Electron density space-filling model

The following picture shows electron density surfaces for ammonia, trimethylamine and quinuclidine. The surfaces are qualitatively very similar to the space-filling models. 

Both space-filling and electron density models yield similar molecular volumes, and both show the obvious differences in overall size. Because the electron density surfaces provide no discernible boundaries between atoms (and employ no colors to highlight these boundaries), the surfaces may appear to be less informative than space-filling models in helping to decide to what extent a particular atom is exposed . This weakness raises an important point, however. Electrons are associated with a molecule as a whole and not with individual atoms. The space-filling representation of a molecule in terms of discernible atoms does not reflect reality, but rather is an artifact of the model. The electron density surface is more accurate in that it shows a single electron cloud for the entire molecule. 

The connection between a molecule s electron density surface, an electrostatic potential surface, and the molecule s electrostatic potential map can be illustrated for benzene. The electron density surface defines molecular shape and size. It performs the same function as a conventional space-filling model by indicating how close two benzenes can get in a liquid or crystalline state. 

Examine space-filling models and electron density surfaces for alkene A and alkene B. For each, which face of the double bond is less hindered Which atoms cause steric hindrance of the alkene Is this reaction controlled by steric hindrance If so, explain which step(s) in the catal3 ic mechanism would be most affected. 

Examine the spin density surface for BHT radical. Is the unpaired electron localized or delocalized Examine BHT radical as a space-filling model. What effect do the bulky tert-butyl groups have on the chemistry of the species (Hint BHT radical does not readily add to alkenes or abstract hydrogens from other molecules.)... 

A low electron density surface roughly shows the outline of a molecule s electron cloud. This surface gives information about molecular shape and volume, and usually looks the same as a van der Waals or space-filling model of the molecule. 

Bond and size surfaces offer some significant advantages over conventional skeletal and space-filling models. Most important, bond surfaces may be applied to elucidate bonding and not only to portray known bonding. For example, the bond surface for diborane clearly shows a molecule with very little electron density concentrated between the two borons. 

Use SpartanView to compare low," medium, and high electron-density surfaces of dimethyl ether (CHjOCH ). Where is the electron density highest near the atomic nuclei, in the bonding region between the nuclei, or far from the nuclei Which electron-density surface most closely resembles the molecule s space-filling model ... 

FIGURE 10.10 (a) Space-filling models, (b) 0.002 e/(ao) electron density isosurfaces, and (c) electrostatic potential energy surfaces for water, ammonia, and methane. 

Isosurfaces of electron density are obtained from the probability density isosurfaces for molecules described in Chapter 6. These are surfaces in three-dimensional space that include all the points at which has a particular value. The value of electron density chosen to define the isosurface is selected by some definite, though arbitrary, criterion. There is broad acceptance of a standard density of 0.002 el ao), where is the Bohr radius. This value is thought to best represent the sizes and shapes of molecules because it corresponds to the van der Waals atomic radii discussed earlier in the context of repulsive forces. These are the same dimensions depicted in space-filling models of molecules. 

Figures 10.10a and b show, respectively, space-filling models and electron density isosurfaces plotted at 0.002 e/(tZo) for water, ammonia, and methane. The electron densities plotted here include all of the electrons in the molecule. They are calculated using state-of-the-art ab initio quantum chemical methods (see discussion in Chapter 6).

Figure 9.11 depicts this fact in three ways a cross-section of a space-filling model an electron density contour map, with lines representing regular increments in electron density and an electron densit > relief map, which portrays the contour map three-dimensionally as peaks of electron density. 

Figure 11 shows a space-filling model of the nonstrained, yet extremely overcrowded, radical PTM-, with its propeller-blade-like pentachlorophenyl groups. It may also be noted that the central trivalent carbon is completely shielded by the six o-chlorines and the three benzene rings. The normally reactive radical site, its central carbon atom where most of the electron spin density resides, is enveloped in a cage of carbons and chlorines, and is unable to approach within the chemical-bonding distance of other molecular species. Furthermore, no molecular distortion, no matter how extensive, would allow any additional medium-sized atom or group to be accommodated on the a-carbon. 

Four models of ethylene The dash-wedge, ball-and-stick, and space-filling models show that the four atoms attached to a carbon-carbon double bond lie in a single plane. The electrostatic potential map shows the electron density (red) above and below the plane that passes through the carbon and hydrogen nuclei. 

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