Ionic and dipolar interactions

Non-covalent bonds include ionic and dipolar interactions, hydrogen bonds, aromatic interactions (7I—7I, cation k and anion 7t), closed shell interactions and van der Waals interactions. 

C=0 C=0, C N C=0, S=0 C=0, C—OH O, and H2O C=0. Because the charge of a dipole is less than that of an ion, charge-dipole and dipole-dipole interactions are weaker than ionic bonds. They are nevertheless key contributors to the overall strengths of drug-receptor interactions, since they occur in any molecule in which electronegativity differences between atoms result in significant bond, group or molecular dipole moments. The key differences between ionic and dipolar interactions relate to their dependence on distance and orientation (Table 21.2). 

The full sum of third-order F-ordered terms is not simple even for ionic and dipolar interactions, but for such interactions it is easy to identify the most important subset of terms for certain regimes of interest. For ionic solutions, for example, the terms of order no higher in concentration than will be the dominant ones. In the symmetric case upon which we are focusing here, these terms are... 

A qualitative description of solvent-solute effects involving ionic and dipolar interactions is given by Kirkwood theory [83], which considers the free energy change for transfer of an ion- or dipole-possessing sphere from a vacuum of unit dielectric constant to a medium of dielectric constant e. The application to rate processes is due to Laidler [84]. Contributions to the volume of activation from dipolar interactions are. 

In the studies described in this section we have neglected (1) the ion-ion interactions, (2) the cross-correlation between the ionic and dipolar subensembles, and (3) the finiteness of the potential well depth for ions. It appears that in future it would be desirable to account for ... 

Compared with the pronounced solvent-induced chemical shifts observed with ionic and dipolar solutes, the corresponding shifts of nonpolar solutes such as tetrame-thylsilane are rather small cf. Table 6-6. A careful investigation of chemical shifts of unsubstituted aromatic, as well as alternant and nonalternant, unsaturated hydrocarbons in aliphatic and aromatic non-HBD solvents by Abboud et al. has shown that the differential solvent-induced chemical shift range (relative to benzene as reference) is of the order of only —1.4...+1.0 ppm (positive values representing downfield shifts) [405]. The NMR spectra of these aromatic compounds have been shown to be sensitive to solvent dipolarity and polarizability, except in aromatic solvents, for which an additional specific aromatic solvent-induced shift (ASIS see later) has been found. There is no simple relationship between the solvent-induced chemical shifts and the calculated charge distribution of the aromatic solute molecules. This demonstrates the importance of quadrupoles and higher multipoles in solute/solvent interactions involving aromatic solutes [405]. 

Clearly, the HNC 3nelds nonzero blocked correlations. Moreover, it is a particularly successful approximation for long-range, electrostatic interactions appearing in ionic and dipolar QA models (see Section 7.7 for a discussion of specific applications). 

In this means of interpretation, geometrical factors, including repulsion and dispersion forces and dipolar interactions, dominate. For example, the existence of thermotropic phases of ionic amphiphiles is driven by the formation of strong ion bonding lattices between the head groups, the molecular shape being a secondary factor. Additional groups, that are capable of association can influence the situation dramatically. Thus, the ability of non-ionic amphiphiles to form the anisotropic liquid state must be discussed separately [116]. In par-... 

As nonconductors of electricity, dielectrics interact with electric fields through their ability to form dipoles, which is called polarization. There are three sources of polarization electric, ionic, and dipolar. 

We consider first the effect of aqueous solvation on all intermolecular stabilizations that derive from the interaction of charged or dipolar species. Because of its small size and significant dipole and quadrupole, water interacts strongly with all ionic and dipolar species. A binding event of charged or dipolar compounds thus proceeds with a significant loss of favorable cohesive interactions between solutes and water. The effect is most profound for ionic interactions similar ameliorations of solute-solute interaction apply to multipole-multipole and dipole-induced dipole interactions. 

The molecules (or atoms, for noble gases) of a molecular solid are held In place by the types of forces already discussed In this chapter dispersion forces, dipolar interactions, and/or hydrogen bonds. The atoms of a metallic solid are held in place by the delocalized bonding described in Section 10-. A network solid contains an array of covalent bonds linking every atom to its neighbors. An ionic solid contains cations and anions, attracted to one another by electrical forces as described in Section 8-. 

The short-range, semiordered water structure that surrounds ionic solutes and polar solutes as a consequence of dipolar and Coulombic interactions. 

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