Symmetry, and molecular orbital calculations

NLFe" ions have been isolated in a number of salts, for both nickel(in) and nickel(lV). Selected examples of these salts are shown in Table 118. NiF6 is diamagnetic and the electronic spectra have been successfully interpreted in O symmetry (ground state Several spectroscopic and mametic studies and molecular orbital calculations have been performed on NiFi" The complex is low-spin with as... 

Semi-empirical molecular orbital calculations have been carried out on the model phosphorane HaPiCHa. Besides the expected transfer of charge, the inclusion of the phosphorus 2>d orbitals showed a significant hyperconjugative interaction between the CHg orbitals and a 2>d orbital of appropriate symmetry on phosphorus. Calculations on cyclopropylidene-phosphorane revealed a similar interaction between the Walsh orbitals of the ring and an in-plane phosphorus 2>d orbital. 

The 327-670 GHz EPR spectra of canthaxanthin radical cation were resolved into two principal components of the g-tensor (Konovalova et al. 1999). Spectral simulations indicated this to be the result of g-anisotropy where gn=2.0032 and gi=2.0023. This type of g-tensor is consistent with the theory for polyacene rc-radical cations (Stone 1964), which states that the difference gxx gyy decreases with increasing chain length. When gxx-gyy approaches zero, the g-tensor becomes cylindrically symmetrical with gxx=gyy=g and gzz=gn. The cylindrical symmetry for the all-trans carotenoids is not surprising because these molecules are long straight chain polyenes. This also demonstrates that the symmetrical unresolved EPR line at 9 GHz is due to a carotenoid Jt-radical cation with electron density distributed throughout the whole chain of double bonds as predicted by RHF-INDO/SP molecular orbital calculations. The lack of temperature... 

Heteroindacenes have been prepared and studied by Hafner and co-workers.198 199 The syntheses of 1,3,5,7-tetra-te/t-butyl-4-azaindacene, its AA-oxide, and l,3,5,7-tetra-tot-butyl-4-phospha-s-indacenes have been recently reported (Scheme 66).200 The 12-jt-electron delocalized systems have been studied by dynamic NMR and X-ray and were subjected to molecular orbital calculations, and there is strong evidence of electron delocalization. However, X-ray crystallographic data for 4-phospha-s-indacene 164 and the 4-7V-oxide 164 show that there is a dual orientation in the crystal this disorder with two different orientations of the molecule does not allow for conclusions regarding bond lengths or delocalization, and the mediated structures show a D2h symmetry rather than C2h with localized double bonds. 

Two groups have studied the bonding in pentadienyl-metal-tricar-bonyl complexes (119, 238) and are agreed that effective overlap between the pentadienyl nonbonding orbital and an orbital of suitable symmetry on the metal (Fig. 17) makes a major contribution to the stability of these complexes. However, the two types of molecular orbital calculation [one an extended Hiickel (119) and the other a parameter-free approximate Hartree-Fock calculation (255)] disagree about the precise ordering of energy levels in this type of complex. 

Electron centre, C033 (C3 symmetry) A pyramidal AB3-type molecular ion with 25 electrons is stabilized by a trivalent impurity such as Y3+ at room temperature and forms a complex of Y3+ - C033-. The shift of -factors from the free electron value ge = 2.0023 and the hyperfine (hfs) constant, A can be estimated by molecular orbital calculation of AB325 -type molecule ... 

The valence orbitals taken for a molecular-orbital calculation of a transition metal complex are the nd, (n + l)s, and (n + l)p metal orbitals and appropriate a and n functions of the ligands. Many of these valence orbitals are not individually basis functions for an irreducible representation in the symmetry under consideration. Symmetry basis functions transforming properly must be constructed, by methods analogous to those used throughout this volume. The results for a number of important symmetries are tabulated in this volume in various places, as follows ... 

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