Interactions electric dipole

This is one approach to the explanation of retention by polar interactions, but the subject, at this time, remains controversial. Doubtless, complexation can take place, and probably does so in cases like olefin retention on silver nitrate doped stationary phases in GC. However, if dispersive interactions (electrical interactions between randomly generated dipoles) can cause solute retention without the need to invoke the... 

The dependence on the electrical field can be approximated either analytically, e.g., with a dipole field interaction, or by simulations including an external field. In either case, the electric field would give rise to a correction term, AG (G), to be added to Equation (3.3). As will also be discussed later, this correction will in most cases be small. 

If stabilization of the transition state (TS) is more effective than that of the ground state (GS), this results in enhancement of reactivity as a result of a decrease in the activation energy (Fig. 3.5), because of electrostatic (dipole-dipole type) interactions of polar molecules with the electric field. 

A molecule must have a permanent dipole moment to be micro-wave active. As it rotates, the changing dipole moment interacts with the oscillating electric field of the electromagnetic radiation, resulting in absorption or emission of energy. This requirement means that homonuclear molecules such as H2 are microwave inactive, but heteronuclear molecules such as SO3, S02, NO and, of course, H20 are active. 

Energy transfer probabilities due to multipolar magnetic interactions also behave in a similar way to that previously discussed for multipolar electric interactions. Thus, the transfer probability for a magnetic dipole-dipole interaction also varies with 1 / 7 , and higher order magnetic interactions are only influential at short distances. In any case, the multipolar magnetic interactions are always much less important than the electric ones. 

The energies of the electric quadrupole (Wg) and magnetic dipole (W ) interactions, which determine the hyperfine structure, are calculated as follows [11,20] ... 

Polar interactions between molecules arise from permanent or Induced dipoles existing in the molecules and do not result from permanent charges as in the case of Ionic interactions. Examples of polar substances having permanent dipoles would be alcohols, ketones, aldehydes etc. Examples of polarizable substances would be aromatic hydrocarbons such as benzene or toluene. It is considered that, when a molecule carrying a permanent dipole comes Into close proximity to a polarizable molecule, the field from the molecule with the permanent dipole induces a dipole in the polarizable molecule and thus electrical interaction can occur. It follows that to selectively retain a polar solute, then the stationary phase must also be polar and contain, perhaps, hydroxyl groups. If the solutes to be separated are strongly polar, then perhaps a polarizable substance such as an aromatic hydrocarbon could be employed as the stationary phase. However, to maintain strong polar interactions with the stationary phase (as opposed to the mobile phase) the mobile phase must be relatively non-polar or dispersive in nature. 

Polarizability is the relative tendency of a charge distribution o(r), that is, the electron cloud of an atom or molecule, to be distorted from its normal shape by an external electric field, F, which may be caused by the presence of a nearby ion or dipole. The interaction of an electronic charge distribution with a uniform electric field gives an energy contribution,... 

Figure 3.1 Creation of an oscillating dipole by interaction between the charge cloud of an atom, (a) an oscillating electric field (b) electromagnetic radiation wave.

In this expansion the dipole-dipole term is the most prominent if donor-acceptor distance R is not too small. The dipole-dipole term represents the interaction between the transition dipole moments Md and MA of donor and acceptor molecules, respectively. The square of these transition dipoles is proportional to the oscillator strengths fy> and fA for radiative transitions in the individual donor and acceptor molecules (equation 3.73). Higher order terms such as electric dipole-electric quadrupole, electric-dipole-magnetic dipole, become important at close approach and may be effective in crystals and highly ordered array of chromophores. 

We assume that the molecule-field coupling is dominated by the dipole transition interaction and represents the resonant continuous electric field e(f) in the form... 

We saw in the previous section that a molecule has a net polarity and an overall dipole moment if the sum of its individual bond dipoles is nonzero. One side of the molecule has a net excess of electrons and a partial negative charge (5-), while the other side has a net deficiency of electrons and a partial positive charge (S+). An ion-dipole force is the result of electrical interactions between an ion and the partial charges on a polar molecule (Figure 10.3). 

Neutral but polar molecules experience dipole-dipole forces as the result of electrical interactions among dipoles on neighboring molecules. The forces can be either attractive or repulsive, depending on the orientation of the molecules... 

Movement of an electron from the ground electronic state of a molecule to an excited state creates a momentary dipole, called an electric transition dipole. Thus, associated with each electric transition is a polarization (electric transition dipole moment) that has both direction and intensity which vary according to the nature of the chromophore and the particular excitation. When two or more chromophores lie sufficiently close together, their electric transition dipoles may interact through dipole-dipole (or exciton) coupling. Exciton coupling arises from the interaction of two (or more) chromophores through... 

Van der Waals postulated that neutral molecules exert forces of attraction on each other which 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 dispersion forces between fluctuating dipoles and induced dipoles. (The term dispersion forces arose because they are largely determined by outer electrons, which are also responsible for the dispersion of light [272].) Except for quite polar materials the London-van der Waals dispersion forces are the more significant of the three. For molecules the force varies inversely with the sixth power of the intermolecular distance. 

For oriented samples, the rotation of the plane-polarized light becomes a tensor - that is, the optical rotation becomes directionally dependent - and includes a contribution from the electric dipole-electric quadrupole polarizability tensor, which is traceless and thus vanishes for freely rotating molecules [30], The term arising from these quadrupolar interactions can be expressed as [30]... 

The theory is based on two observations. First, solute-solvent interactions are characterized by dipole-ion interactions, and so are much weaker than the ion-ion interactions between solvent species. Thus, the presence of the solute dipole should not greatly perturb the liquid from the electrostatic structure of the neat liquid. Second, because the ionic liquid is a conductor, the electric field of the solute must be screened by the solvent. This observation has been confirmed... 

If the system is in the presence of a radiation field, then Hs in Eq. (5.12) is augmented by the dipole-electric field interaction HUR [Eq. (2.10)]. The result is the so-called optical Bloch equations. Note that this approach focuses explicitly on decoherence in the energy representation. 

One of the models for the hydration force, the polarization model,5 assumes that the hydration force is generated by the local correlations between neighboring dipoles present on the surface and in water. The macroscopic continuum theory, in which water is assumed to be a homogeneous dielectric, predicts that there is no electric field above or below a neutral surface carrying a uniform dipolar density. However, at microscopic level the water is hardly homogeneous, and the electric interactions... 

The second difficulty can be removed if one assumes that in the vicinity of an interface the water is organized in icelike layers. The electrical interactions between the water dipoles of successive layers lead indeed to an oscillatory behavior of the polarization [35], If the surface is not perfectly flat, of if the water is not perfectly organized in water layers, the statistical average smooth out the polarization oscillations [35], The latter results have been also supported by molecular dynamics simulation, in which the surface dipoles were allowed to move [36], Let us now examine in detail how the correlation between neighboring dipoles occurs. 

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