Induced dipole effect

Solvent effects on nuclear magnetic resonance (NMR) spectra have been studied extensively, and they are described mainly in terms of the observed chemical shifts, 8, corrected for the solvent bulk magnetic susceptibility (Table 3.5). The shifts depend on the nucleus studied and the compound of which it is a constituent, and some nuclei/compounds show particularly large shifts. These can then be employed as probes for certain properties of the solvents. Examples are the chemical shifts of 31P in triethylphosphine oxide, the 13C shifts in the 2-or 3-positions, relative to the 4-position in pyridine N-oxide, and the 13C shifts in N-dimethyl or N-diethyl-benzamide, for the carbonyl carbon relative to those in positions 2 (or 6), 3 (or 5) and 4 in the aromatic ring (Chapter 4) (Marcus 1993). These shifts are particularly sensitive to the hydrogen bond donation abilities a (Lewis acidity) of the solvents. In all cases there is, again, a trade off between non-specific dipole-dipole and dipole-induced dipole effects and those ascribable to specific electron pair donation of the solvent to the solute or vice versa to form solvates. 

Three categories of relaxation effects are listed as they contribute to gross and fine structure relaxational effects. They include induced dipole effects (Maxwell-Wagner and counterion) and permanent dipole effects (Debye). 

Introducing this induced dipole effect into the expression for the heat of ion-solvent interactions [Eqs. (2.47) and (2.48)], one has... 

Molecular shape complementarity is critical to biomolecular recognition and specificity. Even if the molecules change conformation on binding and water molecules are trapped at the interface, bound complexes show high shape complementarity (31). This shape complementarity is dependent on van der Waals interactions between the binding molecules. Electron-electron repulsion prevents atomic overlap and intermolecular penetration. However, induced dipole effects as atoms approach lead to short-range attractive interactions. 

In addition to the diffusive forces and effects of an external electric field mentioned, ion motion in an IMS drift tube is affected by the electrostatic interactions between the ion and the gas molecules of the supporting atmosphere. The electron cloud surrounding the neutral gas molecule is polarized by the ion, thus inducing a dipole moment in the neutral molecule. This results in an electrostatic interaction between the ion and the neutral molecule the ion-induced dipole effect. Furthermore, molecules that have permanent dipole or quadrupole moments will be attracted to the ion through ion-dipole or ion-quadrupole interactions. 

In this case we expect the dipole-induced dipole contribution to the attraction to be greater than the dipole-dipole contribution. How can the induced dipole effect be greater than the permanent dipole effect This counter-intuitive result stems from the fact that while the dipole-dipole interaction is averaged over orientations that are repulsive as well as attractive, the dipole-induced dipole interaction is attractive at all orientations. 

In the limit as or jl2 0, Eq. (3.54) takes the same form as that for the Lennard-Jones (12 6) potential for pairs of nonpolar molecules. Hence, for the interaction between a polar and a nonpolar molecule the Lennard-Jones potential may be used. However, in order to allow for small induced dipole effects, the combining laws (3.44) are replaced (Hirschfelder et a/., 1954) by... 

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