Dipoles of the molecules

The Raman effect arises when a photon is incident on a molecule and interacts with the electric dipole of the molecule. In classical terms, the interaction can be viewed as a perturbation of the molecule s electric field. In quantum mechanics the scattering is described as an excitation to a virtual state lower in energy than a real electronic transition with nearly coincident de-excitation and a change in vibrational energy. The scattering event occurs in 10 14 seconds or less. The virtual state description of scattering is shown in Figure 1. 

The model presented here develops these ideas and introduces features which make the calculation of mixture properties simple. For a polar fluid with approximately central dispersion forces together with a strong angle dependent electrostatic force we may separate the intermolecular potential into two parts so that the virial coefficients, B, C, D, etc. of the fluid can be written as the sum of two terms. The first terms B°, C°, D°, etc, arise from dispersion forces and may include a contribution arising from the permanent dipole of the molecule. The second terms contain equilibrium constants K2, K, K, etc. which describe the formation... 

Problem 6.9 Show the directions of individual bond dipoles and net dipole of the molecule for (a) 1,1-dichloroethylene, (fc) cis- and t/-a s-l,2-dichloroethylene. 

Surface potential, AV, can be expressed (37) as AV = Ku/jl where K is a constant, n is the number of molecules per square centimeter of film, and ft. is the surface dipole of the molecule. Thus AV/n = Kfi, where the term on the left side of the equation, representing the surface potential per molecule (mv. per molecule), is proportional to the surface dipole, /, of the molecule. When A V/n is plotted against mole fraction of the components, deviation from the additivity line indicates ion-ion or ion-dipole interaction between components (42). 

Ion-Ion or Ion-Dipole Interaction. Figure 5b, shows the general characteristics of mixed monolayers in which ion-ion or ion-dipole interaction takes place—e.g., alkyl phosphate-alkyl trimethylammonium, or steric acid—octadecanol monolayers. The average area per molecule may or may not show a deviation from the additivity rule line, depending upon whether the two components form expanded or condensed mono-layers. However, surface potential per molecule must show a deviation from the additivity line since ion-ion or ion-dipole interactions reduce the average surface dipole of the molecules in mixed monolayers (31, 42). These interactions result in a negative deviation in the plot of log < vs. mole fraction (6). 

The detailed geometry of the liquid crystal formed by the Rhodonine family has not been documented to date. However, Figure 5.5.8-2 illustrates the geometries assumed by related chromophores115. (a) shows the configuration assumed by 2,2 cyanine when formed as a liquid crystalline film on water, (b) shows the same liquid crystalline configuration in more detail. Note the distance between the two Nitrogen atoms which form the dipole of the molecule. 

The deformation of temporary or induced dipoles of the molecules due to the displacement of the average positions of the electrons relative to the nuclei of the molecules (electronic polarizability or mean molecular electron polarizability, ae, or so-called deformation polarizability, ao). 

In spite of the deficiencies of this analogy with a simple plane condenser, there is some little evidence that /x does not change much, as the film is compressed and the molecules crowded, unless there is a re-orientation of the molecules of the film.4 If 6 is the angle of tilt of the dipole of the molecule to the vertical, /z the dipole moment, and D the dielectric constant of the equivalent plane parallel condenser which would give the same poten-... 

The course of reactions in surfaoe films may be followed either by surface pressure or by surface-potential measurements the former detects any change accompanied by a marked re-orientation of the molecules, the latter any re-orientation of the dipoles or any change in the total amount of the dipole of the molecule. Usually surface potential reveals more than surface pressure of the course of any reactions proceeding in the films, but both measurements should be made. 

As with the insoluble films, the amount of the surface potential depends on the vertical component of the dipole of the molecules, and therefore on the orientation of this dipole and of the molecules to the surface. All... 

There are cases, however, including the very common one of an air-water surface, where no ions can possibly pass the boundary thermodynamical equilibrium cannot therefore be set up between the water and air, and adsorption potentials (the surface potentials of Chapters II and III) are permanent. The usual method for measuring surface potentials with a radioactive air-electrode does not appreciably disturb the adsorption potentials the gaseous ions are very few and are attracted into the water by image forces so that no double layer, compensating the double layer in the water due to the dipoles of the molecules in the surface film, can build up in the air. 

The crystal packing of ammoniaborane is quite different from that of ethane, caused by the molecular dipole which should also be responsible for the higher calculated density of the B-N compound (ethane 0.719 gem-3 [78] ammoniaborane 1.142 gcnT3 [20]). The arrangement of the ammoniaborane molecules in the crystal lattice produces a polar environment for the molecules, where the local dipole of the molecules is antiparallel to the environmental dipole (Figure 5). 

The classical treatment of nonpolar dielectric materials is expressed by the Clausius-Mossotti equation. Polar materials in nonpolar solvents are better handled by Debye s modification, which allows for the permanent dipole of the molecule. Onsager made the next major step by taking into account the effect of the dipole on the surrounding medium, and finally Kirkwood treated the orientation of neighboring molecules in a more nearly exact manner. (See Table 2-1.) The use of these four theoretical expressions can be quickly narrowed. Because of their limitations to nonpolar liquids or solvents, the Clausius-Mossotti and Debye equations have little application to H bonded systems. Kirkwood s equation has great potential interest, but in the present state of the theory of liquids the factor g is virtually an empirical constant. The equation has been applied in only a few cases. 

Relaxation time r in Eq. (81) is related to the rotatory diffusion coefficient with reject to the axis normal to the dipole of the molecule... 

The slope of the graph is thus a measure of the permanent dipole of the molecule. Debye (1912) showed that... 

The Comparative Molecular Moment Analysis method based on the 3D - geometrical representation of the molecule ealculates different molecular moments with respect to the -> centre of mass, centre of charge and - centre-of-dipole of the molecule [Silver-man and Platt, 1996 Platt and Silverman, 1996],... 

The Electric-Field-Induced Second-Harmonic Generation (EFISHG) technique makes it possible to measure the molecular hyperpolarizability, p, on liquids or molecular solutions. The centrosymmetry of tire solution is broken by applying a DC electric field to induce an average orientation of the molecules due to interactions of the permanent dipoles of the molecules and the electric field. The energy of a dipole with a permanent dipole fi in an electric field E is given by ... 

Instead of following conventional classifications, it would be reasonable to classify the intermolecular forces into three categories on the basis of their origin [2]. The first is the forces caused by the electronic polarization, i.e. van der Waals attraction, such as London dispersion (electronic polarization-electronic polarization) and Debye interaction (dipole-dipole-induced electronic polarization). The second is the forces caused by the electrostatic charges and/or the dipoles of the molecule these forces are based on the molecular structure and are independent of electronic polarizability. And the last category is the forces caused by exchange of elemental particles, such as an electron (covalent bond) and a proton (hydrogen bond). 

In reality there are several molecular vibrations that can couple to an electronic transition although the basic phenomenology is retained, namely the resonant character of the S St) 0-0 transition and the mirror symmetry of the vibronic satellites. In case of a jr—Mt transition the dominant vibrational modes are those of the polymer backbone, notably of the phenyl ring. An example is the absorption and fluorescence of it-conjugated molecules, such as tetracene, in the gas phase [22], In fluid solution there is interaction between the transient dipole of the molecule with the permanent and induced dipoles of the solvent. It gives rise to (i) a bathochromic shift of the spectra, (ii) a Stokes shift between the... 

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