Localized electron orbital models

The localized molecular orbital model (LMO) (39-41) treats the electrons and nuclei separately. The nuclear contribution is identical to eq. [26] with = Z e, the nuclear charge screened by the irmer shell electrons that are assumed to follow the nuclei. The local units for the contribution from the valence elec-... 

Spherical-domain models of three-center bonds in localized-molecular-orbital models of a nonclassical carbonium ion, B4CI4, and TaeClfJ have been described 49,52) a drawing of a spherical-domain model of the methyl lithium tetramer, (LiCH, is shown in Fig. 31. Large, outer circles represent domains of electron-pairs of C—H bonds. Solid circles represent domains of Li+ ions. Shaded circles represent 4-center lithium-lithium-lithium-carbon bonds — i.e., electron-pair domains that touch, simultaneously, three lithium ions and the kernel of a carbon atom. The... 

Liquefaction the transformation of a gas into a liquid. (18.1) Localized electron (LE) model a model that assumes that a molecule is composed of atoms that are bound together by sharing pairs of electrons using the atomic orbitals of the bound atoms. (13.9)... 

The resonance concept is one way of overcoming some of the limitations of the localized electron pair model, but such cases are treated more naturally by molecular orbital theory, which is not limited to bonds involving two atoms (see Topics C6 and C7). 

The fact that heptacoordinated species in their ground states exMbit pentagonal bipyramidal structures with an unpuckered equatorial plane, cannot be rationalize by VSEPR theory (7,2) in terms of a repelling points on a sphere (POS) model which should result in either a monocapped octahedron or a monocapped trigonal prism. Furthermore, it cannot be explained by conventional bonding schemes involving localized electron orbitals of die central atom to enforce the coplanarity of a central atom and five equatorial ligands. The best explanation to account for this planarity is the... 

When the electron configurations of the elements were worked out, it became clear that the valence electrons of the period 2 elements must be accommodated in just four orbitals, the 2s and the three 2p orbitals. In the localized orbital model it is assumed that each bond can be described by a localized orbital formed by the overlap of one orbital on each of the bonded atoms. According to this model, therefore, a period 2 element can form bonds with at most four ligands so that electron configurations appeared to provide a justification for the octet rule. 

It has also to be remembered that the band model is a theory of the bulk properties of the metal (magnetism, electrical conductivity, specific heat, etc.), whereas chemisorption and catalysis depend upon the formation of bonds between surface metal atoms and the adsorbed species. Hence, modern theories of chemisorption have tended to concentrate on the formation of bonds with localized orbitals on surface metal atoms. Recently, the directional properties of the orbitals emerging at the surface, as discussed by Dowden (102) and Bond (103) on the basis of the Good-enough model, have been used to interpret the chemisorption behavior of different crystal faces (104, 105). A more elaborate theoretical treatment of the chemisorption process by Grimley (106) envisages the formation of a surface compound with localized metal orbitals, and in this case a weak interaction is allowed with the electrons in the metal. 

Scheme la shows the approximate molecular orbital model for the hypervalent X-E-X 3c-4e in EX4, such as SC14. Characters of the three molecular orbitals are bonding (v /i), nonbonding (v /2), and anti-bonding (v /3). Two electrons are in and two in v 2. Electrons in v 2 localize on X of X-E-X and the hypervalent bonds are mainly characterized by v 2. Consequently,... 

Various astoichiometric components (hydrogen, carbon, and others, for example, silicium and aluminum) present may interact with localized and nearly free electrons to differing extents. According to the localized free electron interplay model of metal catalysts developed by Knor 163, 164) the ratio of the two types of electrons may influence the catalytic properties considerably. For example, a subsurface proton attracts nearly free electrons and thus uncovers some localized orbitals. Carbon may interact first with localized electrons 164). This may be one of the reasons why their effects are of opposite character. The collective efforts of catalytic and surface chemists are necessary to bring some clarity to the multitude of problems arising here. 

The truncation procedure for fiill-valence-space and N-electrons-in-N-orbitals SDTQ MCSCF waveflmctions is based on choosing split-localized molecular orbitals as configuration generators since they lead to the greatest number of deadwood configurations that can be deleted. A quite accurate estimation method of identifying the latter has been developed so that the truncation can be performed a priori. The method has been shown to be effective in applications to the molecules HNO, OCO and NCCN where, for instance, the energies of the full SDTQ[N/N] calculations are recovered to better than 1 mh by truncated expansions that require only 11.8%, 10.9% and 6.3%, respectively, of the number of determinants in the full calculations. Similar trends are observed for the FORS 1 model. 

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