varying degrees of sophistication have appeared on fluoroethylenes, but they have yet to attain the accuracy of precise experimental studies. They include the following ab initio calculations on tetra-fluoroethylene, mono-, di-, - and tri-fluoroethylenes, extended Hiickel MO calculations on fluorinated ethylenes and buta-l,3-dienes, semi-empirical calculations on ionic reactivities of fluoro-olefins,i CNDO- and INDO-type calculations on fluoroethylenes, 1,1-dichlorodifluoro- and chlorotrifluoro-ethylene, hexafluoropropene and octafluoroisobutene, on the polymerization of vinyl fluoride,... 

The calculations are of the extended Hiickel type [5] with modified Hii"s [6]. The basis set for palladium consisted of 5s, 5p, and 4d orbitals. The s and p orbitals were described by single Slater type wave functions and the d orbitals were taken as contracted linear combinations of two Slater type wave functions. Only 3s and 3p orbitals were used for phosphorus. 

One of the exceptions is Cu, which does not interact with PI after the deposition, and gives rise to a much weaker complex. Recently, Cain and Matienzo have used the tight binding calculations of the extended Hiickel type to find the relative acid strengths of Cu compounds in the... 

Quantum chemistry for lanthanides and actinides is an active area of current research. The applicable methods range from relativistically parametrized semiempirical extended Hiickel-type approaches to fully relativistic allelectron Dirac-Hartree-Fock calculations with a subsequent correlation treatment. It is emphasized that electron correlation effects and relativistic effects including spin-orbit coupling have to be treated simultaneously in order to avoid errors arising from the nonadditivity of these effects. Considerable progress is expected, especially on the ab initio side of quantum chemical applications, for small lanthanide and actinide systems during the next few years. 

The electronic structure of the transition metal diborides MB 2 (M = Sc, Ti, V, Cr, and Mn) was also investigated by Burdett et al. (169) employing band structure calculations of the extended Hiickel type. The bonding mechanism was discussed, and it was concluded that the interactions of the metal orbitals with those of the planar, graphite-like net of boron atoms and with those of other metal atoms are both important in influencing the properties of the diborides. The authors offer an explanation for the experimentally observed variation in the heat of formation of the diborides, which depends, according to their calculations, on the extent of occupation of the metal-boron orbitals. 

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