Molecular dipole reorientation

Spohr found a significant reduction in the dipole reorientation time for a different model of water (but using the same water/Pt potential). In that paper, the reorientation dynamics are characterized by the spectral densities for rotation around the three principal axes of the water molecule. These calculations demonstrated the hindered rotation of water molecules in the plane parallel to the surface. In addition, a reduction in the frequency of rotation about the molecular dipole for water molecules in the adsorbed... 

The following molecular constants are used in further calculations density p of a liquid the static (es) and optical (n ) permittivity moment of inertia /, which determine the dimensionless frequency x in both HC and SD models the dipole moment p the molecular mass M and the static permittivity 1 referring to an ensemble of the restricted rotators. The results of calculations are summarized in Table XXIII. In Fig. 62 the dimensionless absorption around frequency 200 cm 1, obtained for the composite model, is depicted by dots in the same units as the absorption Astr described in Section B. Fig. 62a refers to H2O and Fig. 62b to D2O. It is clearly seen in Fig. 62b that the total absorption calculated in terms of the composite model decreases more slowly in the right wing of the R-band than that given by Eq. (460). Indeed, the absorption curve due to dipoles reorienting in the HC well overlaps with the curve generated by the SD model, which is determined by the restricted rotators. 

A simple electrostatic model facilitates understanding the physical meaning of the Curie behavior of [Mn(taa)] in the HS phase. Electric polarization P produced by a number of reorienting molecular dipoles /r under a local field Eloc obeys a simple Curie law,... 

In the Marcus nomenclature,41 42 the polarization in polar liquids is u- or e-type. The u-type is due to reorientation of molecular dipoles by an... 

The Models of Debye and Piekara. In 1935 Debye, under the impact of Malsch s measurements," strove to improve his first non-linear theory by including coupling between molecular dipoles. According to him, as wdl as Fowler, the reorientation of a given dipolar molecule by the external electric field is hindered by a field of forces due to the direct environmait... 

Coming to the present volume, one aim has been to provide a basis on which the student and researcher in molecular science can build a sound appreciation of the present and future developments. Accordingly, the chapters do not presume too much previous knowledge of their subjects. Professor Scaife is concerned, inter alia, to make clear what is the character of those aspects of the macroscopic dielectric behaviour which can be precisely delineated in the theoretical representations which rest on Maxwell s analysis, and he relates these to some of the general microscopic features. The time-dependent aspects of these features are the particular concern of Chapter 2 in which Dr. Wyllie gives an exposition of the essentials of molecular correlation functions. As dielectric relaxation methods provided one of the clearest models of relaxation studies, there is reason to suggest that dipole reorientation provides one of the clearest examples of the correlational treatment. If only for this reason, Dr. Wyllie s chapter could well provide valuable insights for many whose primary interest is not in dielectrics. 

A particularly important question involves the understanding of the role of crystal defects in the peculiar electrical behaviour of ice 4. Upon the application of an electric field, the solid becomes polarized by the thermally activated reorientation of the molecular dipoles. Niels Bjerrum postulated the existence of orientational defects, which represent local disruptions of the hydrogen-bond network of ice 4, to explain the microscopic origin of this phenomenon. 

Anisotropic molecules show optically isotropic behavior in the bulk when they are disordered and randomly oriented, for instance in solutions or liquid crystal above the transition temperature. Under the influence of a strong beam, the induced dipole moment of the molecules feels a torque that tends to orient the molecule. The reorientation of the molecular dipoles induces a change in the refractive index. The typical values for molecular susceptibilities and the time-responses vary depending on the type of systems. For small anisotropic molecular systems, x 10 esu, with a time response 10 s. However, in the nematic phase, liquid crystal molecules are strongly correlated, resulting in much higher values, x 10 esu,... 

Figure 8. Rotational reorientation of water molecular dipole direction for 1 = I (Equation 7) (a) "bulk (b) "nonpolar ...

The mixed model of water comprising the librational (LIB) and vibrational (VTB) fractions will be used. We employ four molecular mechanisms a, b, c, and d. The first one, a, refers to the LIB state, in which a permanent dipole reorients in the hat potential formed by tom or weak hydrogen bonds (HB). The last three specific mechanisms (b, c, d), governed by vibrating hydrogen bonds, refer to the VIB state. 

The reorientational dynamics of water molecules has been analyzed by calculating the relaxation times of the molecular dipole vector relative to the laboratory fixed... 

Figure 1.15 Molecular dipoles lend to align parallel to an electric field. Thermal motion will tend to disrupt this reorientation...

In this discussion of dielectric constant the calculation was an equilibrium one and no account was taken of the way in which the equilibrium configuration is achieved. It was only necessary that some mechanism exist for the reorientation of molecular dipoles. In the next section we shall consider the mechanism of dielectric relaxation in detail and from this consideration will come an alternative method for calculating Cg. 

Granicher (1957), which considered only the majority carriers in each region. It is somewhat more difficult to picture the polarization processes in terms of molecular orientations. In the regions where orientational defects provide the relaxation mechanism, their motion in the applied field simply reorients molecules or, if the effective charge associated with the defects is included, then this contributes a polarization in the same direction. In regions where ion states provide the relaxation mechanism, however, motion of these states in the direction of the field tends to orient molecular dipoles antiparallel to the field. The polarizations pro-... 

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