Interaction-induced Dipole Moment

Raman scattering has been discussed by many authors. As in the case of IR vibrational spectroscopy, the interaction is between the electromagnetic field and a dipole moment, however in this case the dipole moment is induced by the field itself The induced dipole is pj j = a E, where a is the polarizability. It can be expressed in a Taylor series expansion in coordinate isplacement... 

As argued above, this result is found to work best for substances in which both the 1,1 and 2,2 forces are either London or dipole-dipole. Even the case of one molecule with a permanent dipole moment interacting with a molecule which has only polarizability and no permanent dipole moment-such species interact by permanent dipole-induced dipole attraction-is not satisfactorily approximated by Eq. (8.46). In this context the like dissolves like rule means like with respect to the origin of intermolecular forces. 

When the oscillating electric held of an incident light ray interacts with a molecule, a small oscillating dipole moment is induced in the molecule as a consequence of its polarisability, a. Polarisability itself is a measure of the change in the dipole moment of a molecule induced by an electric held, and in the simplest case, where the electric held E and induced dipole moment p are in the same direction ... 

If chemisorbed species are polar, or dipole moments are induced by adatom-substrate interactions, there will be a dipole-dipole interaction between two adatoms. The interaction is given by... 

The net strength of a molecule s Debye interactions is a function of its polarizability and the number and magnitude of its local dipole moments. Since induced dipoles tend to be aligned for maximum attraction, the energy of interaction resulting from Debye forces is very nearly additive. 

In this regard, the electrostatic polarization term arises in the case of nonpolar molecules. These nonpolar molecules when within an electric field are polarized, and afterward produce an induced dipole moment. This induced dipole moment, ph interacts with the adsorbent, and the interaction potential is given by... 

This extraction precisely reproduces the same London, Debye, and Keesom interactions, including all relativistic retardation terms that had been effortfully derived in earlier formulations. These interactions are distinguished by whether they involve the interaction of two permanent dipoles of moment //.uipoie, or involve an inducible polarizability aind. A water molecule, for example, has both a permanent dipole moment and inducible polarizability. The contribution of each water molecule to the total dielectric response is a sum of the form of Eqs. (L2.163) and (L2.173) in mks units,... 

As an example, the charged phosphate group on phosphatidylethanol-amine, for example, can interact with the hydrocarbon (CH2) chain of an amino acid—for example, valine—in a peptide. A similar situation would hold in the example to the right for interaction of the hydrocarbon unit in a peptide chain. In both instances the groups with permanent dipole moments can induce a temporary dipole moment in an adjacent molecule. These interactions, however, are very weak and act only at very short distances thus the polarization energies may be of the order of 0.002-0.004 kcal/mol at a distance of 5 A. 

In nonpolar molecules such as carbon tetrachloride, the principal attractive force is the London dispersion force, one of the van der Waals forces (Figure 2-24). The London force arises from temporary dipole moments that are induced in a molecule by other nearby molecules. Even though carbon tetrachloride has no permanent dipole moment, the electrons are not always evenly distributed. A small temporary dipole moment is induced when one molecule approaches another molecule in which the electrons are slightly displaced from a symmetrical arrangement. The electrons in the approaching molecule are displaced slightly so that an attractive dipole-dipole interaction results. 

A close relative of optical spectroscopy is magnetic resonance spectroscopy. Here, the interaction of the magnetic vector of electromagnetic radiation with matter is monitored. Now, the absorption or emission of radiation results from interaction with an intrinsic magnetic (dipole) moment the induced moment resulting from the magnetic polarizability is far too small to play a role (except in circular dichroism). 

FIGURE 10.8 A fluctuation of the electron distribution on one atom induces a corresponding temporary dipole moment on a neighboring atom. The two dipole moments interact to give a net attractive force, called a "dispersion force."... 

The TPA cross section (7>) is proportional to the second-order nonlinear polarizability y. This is derived by consideration of interactions of light with matter causing a change in the dipole moment that is the induced dipole moment /iirld. Induced polarization Pind is proportional to /(ind and the field strength E of the incident light, Eq. (7) [50, 73] ... 

The ion-molecular processes start with scattering in a polarization potential, leading to the so-called Langevin capture of a charged particle and formation of an intermediate ion-molecular complex. If a neutral particle itself has no permanent dipole moment, the ion-neutral charge-dipole interaction is due to dipole moment Pm, induced in the neutral particle by the electric field E of an ion ... 

Dispersion forces are attractive forces between atoms at close distances. Even molecules with no permanent dipole moment have, due to the movement of their electrons, local dipole moments which induce dipoles in the opposite molecule, leading to fluctuating electrostatic attractions. At a closer distance repulsive forces develop due to an unfavorable overlap of the van der Waals spheres of both molecules. These relationships are typically described by the Lennard Jones potential, with an r attractive term and an r repulsive term (Figure 2) [59, 116]. Dipole-dipole interactions and dispersion forces are much weaker than other electrostatic interactions. Nevertheless, if there is a close contact between both molecules over a relatively large surface area, they may sum up to large values of overall interaction energies. 

The syntheses of the ion pairs [ L(y Ae][H2N B(C5F5)3 2] (Ae = Ca, Sr, Ba) were first targeted [17]. The choice of Bochmann s perfluorinated WCA H2N B(QF5)3 2 [18] was motivated by two factors. Firstly, it displays better crystallization properties than the traditional B(QF5)4 while the latter can be considered spherical and often leads to the formation of oily or amorphous materials, H2N(B(C6F5)3 2 possesses a dipole moment that induces an orientation toward the cation and facilitates crystallization processes. Secondly, H2N(B(QF5)3 2 is very robust (due to a pattern of internal H -F stabilizing interactions) and the negative charge is delocalized over an extremely large volume (circa 538 A ) as a result, the catalytic performances with this anion are at least equivalent to those displayed by smaller fluorinated WCAs. 

A molecule with a dipole moment Pdip induces in another molecule of polarizability apoi, a dipole moment which leads to the following energy of interaction ... 

The existance of crystalline forms of noble gases, such as argon, proves that attractive forces must exist between non-polar molecules. In 1930 F. London showed that a collection of nonpolar molecules only lack polarity when viewed over a period of time. An instantaneous photograph of the molecules would show a distortion of the electrons relative to the nucleus sufficient to cause a temporary dipole moment. That instantaneous dipole moment causes induced dipoles in surrounding molecules which results in an attractive force between the non-polar molecules. The potential energy of the interaction was shown by London to be ... 

Structure formation at the atomic level is, in principle, a traditional quantum-chemical many-body problem. Amino acids occurring in bioproteins contain between 7 (glycine) and 24 (tryptophan) atoms, Thus, typical bioproteins consist of hundreds to tens thousands of atoms. In general, the structural properties of macromolecules depend on two classes of chemical bonds covalent and noncovalent bonds. Covalent bonds are based on common electron pairs shared between atoms and stabilize the chemical composition of the molecule. On the other hand, noncovalent bonds are based on much weaker effective interactions due to screening, polarization effects, or dipole moments, partly induced by the surrounding polar solvent. These interactions are responsible for the three-dimensional structure of macromolecules in solvent. 

When a charge approaches a molecule without a static dipole moment, all energies considered so far would be zero. Nevertheless, there is an attractive force, which arises from a charge shift in the nonpolar molecule induced by the charge. This induced dipole moment interacts with the charge. The Helmholtz free energy is... 

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