Molecular orbital calculations electronic structures

With the use of the DV-Xa molecular orbital method, electronic structure calculations have been performed to investigate the impurity effect on material properties. Firstly, calculations were done for F atoms substituted for 0 (oxygen) atoms in copper oxide superconductors. It was found that the population of the atomic orbitals of F atoms is small in HOMO (highest occupied molecular orbital) and a small fraction of charge carriers enters the impurity sites. The F impurities are therefore expected to be effective for pinning magnetic flux lines in Cu oxide superconductors. 

Molecular orbitals were one of the first molecular features that could be visualized with simple graphical hardware. The reason for this early representation is found in the complex theory of quantum chemistry. Basically, a structure is more attractive and easier to understand when orbitals are displayed, rather than numerical orbital coefficients. The molecular orbitals, calculated by semi-empirical or ab initio quantum mechanical methods, are represented by isosurfaces, corresponding to the electron density surfeces Figure 2-125a). 

The simplest molecular orbital method to use, and the one involving the most drastic approximations and assumptions, is the Huckel method. One str ength of the Huckel method is that it provides a semiquantitative theoretical treatment of ground-state energies, bond orders, electron densities, and free valences that appeals to the pictorial sense of molecular structure and reactive affinity that most chemists use in their everyday work. Although one rarely sees Huckel calculations in the resear ch literature anymore, they introduce the reader to many of the concepts and much of the nomenclature used in more rigorous molecular orbital calculations. 

Recently H. L. Jones and D. L. Beveridge have presented molecular orbital calculations on the electronic structure of 2,3-pyrid5me explaining the exclusive formation of 2-aminopyridine from this intermediate [Tetrahedron Letters No. 24, 1577 (1964)]. 

In a series of studies of the spectroscopy and photochemistry of nickel(O) -a-diimine complexes, the structural differences among the complexes NiL2 and Ni(CO)2L (L Q-diimine) have been examined by means of molecular orbital calculations and electronic absorption Raman resonance studies.2471, 472 Summing up earlier work, the noninnocence of a-diimine ligands with a flat — N=C—C=N— skeleton in low-valent Ni chemistry and the course of substitution reactions of Ni° complexes with 1,4-diaza-1,3-dienes or a,a -bipyridine have been reviewed.2473... 

Doms, L., H. J. Geise, C. Van Alsenoy, L. Van den Enden, and L. Schafer. 1985. The Molecular Orbital Constrained Electron Diffraction (MOCED) Structural Model of Quadricyclane Determined by Electron Diffraction Combined with Ab Initio Calculations of Potential and Geometrical Parameters. J. Mol. Struct. 129, 299-314. 

Here, the directions are defined in Fig. 6. In natural N02 the 170 content is quite small so the only observable hyperfine structure will be due to HN, which has a nuclear spin of one. Recent experiments, however, have been carried out using N02 enriched in 170 (34-) Molecular orbital calculations indicate that c2 is reasonably large, i.e., the unpaired electron is expected to have considerable nitrogen p2 character. 

Group 2 complexes are formally electron deficient and conformationally floppy only small energies (often only 1-2 kcal mol-1) are required to alter their geometries by large amounts (e.g., bond angles by 20° or more). In such cases, the inclusion of electron-correlation effects becomes critical to an accurate description of the molecules structures. Both HF/MP2 and density functional theory (DFT) methods have been applied to organoalkaline earth compounds. DFT approaches, which implicitly incorporate electron correlation in a computationally efficient form, are generally the more widely used. Molecular orbital calculations that successfully reproduce bent... 

Molecular orbital calculations have also been carried out to describe the bonding by considering electron delocalization over the entire structure. 

ESR studies have been used extensively to characterize S-N radicals that are persistent in solution at room temperature.32 Typical radicals are cyclic C-N-S systems in which the unpaired electron occupies a delocalized re-orbital. In conjunction with molecular orbital calculations, ESR spectra can provide unique information about the electronic structures of these ring systems. 

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. 

---