Extended Huckel energies

The fact that the electrochemical oxidation of cis-[M2(cp)2(/x-SR)2(CO)4] and of [M2(/i-SR)2(CO)8]2 is reversible despite the amount of structural reorganization involved suggests that these changes require low activation energy. Extended Huckel Molecular Orbital (EHMO) calculations on the model complex (R = H) indicated that the Lowest Unoccupied Molecular Orbital (LUMO) was the u Mo—Mo [ 1- Weakening of the... 

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. ... 

Extended Huckel provides the approximate shape and energy ordering of molecular orbitals. It also yields the approximate form of an electron density map. This is the only requirement for many qualitative applications of quantum mechanics calculations, such as Frontier Orbital estimates of chemical reactivity (see Frontier Molecular Orbitals on page 141). 

CNDO/2 and extended Huckel calculations 74> of 13 (X = S) revealed a small difference in energy between the planar and nonplanar structure, both with bond alternation. These results can be translated into the valence bond structure corresponding to a cyclic thioether. 

With reference to the octahedral energy level diagrams for oxy, car-boxy and deoxy hemoglobin derived from extended Huckel calculations and our earlier work on the natural heme complexes, the results of this study can be discussed. 

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... 

A straightforward orbital correlation, made in Fig. 12, indicates that the thermal process is favored over any excited state process. This has been supported by an extended Huckel calculation (Hoffmann and Woodward, 1965a). An orbital correlation and energy level diagram have also been given for 1,3-dipolar cycloadditions (Eckell et al., 1967), e.g. (26). 

FIG. 1. Potential-energy curves calculated for Ag2 curve A-extended Huckel (EH) calculations, K = 1.3 curve B-EH, Eq. (8) curve C-complete neglect of differential overlap. 

Both combinations of alkyne n orbitals find filled dir orbital symmetry matches in these d4 complexes. Extended Huckel calculations on Mo(HC=CH)2(S2CNH2)2 indicate a large HOMO-LUMO gap of 1.62 eV. These octahedral complexes have proved to be quite robust and resist exchange and substitution reactions in accord with a substantial frontier orbital energy gap (153). 

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. 

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