Dipolar interactions London

Dipolar Interactions London, Keesom, and Debye Forces... 

Dipolar attractions result from the attraction between permanent molecular dipoles. Hydrogen bonding is a special type of dipolar interaction, but only occurs when a hydrogen atom is directly bonded to O, F, or N. London forces are temporary dipolar attractions that result from nonsymmetrical distribution of electrons in an atom or molecule. 

Experimentally, one of the main methods of distinction between the Forster and Dexter mechanisms in an energy transfer is a study of the distance dependence of the observed process. From Equation (2.32) it is evident that the rate of dipole-induced energy transfer, kfen/ decreases as d 6. This is typical of dipolar interactions and is reminiscent of the distance dependence of other such mechanisms, e.g. London dispersion forces. Therefore, the Forster mechanism can operate over large distances, whereas, in contrast, the rate of Dexter energy transfer, kden, falls off exponentially with distance. 

Dispersion Forces The dipolar interaction forces between any two bodies of finite mass, including the Keesom forces of orientation among dipoles, Debye induction forces, and London forces between two induced dipoles. Also referred to as Lifshitz—van der Waals forces. 

The interaction between a solvated peptide or protein and a chemically modified RPC and HIC stationary phase in a fully or partially aqueous solvent environment can be discussed in terms of the interplay of weak physical forces. The main types of physical interactions that are involved in order of relevance and dominance for the establishment of the selective recognition and binding between a peptide or protein and RPC and HIC ligates are (I) hydrophobic interactions and related phenomena mediated by polarized electron donor or electron acceptor processes, (2) Lifshitz-London forces and van der Waals and associated weak dipolar interactions, (3) tt 7t and n ->dipole interactions, (4) hydrogen bond interactions, (5) electrostatic interactions, (6) metal ion coordination interactions, and (7) secondary macromolecular interactions involving force field effects. 

The 3d electrons in a magnetic compound crystal are localized about single ions, so that they can be described fairly adequately by the Heitler-London approach. Usually, as explained in Section II, the orbital magnetic moment is quenched and only spin can contribute to the magnetic moment. When there are two magnetic ions with resultant spin and S, respectively at a reasonable distance from each other, we have the well-known magnetic dipolar interaction,... 

The dispersion forces can be depicted as a coupling between polarijtation of the solute and solvent molecules, so the electron correlation between solvent and solute is the important quantitative effect on the solvation free energy. In the first treatment of this phenomenon London [38] introduced some appropriate approximations that enabled him to relate the dispersion energy of a pair of molecules to their polarizabilities. A more detailed analysis of the dispersion interaction in the reaction field formalism was performed by Linder in the case of a spherical cavity for purely dipolar interactions. In [39] the theory has been generalized to the case of non-spherical cavity and extended to higher order multipole polarizabilities as... 

As mentioned, LMOGs aggregate through intermolecular interactions which indude H-bonding, 7i-K stacking, dipolar interactions, and London dispersion... 

Van der Waals forces are intermolecular and are classified as (i) dipole-dipole interactions, (ii) dipole-induced-dipole interactions and (iii) London dispersion forces which operate between atoms as the result of the nucleus not always being at the centre of mass of the surrounding electrons. The hydrogen bond is regarded as a special form of dipole-dipole interaction, because the positive end of dipolar species containing hydrogen atoms is the relatively unshielded proton. 

Large anions, such as I- and CIO4, have a relatively weak tendency to accept hydrogen bonds. However, they are highly polarizable and interact to a fair extent by dispersion forces (London forces) with the molecules of aprotic solvents, which are also considerably polarizable. Thus, for large anions, the solvation energies in protic solvents (water, alcohols) and those in dipolar aprotic solvents (AN, DMF, DMSO) are not as different as in the case of small anions (Table 2.4). 

There are two types of solute-solvent interactions which affect absorption and emission spectra. These are universal interaction and specific interaction. The universal interaction is due to the collective influence of the solvent as a dielectric medium and depends on the dielectric constant D and the refractive index n of the solvent. Thus large environmental perturbations may be caused by van der Waals dipolar or ionic fields in solution, liquids and in solids. The van der Waals interactions include (i) London dispersion force, (ii) induced dipole interactions, and (iii) dipole-dipole interactions. These are attractive interactions. The repulsive interactions are primarily derived from exchange forces (non bonded repulsion) as the elctrons of one molecule approach the filled orbitals of the neighbour. If the solute molecule has a dipole moment, it is expected to differ in various electronic energy states because of the differences in charge distribution. In polar solvents dipole-dipole inrteractions are important. 

In certain circumstances, it may be necessary to distinguish between the different types of interactions. This can be performed in several ways (Barton, 1983 Van Krevelen, 1990). The most usual method is to make a distinction between dispersion (London), dipolar (Debye Keesom) and hydrogen-bonding components, each one being characterized by its contribution to CED and the corresponding solubility parameter, 8d, 8p, 8h, respectively, such that 8 = (8d + 8p + 8j )1/2. 

It has to be emphasized that more refined approaches have been established, in particular by Van Oss and coworkers (1994). They introduced the so-called Lifschitz-Van der Waals (LW) interactions. These interactions include the dispersion or London forces ( / ), the induction or Debye forces (yD) and the dipolar or Keesom forces (, K), so that ... 

One reason for this at first sight unexpected result is the fact that probably 70... 90% of the solute/solvent interaction term is caused by London dispersion forces, which are more or less equal for the cis and trans isomers. Another important reason is that one has to take into account higher electric moments the trans isomer has a quadrupole moment, and the cis isomer also has moments of a higher order than two. Calculations of solute/solvent interactions of both diastereomers using a reaction field model led to the conclusion that the quadrupolar contribution of the trans isomer is comparable to the dipolar contribution of the cis isomer. It has been pointed out that the neglect of solute/solvent interactions implying higher electric moments than the dipole moment can lead to completely false conclusions [202],... 

Aprotic Solvents, in J. F. Coetzee and C. D. Ritchie (eds.) Solute-Solvent Interactions, Dekker, New York, London, 1969, Vol. 1, p. 219ff. [95] H. Liebig Prdparative Chemie in aprotonischen Ldsungsmitteln, Chemiker-Ztg. 95, 301 (1971). [96] E. S. Amis and J. F. Hinton Solvent Effects on Chemical Phenomena, Academic Press, New York, London, 1973, Vol. 1, p. 271ff. [97] P. K. Kadaba Role of Protic and Dipolar Aprotic Solvents in Heterocyclic Syntheses via 1,3-Dipolar Cycloaddition Reactions, Synthesis 1973, 71. [98] J. H. Hildebrand and R. L. Scott The Solubility of Nonelectrolytes, 3 ed., Reinhold, New York, 1950 Dover, New York, 1964 J. H. Hildebrand and R. L. Scott Regular Solutions, Prentice-Hall, Englewood Cliffs/New Jersey, 1962 J. H. Hildebrand, J. M. Prausnitz, and R. L. Scott Regular and Related Solutions, Van Nostrand-Reinhold, Princeton/New Jersey, 1970. [99] A. E. M. Barton Handbook of Solubility Parameters and other Cohesion Parameters, CRC Press, Boca Raton/Elorida, 1983. [100] M. R. J. Dack, Aust. J. Chem. 28, 1643 (1975). 

For the case of anisotropic dipolar oscillators, the dispersional forces have been discussed by London and De Boer. On neglecting the anisotropy of electron oscillation frequency, the dispersional energy of interaction between two anisotropic molecules becomes, in a dipole-dipole approximation > ... 

Jb) Amsotropic quadrupolar molecules. In dealing with non-dipolar molecules, we have to consider only the parameter which, on taking into account anisotropic London dispersional interactions [equation (90a)], is of the form ... 

A feature of London s paper is its emphasis on the zero-point motion of electrons it is the intermolecular correlation of this zero-point motion that is responsible for dispersion forces. London s Section 9 extends the idea of zero-point fluctuations to the interaction of dipolar molecules. If their moment of inertia is small, as it is for hydrogen halide molecules, then even near the absolute zero of temperature when the molecules are in their non-rotating ground states, there are large fluctuations in the orientation of the molecules and these become correlated in the interacting pair. 

Dispersion Forces, van der Waals postulated that neutral molecules exert forces of attraction on each other that are caused by electrical interactions between three types of dipolar configurations. The attraction results from the orientation of dipoles that may be (1) two permanent dipoles, (2) dipole-induced dipole, or (3) induced dipole-induced dipole. Induced dipole-induced dipole forces between nonpolar molecules are also called London dispersion forces. Except for quite polar materials, the London dispersion forces are the more significant of the three. For molecules the force varies inversely with the sixth power of the intermolecular distance. 

In the last 40 years, techniques to directly measure surface forces and force laws (force vs. separation distance between surfaces) have been developed such as the surface forces apparatus (SFA) [6] and AFM. Surface forces are responsible for the work required when two contacting bodies (such as an AFM tip in contact with a solid surface) are separated from contact to infinite distance. Although the physical origin of all relevant surface forces can be derived from fundamental electromagnetic interactions, it is customary to group these in categories based on characteristic features that dominate the relevant physical behavior. Thus, one speaks of ionic (monopole), dipole—dipole, ion—dipole interactions, electrostatic multipole forces (e.g., quadrupole), induced dipolar forces, van der Waals (London dispersive) interactions, hydrophobic and hydrophilic solvation, structural and hydration forces,... 

The values in Table 2.3 indicate that the most important contribution to van der Waals interactions results from the London dispersion interactions. Keesom dipolar orientation interactions are only operative for strongly polar and hydrogen-bonding substances such... 

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