Electrostatic dipole interactions

The parameter a in Equation (11.6) is positive for electrophobic reactions (5r/5O>0, A>1) and negative for electrophilic ones (3r/0Oelectrochemical promotion behaviour is frequently encountered, leading to volcano-type or inverted volcano-type behaviour. However, even then equation (11.6) is satisfied over relatively wide (0.2-0.3 eV) AO regions, so we limit the present analysis to this type of promotional kinetics. It should be remembered thatEq. (11.6), originally found as an experimental observation, can be rationalized by rigorous mathematical models which account explicitly for the electrostatic dipole interactions between the adsorbates and the backspillover-formed effective double layer, as discussed in Chapter 6. 

Figure 2.6 Schematic diagram to show how an induced dipole forms when polarizable electrons move within their orbitals and cause a localized imbalance of charge (an induced dipole in which the negative electrons on one atom attract the positive nucleus on another). The dotted line represents the electrostatic dipole interaction...

The intensities and the positions of absorption bands may also be influenced by solvent molecules. Electrostatic dipole interactions, and specific interactions that lead to formation of complexes based on hydrogen bonds and of charge-transfer type complexes, result in changes in the spectra of chemical compounds. 

The key piece of evidence for the rationalisation of the anomeric effect in terms of classical electrostatic dipole interactions has been the apparent existence of a reverse anomeric effect. Reversal of the sense of the dipole of the C-X bond, so that X now carries a net positive rather than negative charge, should, on the electrostatic picture, result in X having a more than ordinary tendency to be equatorial. The effect was first observed in glycosyl pyridinium salts and, in what at first sight seemed to be an elegant experiment, the steric demand of the bulky pyridinium salt was apparently eliminated as the source of the effect by the observation that tri-O-acetyl-ot-D-xylopyranosylimidazole changed from 65% to > 95% equatorial on addition of excess acid. Protonation of imidazole will have only a minor effect on its steric requirements, since protonation takes place at a remote site . ... 

As proposed by Munn and Hurst [74, 75], an elegant alternative is to employ the generalization of the electrostatic dipole interaction scheme proposed by SUber-stein for atoms [76, 77] to molecules, which consists in evaluating first the local field via the Lorentz-factor tensor [78] and then the macroscopic linear and NLO susceptibilities from the molecular responses calculated using quantum chemistry methods. To alleviate the limitations of the point dipole approximation, the molecule and its molecular responses are usually partitioned in submolecules [79]. Within this approach, the linear optical susceptibility tensor for a crystal with Z molecules labeled k (or Z submolecules labeled kj) per unit cell with a volume V reads ... 

Stigter and Dill [98] studied phospholipid monolayers at the n-heptane-water interface and were able to treat the second and third virial coefficients (see Eq. XV-1) in terms of electrostatic, including dipole, interactions. At higher film pressures, Pethica and co-workers [99] observed quasi-first-order phase transitions, that is, a much flatter plateau region than shown in Fig. XV-6. 

Consider the interaction of a neutral, dipolar molecule A with a neutral, S-state atom B. There are no electrostatic interactions because all the miiltipole moments of the atom are zero. However, the electric field of A distorts the charge distribution of B and induces miiltipole moments in B. The leading induction tenn is the interaction between the pennanent dipole moment of A and the dipole moment induced in B. The latter can be expressed in tenns of the polarizability of B, see equation (Al.S.g). and the dipole-mduced-dipole interaction is given by... 

The raie gas atoms reveal through their deviation from ideal gas behavior that electrostatics alone cannot account for all non-bonded interactions, because all multipole moments are zero. Therefore, no dipole-dipole or dipole-induced dipole interactions are possible. Van der Waals first described the forces that give rise to such deviations from the expected behavior. This type of interaction between two atoms can be formulated by a Lennaid-Jones [12-6] function Eq. (27)). 

Electrostatic terms other than the simple charge interactions above are commonly included in molecular mechanics calculations. particularly dipole-dipole interactions. More recently, second-order electrostatic interactions like those describing polarizability have been added to some force fields. 

Forces of Adsorption. Adsorption may be classified as chemisorption or physical adsorption, depending on the nature of the surface forces. In physical adsorption the forces are relatively weak, involving mainly van der Waals (induced dipole—induced dipole) interactions, supplemented in many cases by electrostatic contributions from field gradient—dipole or —quadmpole interactions. By contrast, in chemisorption there is significant electron transfer, equivalent to the formation of a chemical bond between the sorbate and the soHd surface. Such interactions are both stronger and more specific than the forces of physical adsorption and are obviously limited to monolayer coverage. The differences in the general features of physical and chemisorption systems (Table 1) can be understood on the basis of this difference in the nature of the surface forces. 

The combination of electrostatic interaction (induced dipole—dipole interaction) with an increase in entropy resulting from the discharge of bound water is fundamental to PVP s abiUty to complex with a variety of large anions. 

Figure 4 shows the measured angle of 105° between the hydrogens and the direction of the dipole moment. The measured dipole moment of water is 1.844 debye (a debye unit is 3.336 x 10 ° C m). The dipole moment of water is responsible for its distinctive properties in the Hquid state. The O—H bond length within the H2O molecule is 0.96 x 10 ° m. Dipole—dipole interaction between two water molecules forms a hydrogen bond, which is electrostatic in nature. The lower part of Figure 4 (not to the same scale) shows the measured H-bond distance of 2.76 x 10 ° m or 0.276 nm. 

The favorability of acid-base reactions is affected, in pa by electrostatic interactions between charged atoms a dipoles within the same molecule. The equilibrium w shift in the direction of an ion that is stabilized 1 intramolecular ion-dipole interactions. 

Decide if ion-dipole interactions are responsible for the observed substituent effects. Obtain the charge on carbon and nitrogen in each cyano group. What evidence is there for a polar CN bond Should the ion (O )-dipole (CN) interaction be stabilizing or destabilizing Can these interactions explain the trends in electrostatic potential (Hint Focus on changes in O—CN distance and in orientation of the cyano group.)... 

From Table 2.5 it is clearly seen that becomes small (less than 0.001 kcal/ mol) beyond a distance of 10 A. The electrostatic interaction reaches the same level of importance at a distance of 30 A. The Table also shows that the interaction between point charges behaves much like a dipole-dipole interaction, i.e. an R dependence. However, the interaction between net charges is very long range even at 100 A separation, there is a 0.34kcal/mol energy contribution. The cut-off distance corresponding to a contribution of 0.001 kcal/mol is of the order of 3000 A ... 

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