Extended Hiickel MO calculations

The circular dichroism curves for a variety of penicillanic acid derivatives have been published and discussed (B-77MI51100) and have been used to support extended Hiickel MO calculations (77T711). 

Voltametric studies (34, 37) have revealed the electron transfers Mo(R2c/fc)4 Mo(R2C fc)4° MoCRjdfc) Mo(R2half wave potentials for the processes 0 +1 and + - + 2 depend upon the nature of R. The dependency can be described by the Taft relation =p2a where a is the Taft constant for the N-bonded substituents R. The rather low values of p indicate that the redox orbitals have mainly a metal character, the mixing of these orbitals with those of the ligands is rather small. This conclusion is in accordance with the interpretation of the electronic spectra and the results of Extended Hiickel MO calculations (37). Only the compound with R = Ph does not fit into the Taft relation, the Ej 2 value is lower than... 

The electron transfer Au(R2voltametric measurements 163). The half-wave potentials of the quasi-reversible process depends on the substituent R according to the Taft relation, as was described for Mo, W and Mn 37). The value of p decreases in the series Au > Mn > Mo = W, which indicates that in this sequence the mixing of ligand orbitals into the redox orbital decreases. The dominant ligand character of the unpaired electron MO in Au(R2dtc)2 relative to those in copper and silver compounds is found from Extended Hiickel MO calculations, as will be discussed later on. 

An extended Hiickel MO calculation supports the assumptions made in the above analysis in that the three t2g orbitals are indeed close together in energy and remain nearly nonbonding metal-based d-orbitals. The detailed agreement is less satisfactory the SOMO is predicted to be primarily dx2 y2 with a small dxz admixture (hybrid 6 of Table 4.13), a result that can be ruled out from our analysis of the ESR results. The EHMO overlap matrix based on the X-ray structure suggests that the molecule is much closer to C2 symmetry than to Cs. If we accept that conclusion, then dxzjdxy hybridization is less likely than dyjdxy, as we tacitly assumed above. 

The chelation of the Cys(l)-X-Y-Cys(2) fragment induces a distortion from D2d symmetry. We examined the variation of the overlap population of Fe-S bond with various Fe-S(C) torsion angles by extended Hiickel MO calculations (22). Restriction of Fe-S torsion angle in Cys(2) due to the preferable conformation of the chelating peptide leads to a C2 distortion of the Fe(in) active site. This distortion was found to increase the overlap population of the corresponding Fe-S bond and thus to cause the positive shift of the redox potential. 

Extended Hiickel MO calculations on Ti(=CH2)L2, where L = H, Cl, Cp, have shown that the completely planar molecule is easily distorted into a flatfish pyramid with Ti at the... 

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. 

Empirical force-field (EFF) calculations failed to predict the A-ring chair conformation of lanost-8-en-3-one, which was indicated by n.m.r. studies. Combined EFF-extended Hiickel MO calculations were successful, however, giving a preference of 1.14 kcal mol over the boat form, although quantitative accuracy is not claimed for the present method of calculation. A treatment of alcohol conformations by molecular mechanics may have applications in predicting the preferred conformations of hydroxy-steroids. 

Compounds such as hypercloso-2 and 12 have provided an entry into the long debated question of the relationship between the structures shown in Fig. 3 and the bicapped square antiprisinatic structure for 10-vertex clusters.23 Extended Hiickel MO calculations on a model system showed that the bicapped square antiprismatic structure is preferred for the 2n+2 skeletal electron system (closo) while the C3v structure was preferred for 2n electron systems (hypercloso).23 25 Thus, the discussion has revolved around the description of compounds 2 and 12 as either hypercloso 2n electron systems23 or as isomers of closo (isocloso) 2n+2 electron systems which apparently violate Wade s rules.24b While simple redox processes would be thought to interconvert these structures, this has not been experimentally shown for underivatized species. As shown in Figure 4, we have now observed, however, the reversible and... 

The latter mechanism is preferred. The intermediate is believed to be the lowest n, a triplet state of the episulfide. This, from extended Hiickel MO calculations has been estimated to lie about 30-40 kcal.mole above the ground state for ethylene episulfide, in close agreement with the experimental activation energy. The decomposition to cis and tra j-2-butene is stereospecific. Its reverse. 

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