Directional electrostatic bonding

Unlike the forces between ions which are electrostatic and without direction, covalent bonds are directed in space. For a simple molecule or covalently bonded ion made up of typical elements the shape is nearly always decided by the number of bonding electron pairs and the number of lone pairs (pairs of electrons not involved in bonding) around the central metal atom, which arrange themselves so as to be as far apart as possible because of electrostatic repulsion between the electron pairs. Table 2.8 shows the essential shape assumed by simple molecules or ions with one central atom X. Carbon is able to form a great many covalently bonded compounds in which there are chains of carbon atoms linked by single covalent bonds. In each case where the carbon atoms are joined to four other atoms the essential orientation around each carbon atom is tetrahedral. 

These effects are attributed to differences in the c-donor character of the C—C bonds as a result of substitution. Electron-attracting groups diminish the donor capacity and promote syn addition. An alternative explanation invokes a direct electrostatic effect arising from the C-X bond dipole. 

Furthermore, the relatively high reactivity of 2-chloropyridine i -oxide as compared to that of the 4-isomer and the detailed inconsistency with theoretical parameters have also been explained in terms of built-in solvation via either direct electrostatic interaction or hydrogen bonding (structures 15 and 16, respectively). 

For acids, the membrane retention actually increases in the case of egg lecithin, compared to soy lecithin. This may be due to decreased repulsions between the negatively charged sample and negatively charged phospholipid, allowing H-bond-ing and hydrophobic forces to more fully realize in the less negatively charged egg lecithin membranes. The neutral molecules display about the same transport properties in soy and egg lecithin, in line with the absence of direct electrostatic effects. These differences between egg and soy lecithins make soy lecithin the preferred basis for further model development. 

All these studies serve to illustrate the dominant importance of hydrogen bonding in stabilizing particular anion inclusion complexes of the type just discussed. The relative importance of any direct electrostatic attraction between the positive cavity and the anionic guest is more difficult to define. Nevertheless, interactions of the latter type are of crucial importance in other systems. 

Miscellaneous Iminium Catalyzed Transformations The enantioselective construction of three-membered hetero- or carbocyclic ring systems is an important objective for practitioners of chemical synthesis in academic and industrial settings. To date, important advances have been made in the iminium activation realm, which enable asymmetric entry to a-formyl cyclopropanes and epoxides. In terms of cyclopropane synthesis, a new class of iminium catalyst has been introduced, providing the enantioselective stepwise [2 + 1] union of sulfonium ylides and ot,p-unsaturated aldehydes.As shown in Scheme 11.6a, the zwitterionic hydro-indoline-derived catalyst (19) enables both iminium geometry control and directed electrostatic activation of sulfonium ylides in proximity to the incipient iminium reaction partner. This combination of geometric and stereoelectronic effects has been proposed as being essential for enantio- and diastereocontrol in forming two of the three cyclopropyl bonds. 

The pHPZC of ferric hydroxide surfaces is about 8 [127], so aqueous Pb2+ should be electrostatically repelled from these surfaces at pH values less than 8. However, as seen in Figure 7.6(a), the Pb2+ present in this aqueous solution is sorbed essentially completely to ferric hydroxide surfaces at pH 6. This behavior suggests that Pb2+ forms direct chemical bonds to these surfaces in order to overcome the repulsive electrostatic forces below the pHpzc of ferric hydroxide. This conclusion based on macroscopic uptake data has been confirmed by direct spectroscopic observation using X-ray absorption fine structure (XAFS) spectroscopy under in situ conditions (i.e., with aqueous solution in contact with a-FeOOH surfaces at ambient temperature and pressure) [133,134]. These studies showed that the aquated Pb(II) ion forms dominantly inner-sphere, bidentate complexes on a-FeOOH surfaces. 

The forces involved in chemistry are essentially electrostatic. They are variants on the Coulomb force. We can distinguish two orders primary forces and secondary forces. Primary forces are those which hold the atoms together in molecules, and the oppositely charged ions in crystalline salts. Respectively, they are known as covalency and electrovalency (or, sometimes, the ionic force). The latter is directly electrostatic, the mutual attraction between Na+ and Cl" in common salt, for example. The former is usually figured as the sharing of an electron-pair between two atoms— Cl-Cl in the chlorine molecule, where the bond stands for a shared pair of electrons. We need quantum mechanics to understand why, in certain circumstances, electron density builds up in the region between the two chlorine atoms. Granted that it does so, we can explain the covalent bond as due to a resultant electrostatic effect. 

We are left with the problem of substituent effects that do not depend on direct mesomeric interactions between the substituent and the reaction center. Effects of this kind can arise in one of two ways. First, the bond between the substituent and the substrate may be polar, and there may also be polar bonds or charged atoms in the substituent itself the charges set up in this way can influence the reaction center either by altering the effective electronegativity of atoms connected with it (inductive effect) or by direct electrostatic interaction across space (field effect). Secondly, the substituent may be attached to a conjugated system which does not itself take part in the reaction, the case exemplified by the Hammett equation (Eq. (105)) here it... 

The relationship between the growth rate and 4 1 can be explained easily if it is considered that the faces with highest dhki also show the highest concentration of chemical and/or electrostatic bonds within the plane of the face and the minimal density of bonds in the direction perpendicular to the plane of the face. When oxides and halides are considered, the previous rule can also be formulated in the following way Faces exposing ions with the lowest coordinative unsaturation are the most stable and least reactive and hence determine the final morphology of the microcrystals under thermodynamic equilibrium conditions. 

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