Dipole moment rigid

The dielectric strength. As, which is proportional to the area under the loss peak, is much lower for the secondary processes, relative to the a relaxation analysed in the next section. This is a common pattern foimd in both polymer materials and glass formers. The P secondary process is even more depleted in linear polymers that contain the dipole moment rigidly attached to the m chmn, such as polycarbonate [78-80] and poly(vinyl chloride) (the behaviour of this polymer was revisited in ref [81] where the secondary relaxation motions are considered as precursors of the a-relaxation motions). Polymers with flexible polar side-groups, like poly(n-alkyl methacrylate)s, constitute a special class where the P relaxation is rather intense due to some coupling vnth main chain motions. 

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. 

Molecules do not consist of rigid arrays of point charges, and on application of an external electrostatic field the electrons and protons will rearrange themselves until the interaction energy is a minimum. In classical electrostatics, where we deal with macroscopic samples, the phenomenon is referred to as the induced polarization. I dealt with this in Chapter 15, when we discussed the Onsager model of solvation. The nuclei and the electrons will tend to move in opposite directions when a field is applied, and so the electric dipole moment will change. Again, in classical electrostatics we study the induced dipole moment per unit volume. 

Note 1. The Free Energy Lost by a Polar Dielectric in an Electrostatic Field. Let Fig. 76 depict a permanent rigid dipole whose axis makes an angle 0 with a uniform field of intensity E. If y is the dipole moment, the potential energy of the dipole in the field is — Ey cos 0. If the dipole is held in this fixed position, any increment dE in the intensity of the field will clearly mean a change in the potential energy of the dipole, equal to — y cos 0 dE. 

Steady-State Fluorescence Depolarization Spectroscopy. For steady state depolarization measurements, the sample is excited with linearly polarized lig t of constant intensity. Observed values of P depend on the angle between the absorption and emission dipole moment vectors. In equation 2 (9), Po is the limiting value of polarization for a dilute solution of fluorophores randomly oriented in a rigid medium that permits no rotation and no energy transfer to other fluorophores ... 

The separation of formal charges in a polar limiting structure like 2b creates a dipole moment of ca. 20 D. Therefore, if such structures were of great importance, quite high dipole moments should be expected for push-pull ethylenes. Data for a reasonable number of mostly symmetrical and rather rigid compounds are known (Table 20). Several high dipole moments are observed, though not in the vicinity of those required for a complete transfer of the double-bond it... 

Parameters required for the description of polyesters I are taken from a recent paper (Abe, A, J. Am. Chem. Soc. 1984, 106, 14) which dealt with the dipole moments of dialkyl esters of dicarboxylic acids. Since the ester groups are all assumed to be in the trans configuration, short-range interactions between consecutive rigid cores are unimportant. As for the rotation around the O-C — C-C bond, the six-state scheme (termed model I in the above paper) is employed. The statistical weight parameter a representing the relative importance of the reversed ester conformations with respect to the normal ones is set equal to unity. The three-state scheme (termed model II) proposed alternatively in the above reference is examined for chains with n = 5 and 6 for comparison. In this model the C O /CC eclipsed form is assumed to be intrinsically more stable than the C 0 /CH form a stabilzation energy E( 1) of 5.0 kj mol-1 is adopted. 

The relaxation behavior of amorphous rigid polymers is studied by using the model PDCMI. Moreover, the conformational characteristics of the polymer are investigated by critically analyzing the dipole moments of the chains. 

The Stake s shift is defined as the difference between the maxima of the luminescence and those of the related absorption spectra. In rigid, non-polar molecules the 0-0 bands are nearly coincident, but in many cases they do not correspond to the maxima of the spectra, so that the Stake s shift is quite large. It increases when the molecular geometry changes substantially between the ground state and the excited state, and increases with solvent polarity when the excited state has a larger dipole moment. 

Compounds with double bonds are important for the theory of dipole moments and dipole moments are of importance for determining structure of compounds with double bonds. There are two reasons for this. All double bonds give some degree of rigidity to the molecule which enables the partial dipole moments to be treated as vectors of known... 

The extremely stable and general sp conformation of esters and carboxylic acids (3a) is one of the remarkable features of stereochemistry. It was observed uniformly2 in all other molecules with the group O—C=0 and also with their heteroatom analogues with S, Se or Te. In contrast with the rigid conformation, the conjugation in the ester molecule, expressed by the formulas 69, is relatively weak and can be just detected in the dipole moment values164. 

Four examples of dipole moments are instructive. First, the dipoles for chloromethane and dichloromethane are 1.87D and 1.60D, respectively. Although two chlorine-carbon bonds are present in the latter, the dipole is not along either but rather bisects the angle between them. This is illustrated schematically using the stylized arrow with its positive end in the form of a cross. The orientation question is shown clearly in the rigid dichlorobenzene framework. The dipole is 2.13D for the ortho-isomer and 0D when the dipoles exactly oppose each other. 

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