Molecular dipole vectors, orientation

For long-chain molecules there are different geometric possibilities for the orientation of molecular dipole vectors with respect to the backbone. Following the notation of Stockmayer (1967), polymers are classified as type A (with dipoles fixed parallel to the mainchain, e.g., ds-l,4-polyisoprene and polyethers), type B [with dipole moments rigidly attached perpendicular to the mainchain e.g., poly (vinyl acetate) and most synthetic polymers], or type C [with a more-or-less flexible polar sidechain e.g.,poly(n-alkyl methacrylate)s]. However, a polymer possessing only one type of dipole moment is an exceptional case. The timescale (speed) of each polarization (and subsequent relaxation) process will determine whether this process will be monitored by a particular dielectric technique. Characteristics and fundamental peculiarities of relaxations generally found in polymers are discussed hereafter. Note that cases where finite polarization is present even in the absence of an external field (e.g., the permanent polarization in ferroelectrics) are not considered. 

The angles ot, p, and x relate to the orientation of the dipole nionient vectors. The geonieti y of interaction between two bonds is given in Fig. 4-16, where r is the distance between the centers of the bonds. It is noteworthy that only the bond moments need be read in for the calculation because all geometr ic features (angles, etc.) can be calculated from the atomic coordinates. A default value of 1.0 for dielectric constant of the medium would normally be expected for calculating str uctures of isolated molecules in a vacuum, but the actual default value has been increased 1.5 to account for some intramolecular dipole moment interaction. A dielectric constant other than the default value can be entered for calculations in which the presence of solvent molecules is assumed, but it is not a simple matter to know what the effective dipole moment of the solvent molecules actually is in the immediate vicinity of the solute molecule. It is probably wrong to assume that the effective dipole moment is the same as it is in the bulk pure solvent. The molecular dipole moment (File 4-3) is the vector sum of the individual dipole moments within the molecule. 

Another means of measuring the properties of insoluble films at the air-water interface is through the use of surface potentials. Surface potential (AF) measures the charge separation created by the vector component of the surfactant s molecular dipole that is perpendicular to the air-water interface. Thus, the surface potential yields information about the orientation of the surfactant molecules. Surface potential values are often expressed alternatively as surface dipole moments /i according to (2), where n is the... 

The second problem of interest is to find normal vibrational frequencies and integral intensities for spectral lines that are active in infrared absorption spectra. In this instance, we can consider the molecular orientations, to be already specified. Further, it is of no significance whether the orientational structure eRj results from energy minimization for static dipole-dipole interactions or from the competition of any other interactions (e.g. adsorption potentials). For non-polar molecules (iij = 0), the vectors eRy describe dipole moment orientations for dipole transitions. 

Dipolar ions like CN and OH can be incorporated into solids like NaCl and KCl. Several small dopant ions like Cu and Li ions get stabilized in off-centre positions (slightly away from the lattice positions) in host lattices like KCl, giving rise to dipoles. These dipoles, which are present in the field of the crystal potential, are both polarizable and orientable in an external field, hence the name paraelectric impurities. Molecular ions like SJ, SeJ, Nf and O J can also be incorporated into alkali halides. Their optical spectra and relaxation behaviour are of diagnostic value in studying the host lattices. These impurities are characterized by an electric dipole vector and an elastic dipole tensor. The dipole moments and the orientation direction of a variety of paraelectric impurities have been studied in recent years. The reorientation movements may be classical or involve quantum-mechanical tunnelling. 

Thus, if van der Waals forces are responsible for the stability of the caffeine-pyrogallol complex we would expect the molecules to be oriented as in V. Bearing in mind, however, the uncertainty as to the exact orientation of caffeine s dipole vector, the superposition of molecular planes might in the final analysis not be so different from I. In that case one would have to look to other evidence to establish whether or not charge transfer forces were involved. 

A molecular dipole moment is the vector sum of the individual bond dipole moments. Molecular dipole moments are not easy to predict because they depend on the bond angles and other factors that vary with the specific molecule. Table 6-1 lists the experimentally measured dipole moments of the halogenated methanes. Notice how the four symmetrically oriented polar bonds of the carbon tetrahalides cancel to give a molecular dipole moment of zero. 

When a polar solvent is placed in a changing electrical field, the molecules must realign so that their dipole vectors maintain the orientation corresponding to minimum energy. Because of intermolecular forces, this process does not occur infinitely fast but on a time scale which depends on the properties of the medium and which is usually on the order of 1-100 ps. Dielectric relaxation experiments provide very useful information about molecular motion in polar liquids and the ability of the solvent molecules to respond to changing electrical conditions. 

If the molecular dipole moment is oriented perpendicularly to the long molecular axis, a material with negative Ae is obtained [31b]. For steric reasons simple lateral monofluorination of an aromatic moiety within the mesogenic core structure does not result in a perpendicular orientation of the molecular dipole moment. This can be achieved only by pair-wise lateral difluorination, which results in mutual cancellation of the respective longitudinal components of the dipole moment vector leaving only the perpendicular contribution. Examples of the most commonly used dielectrically negative materials based on this concept are listed in Table 4.10. 

In these wave packet simulations, the molecular axis of the FHF system is assumed to be aligned along the space-fixed axis Z electric field vector. This assumption involves a maximum interaction of the IR and UV laser pulses with the system. Recalling that the time-dependent interaction potential is given by the scalar product of the electric field vector and the dipole vector, i.e. (t) /j, cos 9, it is clear that for field polarizations perpendicular to the molecular axis [9 = 90°) the interaction of the IR laser pulse with the anion vanishes, and for any molecular orientation different from 0= 0° or 180° the interaction is less efficient. Consider now an ensemble of randomly oriented FHF molecules, as in Fig. 4.13(c). Since the UV pulse is tuned to match the energy gap between anion and neutral... 

Fig. 16. Orientational distribution of the molecular dipole moment on Pt(lOO) (left) and Hg(IIl) (right), cos fi is the angle between the water dipole vector and the surface normal that points into the water phase. Panels a to p on the left are sampled from the distance intervals which are indicated by the cuts through the density profile p(z)/p on the right.

A most widely used method for the prediction of molecular dipole moments is the empirical vector addition of bond moments (45). It involves the concept of point dipoles situated in the directions of the individual bonds artd assumes that the magnitudes of the dipoles are transferable from one molecule to another. For allenes a somewhat modified approach has been established which is of relevance in connection with the discussion of influences of electrostatic field effects on spectroscopic properties of allenes (24 25) (Section III.D). This model uses fixed origines and Hxed orientations for all the different (point dipole) bond moments in substituted allenes. As the origins of the point dipoles the positions of the hydrogen atoms in allene (11) have been used (24). The directions of the dipole moments are assumed to make an angle of 40 with the C==C=C axis (Fig. 6) which represents an average value of experimental directions of dipole moments in differently substituted allenes (24). 

In general, IR absorption is caused by the interaction between the IR electric field vector and the molecular dipole transition moments related to the molecular vibrations. Absorption is at a maximum when the electric field vector and the dipole transition moment are parallel to each other. In the case of perpendicular orientation, the absorption is zero. Directional absorptions are measured using polarised light. The terms parallel and perpendicular refer to the orientation of the polarised beam with respect to a reference axis. For deformation studies, the reference axis corresponds to the stretching direction. 

Infrared spectroscopy is particularly applicable to the study of orientation in polymers [1, 6, 8]. Orientation can be observed in infrared spectroscopy because infrared absorbance is due to the interaction between the electric field vector and the molecular dipole transition moments due to molecular vibrations. 

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