Hiickel calculations fragments

Hiickel calculations place the largest negative charge on the 1,2 and 5,6 positions ( — 0.28e"). The nearest nodal plane to the 1,2 position is at the center of the five-membered ring while the nearest nodal plane to the 5 and 6 carbon atoms places the lithium atom close to both those carbon atoms, as in the HOMO in the third position 3 listed. The first position is actually observed it seems to be favored in that the second HOMO does not contain a node in position 3 but does for position 1. The N—Li—N plane should permit overlap between the 1-11 and 2-9 positions for the two N—Li—N fragments on either side of the five-membered plane. The angle of the N—Li—N plane with respect to the twofold axis of the molecule should therefore be between a maximum of 36° and a minimum of 10°. The observed value is 15°. 

The linear combinations of metal orbitals shown in 2b and 2d have strong metal-metal antibonding character. Such interactions can be found in the molecular orbital diagram depicting the interaction of an Fe3(CO)io fragment with two bridging carbonyls (Fig. 1) which was derived from extended Hiickel calculations. 

In their extended Hiickel calculations, Thorn and Hoffmann treat the carbon portion of the metallacycle as a monoanionic ligand (C5H5 ). It serves as a 4e donor to the metal, and these electrons form the two M—C a-bonds. In the Thorn/Hoffmann analysis, the metallacycle possesses six -electrons and therefore obeys the Hiickel 4n + 2 rule. Four electrons reside in relatively low-lying carbon fragment. Tr-orbitals In and 2ti, Chart 2). ° The remaining two jr-electrons are contributed by the filled metal Axz orbital (or hybrid of appropriate symmetiy), which interacts in a back-bonding fashion with the empty Zti carbon fragment orbital (Chart 2). 

In the mass spectrometer, / -lactams (6) prefer fragmentation to isocyanate and olefin rather than to imine and keten, and extended Hiickel calculations confirm that the preferred path is the one of lowest energy. The mass spectra of a variety of azetidines have also been investigated. ... 

Seeking to understand the interactions between metal centers in complexes such as [(McjSOCH Cu], Mehrotra and Hoffmann carried out extended Hiickel calculations on the fragments [Cu2], [Cuj] ", and [Cu, ], and examined the effects of bridging ligands on the resulting molecular orbitals [18]. 

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. 

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