Repulsion between electrons

Racah parameters The parameters used to express quantitatively the inter-electronic repulsion between the various energy levels of an atom. Generally expressed as B and C. The ratios between B in a compound and B in the free ion give a measure of the nephelauxetic effect. ... 

Ihc complete neglect of differential overlap (CNDO) approach of Pople, Santry and Segal u as the first method to implement the zero-differential overlap approximation in a practical fashion [Pople et al. 1965]. To overcome the problems of rotational invariance, the two-clectron integrals (/c/c AA), where fi and A are on different atoms A and B, were set equal to. 1 parameter which depends only on the nature of the atoms A and B and the ii ilcniuclear distance, and not on the type of orbital. The parameter can be considered 1.0 be the average electrostatic repulsion between an electron on atom A and an electron on atom B. When both atomic orbitals are on the same atom the parameter is written , A tiiid represents the average electron-electron repulsion between two electrons on an aiom A. 

The formalism that we have set up to describe chain flexibility readily lends itself to the problem of hindered rotation. Figure 1.8a shows a sawhorse representation of an ethane molecule in which the angle of rotation around the bond is designated by [Pg.57]

The next step came in the 1950s, with more serious attempts to include formally the effect of electron repulsion between the valence electrons. First came the jT-electron models associated with the name of Pople, and with Pariser and Parr. You might like to read the synopses of their first papers. 

Eclipsing strain (Section 3.6) The strain energy- in a molecule caused by electron repulsions between eclipsed bonds. Fxlipsing strain is also called torsional strain. 

The ligands must be located at the comers of an octahedron to minimize electron-electron repulsion between the electron pairs. To give the greatest stability, the two lone pairs must be as far apart as possible, because lone pairs take up more space than bonding pairs. Placing the lone pairs at opposite ends of one axis, 180° apart, minimizes their mutual repulsion. This leaves the four fluorine atoms in a square plane around xenon ... 

In a seminal work, Koga and Morokuma suggested that the high activation energy of the Bergman cyclization is due to the strong electron repulsion between the... 

Recently, we analyzed the role of electron repulsion relative to bond breaking and antiaromaticity effects on a quantitative basis using Natural Bond Orbital (NBO) analysis.24 Two other destabilizing factors were considered at the initial stage of the cyclization in addition to four-electron repulsion between the filled in-plane acetylenic re-orbitals - distortion/breaking of the acetylenic bonds as a result of their bending, and the fact that, at a distance of ca. 3 A, the in-plane re-orbitals become parallel and reach a geometry that resembles the antiaromatic TS of the symmetry forbidden [2S + 2S] cycloaddition (vide infra). 

In general, type II compounds show greater NO-releasing ability than type I N-nitrosamines. This can be explained by the electronic repulsion between the carbonyl oxygen and nitroso oxygen, or the attraction of the lone-pair electrons at nitrogen, by the carbonyl group both features weaken the N-NO bond. 

The isomerization, itself, originates from the a complex (B in Figure 3). However the total activation energy depends critically on the relative energy of A and B (Figure 3). An alkyne C=C triple bond binds more efficiently to a transition metal complex than a o C-H bond since the % C-C orbital is a better electron-donor and the 71 C-C orbital a better electron acceptor than the a and a C-H orbitals, respectively. However, the difference in energy between the two isomers is relatively low for a d6 metal center because four-electron repulsion between an occupied metal d orbital and the other n C-C orbital destabilizes the alkyne complex. This contributes to facilitate the transformation for the Ru11 system studied by Wakatsuki et al. 

In 1998, Yang and coworkers reported a series of (7 )-carvone derived ketones (63) containing a quaternary center at and various substituents at (Fig. 22) [119]. The ees of fran -stilbene oxide varied with different para and meta substituents when 63b was used as the catalyst. The major contribution for the observed ee difference is from the n-n electronic repulsion between the Cl atom of the catalyst and the phenyl group of the substrate. The substitution at also influences the epoxidation transition state via an electrostatic interaction between the polarized C -X bond and the phenyl ring on franx-stilbene (Table 6, entries 3-7, 10-14). In 2000, Solladie-Cavallo and coworkers reported a series of fluorinated carbocyclic ketones... 

The diastereoselectivity in the ene reaction of O2 with chiral alkenes bearing a stereogenic centre at the a-position with respect to the double bond has been extensively studied. Chiral alkenes which bear a substituent on the asymmetric carbon atom other than the hydroxy or amine functionality afford predominately erythro allylic hydroperoxides. The erythro selectivity was attributed to steric and electronic repulsions between... 

It has been empirically known that the energies of the lowest excited state of tetrahedrally coordinated metals decrease in the order Cr + < Mn + < Fe ". As in the case of 3cf elements, this tendency has been considered to originate from the difference in covalency, which reduced two-electron repulsion between the electrons occupying 3d orbitals. Recently this question was treated using first-principles electronic-structure calculation (Ishii et al. 2002). The same tendencies were found as for the 3d ions. Distance dependent multiplet-energy diagrams for these elements have been obtained (Fig. 5.34), which enable us to envisage the typical shapes of the possible emissions. As in... 

The addition ratio in 68 (86 14) is very different from the ratio of addition of the amide ion to C-3 and C-4 in 3,4-didehydroquinoline (63) (45 55). The less favored addition to C-4 in 68 is probably due to a hindered attack of the amide ion at C-4 because of the Coulomb electron repulsion between the negatively charged incoming amide ion and the sp2 electron pair of the nitrogen at position 5. 

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