Electrostatic potential energy maps

A more sophisticated way to look at charge distribution is the molecular electrostatic potential energy map of the reactant(s). Programs like SPARTAN can compute and display the MEP of a molecule as part of its quantum mechanical repertoire. In an alternate visualization of the MEP, it is common for such graphics programs to display the electron... 

Figure 2 Electrostatic potential energy maps, determined by AMI level calculations using the program CAChe, of (a) 21, (b) 22, (c) 24, and (d) 25. Black regions are of high electron density and whites are areas of low electron density.

The electrostatic potential energy map for a molecule combines information from two different sources into one representation. The size and shape of the molecule come from the spatial distribution of the electron density represented as an isosurface. The electrostatic potential energy that a positive test charge would experience is indicated at each point on that isosurface. Let s construct each of these pieces in turn. 

Section 10.2 on Intermolecular Forces includes an introduction to electrostatic potential energy maps. We define these surfaces very carefully to provide a solid foundation for our students when they encounter these representations in their organic chemistry courses. 

Fig. 1. Electrostatic potential-energy map for HjO in the molecular plane Values are expressed in kcal-mole...

Figure L Molecular topology, atomic numbering, and in-plane molecular electrostatic potential energy maps for a, 9-methylguanine (9-MeG) b, 9-methyladenine (9-MeA) c, 1-methylcytosine (1-MeC) and d, the N 1-deprotonated thymine monoanion (Thy ). Contour levels for the electrostatic potential energy maps are given in kcal/mol as in Ref. 21 and 26.

Fig. 8. Electrostatic molecular potential-energy maps for cytosine in the ring plane (top) and in the plane perpendicular to the ring plane and passing through atoms as indicated (bottom).244...

Electrostatic potential (elpot) maps provide a way to visualize the distribution of electron density in a molecule. Electrostatic potential is defined as the potential energy that a positively charged particle would experience in a molecule s presence. The electrostatic potential is made up of two parts. 

One of the simplest examples of electrostatic catalysis is the acceleration of the 1,5-hydride shift in cyclopentadiene by the influence of Li" " cations. The reaction proceeds via an asymmetric transition state that is 34 kJ mol more stable than the symmetric ground state. It could be shown that this extra stabilization (catalytic rate acceleration) is completely due to the effect of the electrostatically bound cation. Difference in cation complexation energies could be adequately illustrated by molecular electrostatic potential (MEP) maps for the ground and transition states (see Electrostatic Potentials Chemical Applications). The maps are symmetric and asymmetric for the ground and transition states, respectively, and indicate larger cation attraction for the latter. 

Low and/or minimum energy conformations of each sweetener were superimposed to provide optimum steric overlap and electrostatic matching based on electrostatic potential (EP) maps. 

FIGURE 19.3 The free W energies of ionization of ethanol and acetic acid in water. The electrostatic potential maps of ethoxide and acetate ion show the concentration of negative charge in ethoxide versus dispersal of charge in acetate. The color ranges are equal in both models to allow direct comparison. 

Compare electrostatic potential maps for planar and pyramidal forms of 2-methyl-2-propyl anion. For which is the negative charge more delocalized Is this the lower-energy structure For this case, does charge delocalization lead to stabilization Explain. 

Display electrostatic potential maps for both bisected and perpendicular conformers of cyclopropylcarbinyl cation. For which is the charge more delocalized Is the more delocalized cation also the lower-energy cation ... 

What other factors might be responsible for difference in activation energies Compare atomic charges anc electrostatic potential maps for the Sn2 transition states Does the increase in steric crowding lead to enhanced o diminished charge delocalization Explain. How, if at all would this be expected to affect the energy barrier Why ... 

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