Carbon valence electron density

Valence electron density for the diamond structures of carbon and silicon. (Figure redrawn from Cohen M L i. Predicting New Solids and Superconductors. Science 234 549-553.)... 

In an effort to better understand the differences observed upon substitution in carvone possible changes in valence electron density produced by inductive effects, and so on, were investigated [38, 52]. A particularly pertinent way to probe for this in the case of core ionizations is by examining shifts in the core electron-binding energies (CEBEs). These respond directly to increase or decrease in valence electron density at the relevant site. The CEBEs were therefore calculated for the C=0 C 1 orbital, and also the asymmetric carbon atom, using Chong s AEa s method [75-77] with a relativistic correction [78]. 

With respect to the thermodynamic stability of metal clusters, there is a plethora of results which support the spherical Jellium model for the alkalis as well as for other metals, like copper. This appears to be the case for cluster reactivity, at least for etching reactions, where electronic structure dominates reactivity and minor anomalies are attributable to geometric influence. These cases, however, illustrate a situation where significant addition or diminution of valence electron density occurs via loss or gain of metal atoms. A small molecule, like carbon monoxide,... 

The carbides with the NaCl structure may be considered to consist of alternating layers of metal atoms and layers of semiconductor atoms where the planes are octahedral ones of the cubic symmetry system. (Figure 10.1). In TiC, for example, the carbon atoms lie 3.06A apart which is about twice the covalent bond length of 1.54 A, so the carbon atoms are not covalently bonded, but they may transfer some charge to the metal layers, and they do increase the valence electron density. 

A general equation can be derived that describes the variation in direction of the valence electron density about the nucleus. The distortion from sphericity caused by valence electrons and lone-pair electrons is approximated by this equation, which includes a population parameter, a radial size function, and a spherical harmonic function, equivalent to various lobes (multipoles). In the analysis the core electron density of each atom is assigned a fixed quantity. For example, carbon has 2 core electrons and 4 valence electrons. Hydrogen has no core electrons but 1 valence electron. Experimental X-ray diffraction data are used to deri e the parameters that correspond to this function. The model is now more complicated, but gives a better representation of the true electron density (or so we would like to think). This method is useful for showing lone pair directionalities, and bent bonds in strained molecules. Since a larger number of diffraction data are included, the geometry of the molecular structure is probably better determined. 

It is also found that a significant degree of interatomic charge transfer is accompanied by a polarization of the valence electron densities of the atoms in the opposite direction, due to the electric field created by the charge transfer. This affects the measured dipole moments. For example, CO has a nearzero dipole moment because of the very significant polarizations of the atomic densities, particularly that of the carbon atom, which oppose the pronounced charge transfer moment. 

In the vincinity of the planar tetracoordinate carbon atom three maxima of EDD appear (Fig. 2. results for compound II are comparable). The valence electron density associated with the maximum located inside the triangle described by Zr, the carbon atom itself and A1 indicates a 3-center-2-electron-type of bonding between these atoms. 

Figure 19 Total valence electron density, p(r), in the [100] direction from the carbon atom C to the metal atom M (M = Ti, Zr, Nb) and in the [110] direction between the metal atoms (full curves) and the carbon atoms (broken curves). (From Ref. 4. Reproduced with the permission of Critical Reviews in Solid State and Materials Sciences, CRC Press LLC., Critical Reviews in Solid State and Materials Scienees, CRC Press LLC.)...

Figure Al.3.22. Spatial distributions or charge densities for carbon and silicon crystals in the diamond structure. The density is only for the valence electrons the core electrons are omitted. This charge density is from an ab initio pseudopotential calculation [27].

The chemical shift is related to the part of the electron density contributed by the valence electrons, ft is a natural extension, therefore, to try to relate changes of chemical shift due to neighbouring atoms to the electronegativities of those atoms. A good illustration of this is provided by the X-ray photoelectron carbon Is spectmm of ethyltrifluoroacetate, CF3COOCH2CH3, in Figure 8.14, obtained with AlXa ionizing radiation which was narrowed with a monochromator. 

It is apparent from simple valence bond considerations as well as from calculations of rr-electron density, " that isoindoles should be most susceptible to electrophilic attack at carbon 1. This preference is most clearly evident when the intermediate cations (85-87) from electrophilic attack (by A+) at positions 1, 4, and 5 are considered. The benzenoid resonance of 85 is the decisive factor in favoring this intermediate over its competitors. 

Methane has four pairs of valence electrons, each shared in a chemical bond between the carbon atom and one of the hydrogen atoms. The electron density in each C—H bond is concentrated between the two nuclei. At the same time, methane s four pairs of bonding electrons repel one another. Electron-electron repulsion in methane is minimized by keeping the four C—bonds as far apart as possible. 

A similar explanation can be given for the larger Si-O-Si bond angles as compared to C-O-C. Electron density is given over from the oxygen atom into the valence shells of the silicon atoms, but not of the carbon atoms, in the sense of the resonance formulas ... 

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