Extended Hiickel MO

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. 

To examine the effect of covalency on the QS, the values of the QS of some compounds have been calculated with the aid of the Extended Hiickel MO method (i 82, 61). In these calculations the values for the empirical parameters used were obtained from comparison with EPR experiments (see EPR studies). This method is suitable only for molecules with low symmetry, because effectsoof spin-orbit coupling and thermal mixing have been neglected. 

Since the g-matrix has only three principal values and there are almost always many potentially interacting molecular orbitals, there is rarely sufficient information to interpret a g-matrix with complete confidence. When a well-resolved and reliably assigned optical spectrum is available, the energy differences, E0—Em, are known and can be used in eqn (4.11) to estimate the contribution of the corresponding MOs to the g-matrix. Extended Hiickel MO (EHMO) calculations can be useful (but do not trust EHMO energies ), but one is most commonly reduced to arguments designed to show that the observed g-matrix is consistent with the interpretation placed on the hyperfine matrix. 

The SOMO in these radicals is expected from extended Hiickel MO calculations27-29 to be primarily cobalt 3dyz in character. In the Cs symmetry of the radicals, dyz belongs to the a" representation and d-hybridization is possible only with dxy. Assuming that such hybridization is negligible, the g-matrix... 

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. 

An extended Hiickel MO investigation of 1,4-dihydrodiboranaphthalene double- and triple-decker sandwiches134 (e.g., 109 and 110) supports a description of these as 18 and 30 ve complexes, respectively.1343 l,3-dihydro-l,3-diboranaphthalene complexes of cobalt and iron 111-113 have been synthesized and crystallographically characterized.135 Metal complexes of the 1,4-diboratetralene ligand 114, 115 have also been prepared.136... 

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. 

The electronic overlap populations in all three cases were calculated from the one electron Extended Hiickel MO s. For the photocyclizations of 1,2-difuryl ethylenes very similar results were obtained also from minimal basis set ab-initio wavefunctions l The possibility of obtaining useful reactivity analyses from wave-functions which are easily available even for large systems could prove to be an important practical consideration for further applications of this method. The dependence on Sri sj ill (5) ensures that electronic overlap populations show the desirable physical characteristics for their use as reactivity measures strong falling-off with increasing interatomic distance and proper directional dependence. This last point is of particular significance for bond formation in polyenes. Thus for two C 2 p atomic... 

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. 

Platinum (n)—Group VII and Hydride Donors. SCF—Xa calculations have been made for the ions [MC14]2 (M = Pd or Pt) (see p. 384).13 In addition the extended Hiickel MO method has been employed to calculate activation energies of the halide-... 

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