Negative electrostatic potential

FIGURE 13.5 Isosurface plots, (a) Region of negative electrostatic potential around the water molecule. (A) Region where the Laplacian of the electron density is negative. Both of these plots have been proposed as descriptors of the lone-pair electrons. This example is typical in that the shapes of these regions are similar, but the Laplacian region tends to be closer to the nucleus. 

FIGURE 3 10 Bent bonds in cyclopropane (a) The orbitals involved in carbon-carbon bond formation overlap in a region that is displaced from the internuclear axis (b) The three areas of greatest negative electrostatic potential (red) correspond to those predicted by the bent bond description... 

Nucleophilic atoms can also be identified by inspection of electrostatic potential maps. Reactive sites appeal as negative electrostatic potentials. Examine electrostatic potential maps for trimethylamine, methyl fluoride, and phenol. Identify the most nucleophilic atom in each molecule. Are these the same as you identified above using Lewis structures Are all sides of the nucleophilic atoms equally electron rich, or only particular regions ... 

Enhanced nucleophilicity is often correlated with more negative electrostatic potential. Which of the three molecules listed above is most nucleophilic according to this criterion Which is least nucleophilic (The least nucleophilic molecule does not, in fact, undergo Sn2 reactions.)... 

The endo pyramidization results in the high exo reactivity and, in fact, this pyramidi-zation was confmned by Wipff and Morokuma [37], The orbital distortion also implies large negative electrostatic potential in the exo face. Very recently, Abbasoglu and Yilmaz [38] calculated a derivative of norbomene, i.e., endo tricyclo[3.2.1.0 ] oct-6-ene and confirmed the exo extension of the HOMO accompanied by the endo pyramidization, the large negative electrostatic potential on the exo face, and the preferential exo addition of Br. ... 

Fig. 3 Due to the anisotropic distribution of the electron density, halogen atoms show a negative electrostatic potential and a larger radius (rmax) in the equatorial region and a positive electrostatic potential and a smaller radius (rmjn) in the polar region. As a consequence of this, halogens behave as nucleophiles at the equator and as electrophiles at the pole...

Formally, divalent oxygen has two lone pairs, both of which will normally produce regions of negative electrostatic potential, overlapping to some extent. In many instances, each of these regions will have a most negative point, a Fs.min- On occasion, however, due to extensive overlapping or some other factor, only one Vs,min can be identified. This will be seen in the next section, for hydroxylamine. 

For our example, this means that the benzenesulfonate ions are adsorbed in a surface layer and that the sodium ions form an atmosphere of counter ions in which their concentration gradually decreases with the distance from the benzenesulfonate layer. A small separation between the negative and the positive charges has been created at the interface between the mobile and the stationary phases. As in a plate capacitor, this charge separation creates a difference in electrostatic potential between the charged stationary phase and the surrounding mobile phase. By convention, the surface has a negative electrostatic potential in this case. 

Other long-range effects which may be exploited for metal binding are those involving the so-called macrodipoles of a helices, that is, the sizable electrostatic potentials that characterize the amino and carboxy termini of a helices. The amino-terminal portion of an a helix is characterized by a positive electrostatic potential and is implicated in the binding of phosphate anions to proteins (Hoi et al., 1978) the carboxy-terminal end of the helix is likewise characterized by a negative electrostatic potential and may be similarly implicated in cation binding. Work... 

Both of these surfaces convey structure. The size surface reveals the size and shape of benzene, while the negative potential surface delineates in which regions surrounding benzene a particular (negative) electrostatic potential will be felt . 

Fluorine substitution has greatly diminished the (negative) electrostatic potential for the internal double bond, but has had little effect on the potential for the external double bond. The change in selectivity (toward favoring addition onto the external double bond) is a direct consequence given that carbene addition is electrophilic addition. 

Full geometric optimization of c c/o-lricyclo 3.2.1.024loct-6-ene (endo-TCO) by ab initio and DFT methods has demonstrated the endo-pyramidalizalion of the double bond and non-equivalence of its two faces, with the exo face having regions with much higher electron density (qi, HOMO) and more negative electrostatic potential. 

Figure 8.56 (a) Calculated negative electrostatic potential for a coordinated Cl- ligand in the model... 

Figure 1. (a) X-ray crystal structure of horse-heart ferricytochrome c.8 All protein atoms are shown in the C.-P.-K. form, while the heme group is shown in the stick form. All Arg and Lys residues are colored blue, while Glu and Asp are colored in red, to contrast the destribution of the most ionizable side chains, (b) The X-ray crystal structure of horse heart ferricytochrome c in complex with horse cytochrome c peroxidase (cep).9 The peroxidase is shown as a molecular surface model, with blue regions depicting positive and red representing negative electrostatic potential. Note the cluster of negative potential on ccp that surrounds the contact interface. 

Table 6.1 lists a series of molecules containing Group IV, V, VI and VII atoms. In each instance, we have given the most positive and the most negative electrostatic potentials, the Vs,max and the Vs,min, on the surfaces of these atoms, as defined by the 0.001 au contour of p(r). The Vs,max correspond to positive a-holes, and are located approximately on the extensions of covalent bonds to the atoms. The Vs,min are usually due to lone pairs. The calculations were carried out at the density functional B3PW91/6-31G(d,p) level, using the Gaussian 03 code [55]. 

Fig. 8 The maps of electrostatic potential A - 1,3,5-trinitrobenzene B - siloxane site of clay minerals. Red color - area of negative electrostatic potential blue color - area of positive electrostatic potential.

The nature of the X X interaction in trimer synthon 44 is illustrated in Fig. 20.1.7. The C-X bond in a halo-substituted phenyl ring is polarized, so that there are regions of positive and negative electrostatic potentials around the X atom. The cyclic interaction of three C-X groups optimizes electrostatic potential overlap in the halogen trimer system. 

More work is needed to clearly establish the role of outer-sphere association in DNA-platination. We can infer its influence on the rate of platination according to the relation kp = kK0 [N]/(l + K0 [N]) (with N = nucleotide-binding sites of Pt, i.e., N G, [N] [Pt]) (Scheme 3). It could also influence the selectivity of platination via selective association between the cationic species and the sites of higher negative electrostatic potential. To test this hypothesis one will have to analyze the influence of various sequences, of different types of platinum ligands, and of the ionic status of the DNA medium. 

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