Chemical bonding electron density plots

From the plots of the rr-charge densities of the carbon atoms of the purine and pyrimidine residues of nucleosides versus chemical shift values a gross correlation is obvious. Different calculations of the -densities, however, yield different plots. The n-electron densities calculated by Hoffmann and Ladik [763] by the HMO method or those calculated by Veillard and Pullman [764] by the Pariser-Parr SCF method show the strongest deviation for C-4, most probably because amplified C —N bond polarization terms have to be used for the calculations. 

The 13C-chemical shifts of tropylium salts 218, 329, and 333 (Table XX) were plotted against the electron densities calculated by the HMO method. Most points fall on straight lines, except those for oxygen-bonded carbons (C-2, C-8a), which exhibit more deshielding than expected from the calculated electron densities. 

In Fig. Ila e(f) is plotted for the two upper valence bands. Distinct differences in the electron distribution for these electronic bands compared to the ones of the lower valence bands (Fig. 11b) can be found, although a gradient in the charge density from the nonalkali to the alkali sublattice still exists. For the electronic states of the upper valence bands, however, the chemical bond possesses a metallic-like component. This metallic... 

The interaction of a Pd4 (C4v) cluster with the oxide surface was analyzed in more detail with the help of electron density difference plots and other theoretical tools, such as population analysis, core level shifts as well as induced and dynamic dipole moments [175]. Three interaction mechanisms were found to contribute to different extent metal polarization with the subsequent electrostatic attraction, Pauli repulsion, and covalent orbital interactions. Electrostatic interactions make up a sizeable fraction of the adhesion energy the polarization of the metal adsorbate by the surface electric field provides an important bonding mechanism. For the adsorption of Pd on-top or in the vicinity of the surface Mg " cations this electrostatic interaction accounts for almost the entire adsorption energy, albeit counteracted by Pauli repulsion. For adsorption on-top 0 , on the other hand, mixing of adsorbate and substrate orbitals becomes noticeable. This hybridization or covalent bonding at the interface with the oxide anions is complemented by electrostatic polarization. Further work is required to establish in a more quantitative way the relative importance of electrostatic and chemical bonding contributions. However, in line with our other studies of... 

We hope we have demonstrated that the usual delocalized molecular orbitals obtained from self-consistent-field calculations are about as readily understood and interpretable in chemical terms as are the localized orbitals which some people have taken great pains to derive (32) from these delocalized ones. We have demonstrated also through the plots of electron density how these delocalized molecular orbitals are made up from atomic orbitals and how they may be discussed in terms of their atomic-orbital composition. This is particularly interesting for second-row atoms such as sulfur because of the diffuse nature of the parts of the valence-shell atomic orbitals which are involved in bonding. 

Figure 10 (a) Molecular structure of 118 shown with displacement ellipsoids of 50% probability (b) deformation density plot (positive contours as solid lines, negative contours dashed lines contour interval 0.05 eA (c) total electron density (exponential contours) with Co-C, Fe-C, and C-0 bond paths shown (d) Laplacian of the electron density, V, (exponential contours negative contours as solid lines, positive contours as dashed lines). Reproduced from Macchi, P. Garlaschelli, L. Sironi, A. J. Am. Chem. Soc. 2002, 124, 14173-14184 with permission of the American Chemical Society. 

The different behavior of tertiary and quaternary carbon atoms seems to be due to either the complete neglect of overlap in these calculations or to polarization effects of the carbon-nitrogen bonds, Similar results are obtained for a series of 5-halouracils by plotting the 13C NMR chemical shifs versus 7r-electron charge densities calculated by the extended Hiickel theory [756], Though for several nitrogen heterocycles a better correla-... 

When a cationic center becomes sufficiently electrophilic, it may draw on electrons from neighboring Jt- and o-bonds and thus delocalize positive charge density. The onset of participation of n- and o-bonds can be detected as a departure from linearity in a Hammett-type plot as the electron-withdrawing ability of the aryl substituent increases. In stable ion studies, 13C NMR chemical shifts are generally used as a structural probe reflecting the charge density at the cation center (in closely related homologous cations, other factors that may affect chemical shift may be assumed constant). 

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