Reorientation dipole

The dependence of k on viscosity becomes even more puzzling when the time scale of motion along the reaction coordinate becomes comparable to that of solvent dipole reorientation around the changing charge distribution... 

Hochstrasser R M ef a/1991 Anisotropy studies of ultrafast dipole reorientations Proc. Indian Acad. Sc/. (Chem. Sci.) 103 351-62... 

Data for the pc-Au/DMF + LiC104 interface have been collected by Borkowska and Jarzabek.109 The value of ffa0was found to be 0.27 V (SCE in H20) and the roughness factor / = 1.3 (Table 8). Unlike Hg, Bi, In(Ga), and Tl(Ga) electrodes and similarly to the Ga/DMF interface, the inner-layer capacity for pc-Au in DMF depends weakly on a, and thus the effect of solvent dipole reorientation at pc-Au is less pronounced than at In(Ga), Bi, and other interfaces. 

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... 

At low frequencies, the dipole moments of polymers are able to remain in phase with changes in a strong electric field resulting in low power losses. However, as the frequency increases the dipole reorientation may not occur sufficiently rapid to maintain the dipole in phase with the electric field and power losses occur. 

From frequency dependent dielectric loss measurements, the transitions associated with solvent dipole reorientations occur on a timescale of 10-n -10-13 s. By contrast, the time response of the electronic contribution to the solvent polarization is much more rapid since it involves a readjustment in electron clouds . The difference in timescales for the two types of polarization is of paramount importance in deciding what properties of the solvent play a role in electron transfer. The electronic component of the polarization adjusts rapidly and remains in equilibrium with the charge distribution while electron transfer occurs. The orientational component arising from solvent dipoles must adopt a non-equilibrium distribution before electron... 

The energy spacings between levels associated with solvent dipole reorientations are small, 1-10 cm-1. Since the spacings are well below kBT at room temperature ( 200 cm-1), the contribution of the solvent to the energy 6f activation for electron transfer can be treated classically. The results of classical treatments, where the solvent is modelled as a structureless dielectric continuum, will be discussed in later sections. 

In principle, solvent trapping is also included in the vibrational overlap intergral in equation (25). As noted above, the solvent dipole reorientational motions associated with solvent trapping can be treated as a series of collective vibrations of the medium using approaches devised for treating the collective vibrations of ions or atoms that occur in a crystalline lattice.32-33 However, the problem is mathematically intractable because of the many solvent molecules involved which lead to many normal modes and the existence of a near continuum of energy levels. In addition,... 

A detailed comparative study of dielectric behaviour of smectic and nematic polymers was carried out for polymers of acrylic and methacrylic series, containing identical cyanbiphenyl groups (polymers XI and XII) 137 138>. The difference in structural organization of these polymers consists in a more perfect layer packing of smectic polymer XI (see Chaps. 4.1 and 4.2) with antiparallel orientation of CN-dipoles. This shifts the relaxation process of CN-dipole reorientation to a low frequency region compared to nematic polymer XII. Identification of Arrhenius plots for dielectric relaxation frequencies fR shows that for a smectic polymer the value of fR is a couple of orders lower than for a nematic polymer (Fig. 21). Though the values... 

B. Spectral Function of a Dipole Reorienting in a Local Axi symmetric Potential... 

The depth of any reasonable potential well should of course be finite. Moreover, the recorded spectrum of such an important liquid as water comprises two absorption bands One, rather narrow, is placed near the frequency 200 cm, and another, wide and intense band, is situated around the frequency 500 or 700 cm-1, for heavy or ordinary water, respectively. In view of the rules (56) and (57), such an effect can arise due to dipoles reorientation of two types, each being characterized by its maximum angular deflection from the equilibrium orientation of a dipole moment.20 The simplest geometrically model potential satisfying this condition is the rectangular potential with finite well depth, entitled hat-flat (HF), since its form resembles a hat. We shall demonstrate in Section VII that the HF model could be used for a qualitative description of wideband spectra recorded in water21 and in a nonassociated liquid. 

This section presents the continuation of Section V. In the latter a new model [10] termed the hat-curved model was described, where a rigid dipole reorients in a hat-like intermolecular potential well having a rounded bottom. This well differs considerably from the rectangular one, which is extensively applied to polar fluids. Now the theory of the hat-curved model will be generalized, taking into account the non-rigidity of a dipole that is, a simplified polarization model of water is described here. 

Figure 43. Wideband FIR spectra calculated for the composite hat-curved-cosine-squared potential model (solid lines) dashed-and-dotted lines mark the contribution due to dipoles vibrating in the shallow CS well. Water H20 (a, c, e) and water D20 (b, d, f) at 22.2°C. Absorption coefficient (a-d) and dielectric loss (e, f) in Figs, a, b, e, f, dashed lines refer to the experiment [17, 51, 54]. In Figs, c, d dahsed lines mark the contribution to absorption due to dipoles reorienting in a deep hat-curved well.

Change of g influences insignificantly the resonance line pertinent to the ensemble of rigid dipoles reorienting in a hat-like potential well, since the resonance absorption/loss peaks are usually placed at much higher frequencies than the Debye loss peak. 

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. 

The specific low-frequency dielectric losses are found out in composite films, containing M nanoparticles. It is assumed that these losses are caused by interaction of an electromagnetic field with the dipoles reorientation in the environment connected with tunnel electrons transfer between the nanoparticles or traps of the environment. 

The new interaction is unstable. To reach stability, fluorophore molecules will use some of their energy to reorient the dipole of the microenvironment (solvent and surrounding amino acids). Dipole reorientation is called relaxation. Emission occurs after the relaxation phenomenon. 

RIO. Emission from a nonrelaxed state means that it occurs before the dipole reorientation of the solvent molecules or the dipole of the amino acids of the fluorophore binding site. Emission from a non-polar medium means that the fluorescence occurred after the inducing by the fluorophore of a dipole in the medium. 

When the electric field is higher than the coercitive field strength, the spontaneous polarisation is switched and the dipoles reorientate along the field lines. This process of domain switching for Eei=Ec can be described in three steps i) nucleation of an anti-parallel domain, ii) domain growth and iii) saturation of the polarisation [461, 462]. 

With an alternating current (AC) field, the dielectric constant is virtually independent of frequency, so long as one of the multiple polarization mechanisms usually present is active (see Section 8.8.1). When the dominating polarization mechanism ceases as the frequency of the applied field increases, there is an abmpt drop in the dielectric constant of the material before another mechanism begins to dominate. This gives rise to a characteristic stepwise appearance in the dielectric constant versus frequency curve. For each of the different polarization mechanisms, some minimum dipole reorientation time is required for reahgnment as the AC held reverses polarity. The reciprocal of this time is referred to as the relaxation frequency. If this frequency is exceeded, that mechanism wUl not contribute to the dielectric constant. This absorption of electrical energy by materials subjected to an AC electric held is called dielectric loss. 

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