Extended Huckel calculations

An extended Huckel calculation is a simple means for modeling the valence orbitals based on the orbital overlaps and experimental electron affinities and ionization potentials. In some of the physics literature, this is referred to as a tight binding calculation. Orbital overlaps can be obtained from a simplified single STO representation based on the atomic radius. The advantage of extended Huckel calculations over Huckel calculations is that they model all the valence orbitals. 

AuCN has a similar structure to AgCN and likewise dissolves in excess cyanide to form Au(CN)J this is important in the extraction of gold. It has been characterized as various salts (Tl, K, Bu4N, Cs) with Au-C 1.964A (Bu4N salt [91]). The thallium salt has short Au-Au (3.10A) and Au-Tl (3.50 A) interactions extended-Huckel calculations indicate the importance of relativistic effects in these covalent interactions. Isocyanides form stable complexes ... 

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. 

Fig. 10. Schemes of MO s for technetium compounds according to the results of extended Huckel calculations [58]...

The interactions between the [W(calix)] metal fragment and the ethylene moiety has been analyzed using the extended Huckel calculations (see Chart 4).22b As supported by the structural parameters of 31, the dxz is no longer available for -it donation from the calixarene oxygens in the xz plane, thus explaining the strong C2v distortion of the [ [ Bu -cal ix 14]-(0)4 W] unit observed in the X-ray structure of 31, with the two WO bonds in the metal-ethylene plane ca. 0.2 A longer than... 

The dipolar parts of the analyzed hfs tensors have been compared with calculated values obtained from first order expressions of the electron-nuclear dipole interaction (5.3)57. The coefficients of the atomic orbitals used in this computation, which considers all two- and three-center contributions, are obtained from an extended Huckel calculation (ethyl groups replaced by protons). It has been found that almost 100% of the unpaired electron is located on the CuS4 fragment so that the replacement of the ethyl groups by protons is of minor importance for the calculation of the atomic orbital coefficients. The experimental and theoretical hfs data, summarized in Table 8, are found to... 

A. M. Schaffer and M. Gouterman, Porphyrins XXV. Extended Huckel calculations on location and specUal effects of free base protons, Theoret. Chim. Acta 25, 62-82 (1972). 

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 CH2 group of cycloproparenes is relatively acidic. Extended HUckel calculations predict that the benzocyclopropenyl anion 294 should be a resonance-stabilized species, contrary to the cyclopropenyl anion 295 or cyclohepta-trienide (296), which are at least potentially antiaromatic. This prediction has been experimentally verified benzocyclopropene (1) may be deprotonated with BuLi, and the intermediate anion 294 has been trapped with trimethylsilane to afford 236. From the rate of hydrolysis of 236, the pX of 1 has been estimated to 36, i.e., some 5 units below that of toluene (pXj = 41). Theoretical calculations (STO-3G) give a pK of 33 for 1. Metallation at the CHj group of cycloproparenes is the key step for the synthesis of alkylidenecycloproparenes (see above) however, it should be noted that so far, no benzocyclopropenyl anions have... 

An extended Huckel calculation of the quadrupole splitting in Fe(R2-Dtc)2Cl and [Fe(R2Dtc)2] 2 has been reported (384). The abnormally large... 

Table 7.1 Atomic orbital parameters used in extended Huckel calculations. Single zeta STO functions are used for B and C and double zeta STO functions are used for the transition metals...

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). 

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