Dipole moments vibrationally induced

Raman scattering has been discussed by many authors. As in the case of IR vibrational spectroscopy, the interaction is between the electromagnetic field and a dipole moment, however in this case the dipole moment is induced by the field itself The induced dipole is pj j = a E, where a is the polarizability. It can be expressed in a Taylor series expansion in coordinate isplacement... 

A similar condition must be fulfilled for a vibration to be observed in the Raman spectrum. When a molecule is exposed to an electric field, electrons and nuclei are forced to move in opposite directions. A dipole moment is induced which is proportional to the electric field strength and to the molecular polarizability a. A molecular vibration can only be observed in the Raman spectrum if there is a modulation of the molecular polarizability by the vibration. 

Fig. 4.—Depolarization due to the anisotropy of the polarizability. a) In an isotropic molecule the induced dipole moment vibrates in the direction of the electric vector of the light wave.

The surface potential at the interfacial layer is controlled by the surface excess of ions, their charges and polariTation, packing density, and dipole moment or induced dipole moment it can be determined by special techniques, that is, the vibrating plate method, combined with a Kelvin probe. Experimental data together with the most suitable prediction models may include the ion concentration in the bulk or ionic strength, and, when dealing with a monolayer, the ion number density in the monolayer, the monolayer thickness, and ion diameter. 

In our presentation of the atomic polar tensor formulation we shall follow the notation introduced by Person and Newton [33] since it is now generally accepted. The dipole moment changes induced by vibrational distortions are represented as functions of individual atom displacements... 

Measurements of Stark splittings in microwave and radiofrequency spectra allow tliese components to be detennined. The main contribution to tire dipole moment of tire complex arises from tire pennanent dipole moment vectors of tire monomers, which project along tire axes of tire complex according to simple trigonometry (cosines). Thus, measurements of tire dipole moment convey infonnation about tire orientation of tire monomers in tire complex. It is of course necessary to take account of effects due to induced dipole moments and to consider whetlier tire effects of vibrational averaging are important. 

As the isoquinoline molecule reorients in the order listed above, the absorption of infrared radiation by the in-plane vibrational modes would be expected to increase, while that of the out-of-plane modes would be predicted to decrease (in accordance with the surface selection rule as described above). In the flat orientation there is no component of the dipole moment perpendicular to the surface for the in-plane modes, and under the surface selection rule these modes will not be able to absorb any of the incident radiation. However, as mentioned above, infrared active modes (and in some cases infrared forbidden transitions) can still be observed due to field-induced vibronic coupled infrared absorption (16-20). We have determined that this type of interaction is present in this particular system. 

The relative changes in intensity of the vibronic bands in the pyrene fluorescence spectrum has its origin in the extent of vibronic coupling between the weakly allowed first excited state and the strongly allowed second excited state. Dipole-induced dipole interactions between the solvent and pyrene play a major role. The polarity of the solvent determines the extent to which an induced dipole moment is formed by vibrational distortions of the nuclear coordinates of pyrene (Karpovich and Blanchard, 1995). 

Raman and IR spectroscopies are complementary to each other because of their different selection rules. Raman scattering occurs when the electric field of light induces a dipole moment by changing the polarizability of the molecules. In Raman spectroscopy the intensity of a band is linearly related to the concentration of the species. IR spectroscopy, on the other hand, requires an intrinsic dipole moment to exist for charge with molecular vibration. The concentration of the absorbing species is proportional to the logarithm of the ratio of the incident and transmitted intensities in the latter technique. 

Closing naphthyl ring. The induced dipole moment is then chirally disposed in relation to the inducing NH2 dipole moment. This mechanism, referred to as the dynamic polarization model (45), is shown to explain most of the observed VCD intensity in the synunetric NH2 stretching mode, >>nh2- The anisotropy ratio for this VCD band is ca. 10. Since an NH. . . x type hydrogen bond is possible in this molecule, a description based on vibrationally induced charge flow (currents) may also be riuitfiil, similar to that proposed for a-phenyleth-ylamine. Sect. FV-B-2. 

The polarizability tensor of a molecule related the components of the induced dipole moment of the molecule to the components of the electric field doing the inducing. It therefore has 9 components, axx, ctxy, etc., only 6 of which are independent. The theory of the Raman effect shows that a vibrational transition, from the totally symmetric ground state to an excited state of symmetry species F, will he Raman active if at least one of the following direct products contains the totally symmetric representation ... 

C-H stretching motion in CH4 do not induce a dipole moment, and are thus infrared inactive non-totally-symmetric vibrations can also be inactive if they induce no dipole moment. 

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