Electrostatic effect

Electrostatic effects have been capitalized upon in the creation of nucleophiles in common synthetic transformations. For example, alkyl anions can be readily created in direct proximity to positive phosphorus, nitrogen, and sulfur centers, creating what are known as ylides. The pK of PhsP+CHs is 22.4, deprotonation of which gives the common phosphorus ylide exploited in the Wittig reaction. Table 5.6G shows a handful of other examples, where pXgS as low as 8 can be obtained when in conjunction with other effects. 

A fascinating effect taught in beginning organic chemistry courses is the influence of the hybridization state of carbon on C-H acidities. Recall that alkynes are more acidic than 

Decreasing acidity with decreasing s character in carbon hybridization 

This electronegativity difference due to the hybridization states of carbon should manifest itself in other acidity trends. Indeed, it does. Examine the trends in acidities of alcohols and carboxylic acids given in Tables 5.5F and G, respectively. Proximity of the acidic group to an alkynyl group results in the strongest acid, whereas a simple alkyl group gives the weakest acid. 

Hybridization effects are evident in the acidities of protonated amines also. The pfCa of a protonated nitrile is around -12 (an sp nitrogen), while that of pyridinium is 5.23 (sp nitrogen), and alkyl ammoniums are around 10 to 11 (sp nitrogen). 

The charged micelles give rise to strong local electrical fields in the solution, which will influence the distribution and motion of other ionic entities. The micelles also repel each other reducing the translational mobility. These electrostatic interactions influence the energetics of the micellization process substantially. This is seen from the comparatively high values of the CMC for ionic amphiphiles and by the fact that the addition of salt decreases the CMC. 

It is thus clear that a treatment of the micellization process of ionic amphiphiles must include a discussion of electrostatic effects. Furthermore, even for zwitterionic and nonionic surfactants, the electrostatic effects play a role. The favorable interaction between the polar groups of these amphiphiles and the solvent water is probably mainly of an electrostatic origin. 

Whenever possible, we will attempt to identify the effects of interactions between relatively few frontier orbitals, which describe the distribution of relatively few electrons of a molecule. We effectively ignore the vast numbers of the rest of the electrons in the system as well as the nuclear charges. In some cases this neglect is not justified and may lead to misleading results. When a molecule or parts of it are charged, coulombic interactions may dominate. The electrostatic energy of interaction of each pair of centers, i and j, with net charges, qt and qj, respectively, separated by a distance Ry in a medium with dielectric constant em has the form 

Electrostatic effects cannot be ignored whenever a process takes place that changes the numbers of charged species. Heterolytic cleavage of a bonds (see Chapter 4) would 

We will not attempt to quantify electrostatic effects but will need to be aware of possible influences as we consider our orbital interaction diagrams. Fortunately, the directions of electrostatic influences are easy to deduce from equation (3.48). 

The relative rate of hydrolysis in acid of any peptide bond and hence the yield of a given peptide is determined mainly by the number of hydrogen ions that can approach the bond. While the rate probably depends on a number of different factors, we may consider two which probably play a major role, namely, electrostatic effects and steric effects. 

Similar conclusions may be drawn from the rates of hydrolysis of gramicidin (Synge, 1945 Christensen and Hegsted, 1945) the effect being more marked at 37° than at the boiling temperature. On the contrary, however, it was found that the yield of free amino acids during the hydrolysis of ovalbumin with 1 N HCl was almost exactly theoretical (Warner, 1942b). 

An additional effect of a similar nature can be expected to arise as a result of the charge transfer from the ion to the cluster. Detailed analysis shows that an extra 

The results for fluoride are likely also to include some repulsion between the ion and the negative charge transferred to the cluster. Nevertheless, because the charge transferred to the cluster is much smaller, it is dominated by the effect of the polarization of the metal because of the charge remaining on the ion. This might indicate that results for fluoride are much closer to those expected for the infinite surface than for the other ions. For this unique ion the response of the cluster to the presence of the ion is similar to that expected for the infinite neutral metal surface. 

The results from analogous calculations of the electrostatic interaction between ions with the Agu and Aui2 clusters are presented in Tables 3 and 4, respectively as 

It is apparent that the AF app values are characterized by a clear trend, much better defined than that found for the original DFT results. The adsorption of halide ions on the different metals is strongest on Au, much weaker on Cu and 

One must, of course, realize that all the energy values discussed here are very crude estimates and cannot be treated as true values of the ion-metal interaction. Firstly, the effect of the polarization of the metal by the ion should be evaluated. Also, careful tests are needed to determine whether the repulsion between the charge remaining on the ion and that transferred to the cluster is indeed an artifact of the cluster used the true contribution of this component to the total ion-metal interaction should also be determined. Thus, much more extensive testing of electrostatic effects in cluster-model calculations are necessary if the ion-metal interac- 

The same phenomenon is responsible for the decrease in the ionization constant of a polymeric acid such as poly(acrylic acid) with increasing degree of ionization [Kawaguchi et al., 1990, 1992 Morawetz and Wang, 1987]. This effect is somewhat moderated for acrylic acid-ethylene copolymers, since charge density on the ionized polymer is decreased. The basicity 

Charge on a polymer molecule can also affect reactivity by altering the concentration of the small molecule reactant within the polymer domains. The reaction of a charged polymer with a charged reactant results in acceleration for oppositely charged species and retardation when the charges are the same. For example, the rate constant for the KOH saponification of poly(methyl methacrylate) decreases by about an order of magnitude as the reaction proceeds [Plate, 1976]. Partially reacted poly(methyl methacrylate) (IV) repells hydroxide ion, while 

Related phenomena include the effect of a polymer on the reaction between two small reactants. Acceleration occurs when the polymer attracts both reactants (both reactants 

The rate increases with polyanion concentration, reaches a maximum, and then decreases with further increase in polyanion concentration [Morawetz and Vogel, 1969]. Retardation occurs for reactions between ions of opposite charge because the polymer attracts one ion but repels the other. 

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Most of the Langmuir films we have discussed are made up of charged amphiphiles such as the fatty acids in Chapter IV and the lipids in Sections XV-4 and 5. Depending on the pH and ionic strength of the subphase, electrostatic effects can become quite important. Here we develop the theoretical foundation for charged films with the Donnan relationship. Then we mention the influence of subphase pH on film behavior. 

The development of efficient algorithms and the sophisticated description of long-range electrostatic effects allow calculations on systems with 100 000 atoms and more, which address biochemical problems like membrane-bound protein complexes or the action of molecular machines . 

The model adopted by Ri and Eyring is not now acceptable, but some of the more recent treatments of electrostatic effects are quite close to their method in principle. In dealing with polar substituents some authors have concentrated on the interaction of the substituent with the electrophile whilst others have considered the interaction of the substituent with the charge on the ring in the transition state. An example of the latter method was mentioned above ( 7.2.1), and both will be encountered later ( 9.1.2). They are really attempts to explain the nature of the inductive effect, and an important question which they raise is that of the relative importance of localisation and electrostatic phenomena in determining orientation and state of activation in electrophilic substitutions. 

Fabric filters are limited by physical size and bag-life considerations. Some sacrifices in efficiency might be tolerated if higher air-cloth ratios could be achieved without reducing bag life (improved pulse-jet systems). Improvements in fabric filtration may also be possible by enhancing electrostatic effects that may contribute to rapid formation of a filter cake after cleaning. 

The electrostatic effect can be incorporated into wet scrubbing by charging the particulates and/or the scrubbing-liquor droplets. Electrostatic scrubbers may be capable of achieving the same efficiency for fine-particulate removal as is achieved by high-energy scrubbers, but at substantially lower power input. The major drawbacks are increased maintenance of electrical equipment and higher capital cost. 

In HT motors even electrostatic effects between the windings of the stator and the rotor due to stator voltage... 

In Chapter 4, we will discuss the relative importance of inductive effects and field effects on reactivity. Generally, field effects appear to be the dominant mechanism for the transmission of electrostatic effects of polar bonds to other parts of a molecule. 

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. 

Similarly, carboxylic acid and ester groups tend to direct chlorination to the / and v positions, because attack at the a position is electronically disfavored. The polar effect is attributed to the fact that the chlorine atom is an electrophilic species, and the relatively electron-poor carbon atom adjacent to an electron-withdrawing group is avoided. The effect of an electron-withdrawing substituent is to decrease the electron density at the potential radical site. Because the chlorine atom is highly reactive, the reaction would be expected to have a very early transition state, and this electrostatic effect predominates over the stabilizing substituent effect on the intermediate. The substituent effect dominates the kinetic selectivity of the reaction, and the relative stability of the radical intermediate has relatively little influence. 

Although the nature of the general polar effect suggested by Kamernitzsky and Akhrem " to account for axial attack in unhindered ketones is not clear, several groups have reported electrostatic interactions affect the course of borohydride reductions. Thus the keto acid (5a) is not reduced by boro-hydride but its ester (5b) is reduced rapidly further, the reduction of the ester (6b) takes place much more rapidly than that of the acid (6a). Spectroscopic data eliminate the possibility that in (5a) there is an interaction between the acid and ketone groups (e.g. formation of a lactol). The results have been attributed to a direct repulsion by the carboxylate ion as the borohydride ion approaches. " By contrast, House and co-workers observed no electrostatic effect on the stereochemistry of reduction of the keto acid (7). However, in this compound the acid group may occupy conformations in which it does not shield the ketone. Henbest reported that substituting chlorine... 

Electrostatic effects have long been recognized in commercial HPLC columns for SEC of proteins (15,21,22). The usual remedy is to add 100 mM salt to the mobile phase. This works here too the Lys and Asp peaks collapse into the Gly peak with 100 mM salt (Eig. 8.8). High concentrations of sodium sulfate were added to determine the role played in SEC by hydrophobic interactions (sodium sulfate, a structure-forming salt, strengthens such interactions). Sodium sulfate increased the retention only of the most hydrophobic amino acids to any extent, and then only when the concentration approached 1 M. Clearly, hydrophobic interaction cannot account for the elution order of amino acids on PolyHEA. 

In the models discussed thus far in this section, emphasis has been placed on electrostatic effects and solvent polarity. An alternative view that to some extent takes other forces into account begins with the idea that, in order to dissolve a solute molecule in a solvent, energy is required to create a cavity in the solvent the solute is then inserted into this cavity. In Section 8.2 we saw that the energy to create a cavity can be expressed as a product of the surface area of the cavity and the surface tension of the solvent. An equivalent expression is obtained as the product of the volume of the cavity and the pressure exerted by the solvent, and we now explore this concept. 

So far, all of the calculations we ve done have been in the gas phase. While gas phase predictions are appropriate for many purposes, they are inadequate for describing the characteristics of many molecules in solution. Indeed, the properties of molecules and transition states can differ considerably between the gas phase and solution. For example, electrostatic effects are often much less important for species placed in a solvent with a high dielectric constant than they are in the gas phase. 

Destabilization of the ES complex can involve structural strain, desolvation, or electrostatic effects. Destabilization by strain or distortion is usually just a consequence of the fact (noted previously) that the enzyme is designed to bind the transition state more strongly than the substrate. When the substrate binds, the imperfect nature of the fit results in distortion or strain in the substrate, the enzyme, or both. This means that the amino acid residues that make up the active site are oriented to coordinate the transition-state structure precisely, but will interact with the substrate or product less effectively. 

Compare energies for both diaxial and diequatorial chair conformers of trans-2-fluorocyclohexanol (X = OH). Which conformer is preferred Does the preferred conformer minimize steric repulsion Is it reasonable to attribute the conformational preference solely to steric effects Explain. Examine dipole moments for the two conformers. Does the preferred conformer minimize electrostatic repulsion (or maximize electrostatic attraction) Is it reasonable to attribute the conformational preference solely to electrostatic effects Explain. 

Now, examine the orbital on cyclohexanone lithium enolate most able to donate electrons. This is the highest-occupied molecular orbital (HOMO). Identify where the best HOMO-electrophile overlap can occur. Is this also the most electron-rich site An electrophile will choose the best HOMO overlap site if it is not strongly affected by electrostatic effects, and if it contains a good electron-acceptor orbital (this is the lowest-unoccupied molecular orbital or LUMO). Examine the LUMO of methyl iodide and trimethylsilyl chloride. Is backside overlap likely to be successful for each The LUMO energies of methyl iodide and trimethylsilyl chloride are 0.11 and 0.21 au, respectively. Assuming that the lower the LUMO energy the more effective the interaction, which reaction, methylation or silylation, appears to be guided by favorable orbital interactions Explain. 

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