Polarity induced dipole

Macroscopic solvent effects can be described by the dielectric constant of a medium, whereas the effects of polarization, induced dipoles, and specific solvation are examples of microscopic solvent effects. Carbenium ions are very strong electrophiles that interact reversibly with several components of the reaction mixture in addition to undergoing initiation, propagation, transfer, and termination. These interactions may be relatively weak as in dispersive interactions, which last less than it takes for a bond vibration (<10 14 sec), and are thus considered to involve "sticky collisions. Stronger interactions lead to long-lived intermediates and/or complex formation, often with a change of hybridization. For example, onium ions are formed with -donors. Even stable trityl ions react very rapidly with amines to form ammonium ions [41], and with water, alcohol, ethers, and esters to form oxonium ions. Onium ion formation is reversible, with the equilibrium constant depending on the nucleophile, cation, solvent, and temperature (cf., Section IV.C.3). 

Obviously the larger the values of y and xYkl the higher the second- and third-order polarization created in the sample, respectively. In polymeric materials, the origin of the optical nonlinearity can be traced to the molecular constituents, and therefore, one can identify polarization due to molecular units. Accordingly, the interaction of radiation with materials can be expressed in terms of the induced molecular polarization (induced dipole moment) as follows [3] ... 

All atoms and molecules can be polarized by an electric field. The polarization (induced dipole of a unit volume) is P = aE where a is molecular polarizability. For spherically symmetric atoms or molecules (like C60 fullerenes) the polarizability is a scalar quantity (tensor of zero rank) and P E. In general case of lath-like molecules, QLij is a second rank tensor (9 components) and Py = a,yE/. By a proper choice of the reference frame the tensor can be diagonalized... 

Of course, it should be remembered that the only type of electrostatic charge being considered is Coulombic. Many nonionic solutes will interact with solution molecules through other types of electrostatic interactions such as polar, induced dipole, hydrogen bonding, etc. These other electrostatic interactions would lead to a nonunity value for, in general. 

Electroiuc polarization arises from the temporary distortion of electron clouds with respect to the nuclei with which they are associated, upon the application of an external electric field. Electroiuc polarization can occur to all materials and also is called atom or atomic polarization. Electronic polarization induces dipole moments. 

ANGLE AVERAGING IS USED TO ACCOUNT FOR AN INDUCED DIPOLE IN POLAR-NONPOLAR PAIR. 

An interesting alternative method for formulating f/(jt) was proposed in 1929 by de Boer and Zwikker [80], who suggested that the adsorption of nonpolar molecules be explained by assuming that the polar adsorbent surface induces dipoles in the first adsorbed layer and that these in turn induce dipoles in the next layer, and so on. As shown in Section VI-8, this approach leads to... 

The average cloud is spherically synnnetric with respect to the nucleus, but at any instant of time there may be a polarization of charge givmg rise to an instantaneous dipole moment. This instantaneous dipole induces a corresponding instantaneous dipole in the other atom and there is an interaction between the instantaneous dipoles. The dipole of either atom averages to zero over time, but the interaction energy does not because the instantaneous and induced dipoles are correlated and... 

Su T, Viggiano A A and Paulson J F 1992 The effect of the dipole-induced dipole potential on ion-polar molecule collision rate constants J. Chem. Phys. 96 5550-1... 

Polarization is usually accounted for by computing the interaction between induced dipoles. The induced dipole is computed by multiplying the atomic polarizability by the electric field present at that nucleus. The electric field used is often only that due to the charges of the other region of the system. In a few calculations, the MM charges have been included in the orbital-based calculation itself as an interaction with point charges. 

Induced dipole/induced dipole forces are the only intermolecular attractive forces available to nonpolar molecules such as alkanes In addition to these forces polar molecules engage m dipole-dipole and dipole/mduced dipole attractions The dipole-dipole attractive force is easiest to visualize and is illustrated m Figure 4 3 Two molecules of a polar substance experience a mutual attraction between the positively polarized region of one molecule and the negatively polarized region of the other As its name implies the dipole/induced dipole force combines features of both the induced dipole/mduced dipole and dipole-dipole attractive forces A polar region of one mole cule alters the electron distribution m a nonpolar region of another m a direction that produces an attractive force between them... 

Neither bromine nor ethylene is a polar molecule but both are polarizable and an induced dipole/mduced dipole force causes them to be mutually attracted to each other This induced dipole/mduced dipole attraction sets the stage for Br2 to act as an electrophile Electrons flow from the tt system of ethylene to Br2 causing the weak bromine-bromine bond to break By analogy to the customary mechanisms for electrophilic addition we might represent this as the formation of a carbocation m a bimolecular elementary step... 

In general arenes resemble other hydrocarbons in their physical properties They are nonpolar insoluble in water and less dense than water In the absence of polar sub stituents mtermolecular forces are weak and limited to van der Waals attractions of the induced dipole/mduced dipole type... 

Now let us examine the molecular origin of Molecular polarity may be the result of either a permanent dipole moment p or an induced dipole moment ind here the latter arises from the distortion of the charge distribution in a molecule due to an electric field. We saw in Chap. 8 that each of these types of polarity are sources of intermolecular attraction. In the present discussion we assume that no permanent dipoles are present and note that the induced dipole moment is proportional to the net field strength at the molecule ... 

An electric dipole consists of two equal and opposite charges separated by a distance. AH molecules contain atoms composed of positively charged nuclei and negatively charged electrons. When a molecule is placed in an electric field between two charged plates, the field attracts the positive nuclei toward the negative plate and the electrons toward the positive plate. This electrical distortion, or polarization of the molecule, creates an electric dipole. When the field is removed, the distortion disappears, and the molecule reverts to its original condition. This electrical distortion of the molecule is caHed induced polarization the dipole formed is an induced dipole. 

A polar molecule can also induce a dipole on a neighbouring molecule that possesses no permanent dipole. The resultant intermolecular attraction between the permanent and the induced dipole is spoken of as the induction force. Its magnitude is small and independent of temperature. 

The induced counter-dipole can act in a similar manner to a permanent dipole and the electric forces between the two dipoles (permanent and induced) result in strong polar interactions. Typically, polarizable compounds are the aromatic hydrocarbons examples of their separation using induced dipole interactions to affect retention and selectivity will be given later. Dipole-induced dipole interaction is depicted in Figure 12. Just as dipole-dipole interactions occur coincidentally with dispersive interactions, so are dipole-induced dipole interactions accompanied by dispersive interactions. It follows that using an n-alkane stationary phase, aromatic... 

Two Molecules Interacting and Held Together by Dispersive Forces and Polar Forces from Permanent and Induced Dipoles... 

Figure 12. Polar Interactions Dipole-Induced Dipole Interactions...

Induced-dipole/induced-dipole attraction (Section 2.17) Force of attraction resulting from a mutual and complementary polarization of one molecule by another. Also referred to as London forces or dispersion forces. 

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