Intervening medium, effect

The closest contact distance, do, is normally taken to be 3 A (van der Waals contact of the edges of the donor and acceptor). The value of p is a measure of the effectiveness of the intervening medium in coupling the donor and acceptor [3, 30]. 

Clearly, if one takes a smaller value of D, one gets a higher value of IVj j, for a given distance Rufj. Kirkwood and Westheimer (1938), Westheimer and Kirkwood (1938), and Westiieimer and Shookhoff (1939) indeed argued that one should take a much smaller dielectric constant, since the intervening medium between the two protons more closely resembles a hydrocarbon liquid rather than water. In fact, for any dicarboxylic acid one can define an effective dielectric constant to fit the experimental value of W, by an equation of the form (4.8.13), with Dg being dependent on the proton-proton distance, the type and size of the acid and the solvent. 

In reaching this conclusion we have assumed that no time lag affects the field that establishes the attraction between the particles. We have also considered particles under vacuum so no intervening medium enters the picture. Each of these simplifying approximations has the effect of overestimating the van der Waals attraction between particles at large separations from one another and embedded in a medium. We consider presently the effect of a time lapse between the interaction of a field with two different particles the effect of the medium is discussed in Section 10.8. 

The first difficulty could be explained by observing that water is not homogeneous at a molecular scale [34], The interactions with remote dipoles are screened by the intervening water molecules, because the effective dielectric constant of their interaction is comparable with the macroscopic dielectric constant (a—80). In contrast, the interaction between neighboring dipoles is much less screened, because there is no intervening medium between than. While assuming a constant e for all interactions would predict a vanishing electric field near the surface, the local value of an effective e leads to a net electric field, which can polarize the water molecules above the surface. 

Fig. 1 shows the interference effects that occur when a thin film sample is irradiated on a substrate surface. (To clearly demonstrate the phenomenon, the reflective interference is illustrated here with angular incident light.) The reflection of vertically incident light of a specific wavelength depends on the film thickness, which can then be computed if the refractive indices for the intervening medium, film and substrate are all known [1]. 

We have identified the free-energy increase arising from the formation of unit area of new surface with the surface tension and have pointed out that when the surfaces are immersed in a fluid (liquid or vapour) this free energy is modified (i.e. the surface tension of the approaching surfaces is decreased). One contribution to this effect arises from the screening of London— van der Waals forces by the intervening medium. A second contribution arises from adsorption by the surfaces of one of the components of the fluid phase. 

Many of the available computations on radicals are strictly applicable only to the gas phase they do not account for any medium effects on the molecules being studied. However, in many cases, medium effects cannot be ignored. The solvated electron, for instance, is all medium effect. The principal frameworks for incorporating the molecular environment into quantum chemistry either place the molecule of interest within a small cluster of substrate molecules and compute the entire cluster quantum mechanically, or describe the central molecule quantum mechanically but add to the Hamiltonian a potential that provides a semiclassical description of the effects of the environment. The 1975 study by Newton (28) of the hydrated and ammoniated electron is the classic example of merging these two frameworks Hartree-Fock wavefunctions were used to describe the solvated electron together with all the electrons of the first solvent shell, while more distant solvent molecules were represented by a dielectric continuum. The intervening quarter century has seen considerable refinement in both quantum chemical techniques and dielectric continuum methods relative to Newton s seminal work, but many of his basic conclusions... 

When considering long-range electron transfer in proteins, there has been some discussion of the influence of the intervening medium and in particular of whether the electron passes along a particular path or makes significant use of particular protein side chains. For instance it was postulated that aromatic side chains of proteins play an important role. However a recent study by Moser et of electron transfer in the bacterial photoreaction center reveals no such effects in that case. 

The equation for the total van der Waals interaction between two atoms or molecules [Eq. (4.40)] includes a factor for corrections due to changes in the dielectric characteristics of an intervening medium other than vacuum. That aspect of the theory can be of great importance both quantitatively and qualitatively and has significant ramifications in practical systems. A full discussion of the theoretical aspects of the effects of medium on van der Waals interactions is beyond the scope of this book, but the reader is referred to the work by Israelachvili for further enlightenment. From a practical standpoint, however, several important points arise from an analysis of the dispersion force equation for media of differing dielectric constants. The relevant points include the following ... 

The equations for surface interactions given above were derived for the situation in which the interacting units were separated by a vacuum. Obviously, for practical purposes, that usually represents a rather unrealistic situation. Real life dictates that in all but a few situations, interacting units be separated by some medium that itself contains atoms or molecules that will impose their own effects on the system as a whole. How will the relevant equations be modified by the presence of the intervening medium ... 

One must keep in mind that the preceding discussion was couched in terms of interactions in a vacuum or other inert environment, which is not a very practical situation for most applications. In order to understand real colloidal systems, one must take into consideration the effects of an intervening medium, the continuous phase, on the above interactions. 

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