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Administrator<br />

Density surface is calculated from scratch for each<br />

molecule. The Total Charge Density is generally<br />

more accurate than the Space Filling display.<br />

For Total Charge Density surfaces, the properties<br />

available for mapping are Molecular Orbital, Spin<br />

Density, Electrostatic Potential, and Partial<br />

Charges. The color scale uses red for the highest<br />

magnitude and blue for the lowest magnitude of the<br />

property. Neutral is white.<br />

You can choose the orbital to map onto the surface<br />

with the Molecular Orbital tool on the Surfaces<br />

menu. The orbital number appears in parentheses<br />

in the HOMO/LUMO submenu.<br />

Total Spin Density<br />

The total spin density surface describes the<br />

difference in densities between spin-up and<br />

spin-down electrons in any given region of a<br />

molecule’s space. The larger the difference in a<br />

given region, the more that region approximates an<br />

unpaired electron. The relative predominance of<br />

spin-up or spin-down electrons in regions of the<br />

total spin density surface can be visualized by color<br />

when total spin density is mapped onto another<br />

surface (total charge density). Entirely spin-up<br />

(positive value) electrons are red, entirely<br />

spin-down (negative) blue, and paired electrons<br />

(neutral) are white.<br />

The total spin density surface is used to examine the<br />

unpaired electrons of a molecule. The surface exists<br />

only where unpaired electrons are present. Viewing<br />

the total spin density surface requires that both Spin<br />

Density and Molecular Surfaces are calculated by<br />

MOPAC or Gaussian using an Open Shell<br />

Wavefunction.<br />

Molecular<br />

Electrostatic Potential<br />

The molecular electrostatic potential (MEP)<br />

represents the attraction or repulsion between a<br />

molecule and a proton. Attraction is represented by<br />

negative values and repulsion is indicated by<br />

positive values. Experimental MEP values can be<br />

obtained by X-ray diffraction or electron diffraction<br />

techniques, and provide insight into which regions<br />

of a molecule are more susceptible to electrophilic<br />

or nucleophilic attack. You can visualize the relative<br />

MEP values by color when MEP is mapped onto<br />

another surface (total charge density). The most<br />

positive MEP value is red, the most negative blue,<br />

and neutral is white.<br />

Molecular Orbitals<br />

Molecular orbital (MO) surfaces visually represent<br />

the various stable electron distributions of a<br />

molecule. According to frontier orbital theory, the<br />

shapes and symmetries of the highest-occupied and<br />

lowest-unoccupied molecular orbitals (HOMO and<br />

LUMO) are crucial in predicting the reactivity of a<br />

species and the stereochemical and regiochemical<br />

outcome of a chemical reaction.<br />

To set the molecular orbital being displayed:<br />

• From the Surfaces menu, point to Molecular<br />

Orbital to see the HOMO/LUMO options.<br />

Select the orbital.<br />

You can specify the isocontour value for any<br />

computed MO surface using the Isocontour tool on<br />

the Surfaces menu. The default isocontour value for<br />

a newly computed surface is the value you last<br />

98 •Displaying Models <strong>CambridgeSoft</strong><br />

Molecular Surface Displays

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