Van der Waals forces induction

Absorption, metaboHsm, and biological activities of organic compounds are influenced by molecular interactions with asymmetric biomolecules. These interactions, which involve hydrophobic, electrostatic, inductive, dipole—dipole, hydrogen bonding, van der Waals forces, steric hindrance, and inclusion complex formation give rise to enantioselective differentiation (1,2). Within a series of similar stmctures, substantial differences in biological effects, molecular mechanism of action, distribution, or metaboHc events may be observed. Eor example, (R)-carvone [6485-40-1] (1) has the odor of spearrnint whereas (5)-carvone [2244-16-8] (2) has the odor of caraway (3,4). 

The selection of the solvent is based on the retention mechanism. The retention of analytes on stationary phase material is based on the physicochemical interactions. The molecular interactions in thin-layer chromatography have been extensively discussed, and are related to the solubility of solutes in the solvent. The solubility is explained as the sum of the London dispersion (van der Waals force for non-polar molecules), repulsion, Coulombic forces (compounds form a complex by ion-ion interaction, e.g. ionic crystals dissolve in solvents with a strong conductivity), dipole-dipole interactions, inductive effects, charge-transfer interactions, covalent bonding, hydrogen bonding, and ion-dipole interactions. The steric effect should be included in the above interactions in liquid chromatographic separation. 

In any exact calculation, corrections must be applied for the van der Waals forces (Section 51), and for induction effects (Section 41). Both corrections can be partially applied, but the calculation then becomes considerably more complicated. They will not be considered further here, but, neglecting the fact that agreement between calculated and observed heats of formation is not exact, we will try to obtain a general picture of the change of heat of formation when an ion in a compound is replaced by another with different radius, charge and, eventually, different electronic structure. 

Table 2 contains some typical values for these three van der Waals forces for a few atoms and simple molecules. Beginning with the noble gases that are nonpolar and have no induction or orientation forces, we can see that the polarizability increases as the size increases and that the dispersion forces also increase. The symmetrical molecule hydrogen has no permanent dipole and only a small dispersion force between its molecules. 

Van der Waals interactions are noncovalent and nonelectrostatic forces that result from three separate phenomena permanent dipole-dipole (orientation) interactions, dipole-induced dipole (induction) interactions, and induced dipole-induced dipole (dispersion) interactions [46]. The dispersive interactions are universal, occurring between individual atoms and predominant in clay-water systems [23]. The dispersive van der Waals interactions between individual molecules were extended to macroscopic bodies by Hamaker [46]. Hamaker s work showed that the dispersive (or London) van der Waals forces were significant over larger separation distances for macroscopic bodies than they were for singled molecules. Through a pairwise summation of interacting molecules it can be shown that the potential energy of interaction between flat plates is [7, 23]... 

The physisorption of alkanes in zeolites is essentially due to van der Waals forces. The main contributions to the stabilization are the induction energy, roughly speaking the electrostatic interaction of the dipole moment induced by the electric field of zeolite with the framework charge distribution and the... 

Van der Waals forces comprise an attractive part, the induction or London forces, and repulsion between the atoms at short distance. For short contacts, these forces are usually treated according to the atom-atom method, which has recently been extensively reviewed by Pertsin and Kitaigorodsky (1987). This approach is based on empirical parameters. The interaction between two molecules is represented as the sum of all the interactions of all atoms of one molecule with all atoms of the other one. The atom-atom interactions are added as long as the summation converges, usually between 6 and 10 A. Two different analytical forms are usually assumed ... 

Physisorption or physical adsorption, which is a weak binding based on van der Waals forces, e.g. dipole, dispersion or induction forces. These forces are weaker than the intramolecular binding forces of molecular species. Therefore, physi-sorbed molecules maintain their chemical identity. 

The van der Waals forces are the interactive forces between dipoles of molecules. The dispersion force, a type of van der Waals force between dipoles that arises from thermal fluctuation or electric field induction, is always present between molecules. The van der Waals forces (FV(jw) between tip and sample can be estimated by the following equation, if we assume the tip is a sphere with radius, R. 

Surface tension of any fluid can be related to various interaction forces, e.g., van der Waals, hydrogen bonding, dipole, and induction. The above analyses of the alkanes thus provide information about the van der Waals forces only. In other homologous series, such as alcohols, we can expect that there are both van der Waals and hydrogen bonding contributions. We can thus combine these two kinds of homologous series of molecules and analyze the contribution from each kind of interaction. 

Pi-pi stacking interactions (183-185) An interaction between two aromatic groups arising from numerous forces, primary of which are attractive van der Waals forces. There are also contributions from electrostatics, induction, and exchange-repulsion. 

If no ion is present in the system (q, = 0) then the above expression gives the sum of the Keesom-dipolar orientation and Debye-induction contributions to the total van der Waals forces between two molecules. A third contribution to van der Waals forces is also present,... 

Van der Waals interactions between collector molecules and minerals Van der Waals force includes the molecular effects of orientation, induction and dispersion. The dispersion effect is generally the strongest and the induction is the weakest [16]. The relationship between the molecular structure and the van der Waals interaction can... 

The solid material or adsorbent provides a very large surface area and has the ability to absorb chemical substances on its surface through such physical and chemical interactions as (i) Van der Waals forces, (ii) Inductive force, (iii) Hydrogen bonding, (iv) Charge transfer and (v) Covalent bonding. 

Apropos of nomenclature the forces between closed-shell molecules (exchange repulsion, electrostatics, induction, and dispersion) are nowadays usually referred to as van der Waals forces. A stable cluster consisting of closed-shell molecules bound by these... 

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. 

Dispersion Forces, van der Waals postulated that neutral molecules exert forces of attraction on each other that are caused by electrical interactions between dipoles. The attraction results from the orientation of dipoles due to any of (1) Keesom forces between permanent dipoles, (2) Debye induction forces between dipoles and induced dipoles, or (3) London-van der Waals forces between induced dipoles and induced dipoles. Except for quite polar materials, the London-van der Waals dispersion forces are the most significant of the three. For molecules the force varies inversely with the sixth power of the intermo-lecular distance. 

How can electrically neutral and nonpolar molecules form a crystal What are the forces between the molecules with filled orbitals which bind them together in a crystal These forces are called inductive, dispersive, or, as a generic term, van der Waals forces. They are responsible for van der Waals bonding [Ml]. 

The van der Waals forces in the strict sense, also called dispersive forces, are the attractive forces between two neutral, nonpolar molecules, for example anthracene molecules, which thus have no static dipole moments. Were the charge dishibution within the molecules rigid, then there would indeed be no interactions between them. However, due to their temporally fluctuating charge distributions, they also have fluctuating dipole moments and these can induce dipoles in other molecules, compare Fig. 2.3. This results in an attractive force, as we aheady calculated in the section on inductive forces. To distinguish the two cases (of a permanent dipole and fluctuating dipoles), these forces due to fluctuations are also termed dispersive forces. 

Whether the van der Waals forces arise from the zero-point oscillations and their mutual effects, from the orientation of existing permanent moments, or from the induction of new ones, they are all essentially of the nature of dipole attractions—to which multipole interactions of higher order may be added as correction terms. 

There are three primary categories of secondary forces, also called van der Waals forces dispersion forces, induction forces, and dipole forces—from weakest to strongest. These forces are highly sensitive to the distance between the molecules, with an approximate range of action between 3 and 5 A (1 A = 1 x 10 m). This... 

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