Orientation accidental

Note here that in some instances, even when the molecule has lower symmetry, the value of E can be so small as to be indistinguishable from zero, especially with a randomly oriented sample. In that case again, only four of the expected six lines may be observed. With this caution in mind, we can see that a non-zero E value may be interpreted confidently as indicative of a carrier with low symmetry, but the converse, an approximately zero value, could be due to true symmetry, or to an accidental equivalence of two axes. We return to this point in the discussion... 

The frictional coefficient of an asymmetric particle depends on its orientation. At low velocities such particles are in a state of random orientation through accidental disturbances, and the resistance of the liquid to their motion can be expressed in terms of a frictional coefficient averaged over all possible orientations. For particles of equal volume the frictional coefficient increases with increasing asymmetry. This is because, although the resistance of the liquid is reduced when the asymmetric particle is end-on to the direction of flow, it is increased to a greater extent with side-on orientations, so that on average there is an increase in resistance. The frictional coefficient is also increased by particle solvation. 

A TP s molecular orientation can be accidental or deliberate. Accident can occur during the processing where unwanted excessive frozen-in stresses develop, however with the usual proper process control, there is no accidental orientation. The frozen-in stresses with certain TPs can be extremely damaging with products being subjected to environmental stress cracking or crazing in the presence of heat, chemicals, etc. 

A molecule which is a symmetric top on account of its symmetry (accidentally symmetric tops are not considered), exhibits two types of normal vibrations, because the oscillating dipole moment may either be oriented parallel to the top axis or perpendicularly to it. The infrared selection rules for a so-called parallel band are... 

The solvent surrounding a polar molecule polarizes itself generating a reaction field at the chromophore position [51, 52]. The electronic polarization of the solvent is very fast and, as such, only results in a renormalization of the electronic states [84]. The slow orientational component of the solvent polarization, as occurring in polar solvents instead plays essentially the same role as internal vibrations, the main difference being that the relevant solvation coordinate is a very slow, actually overdamped coordinate [74, 85]. The similarity of polar solvation and vibrational coupling is not accidental in pp chromophores molecular vibrations induce a flux of electronic charge back and forth between the D and A sites and hence plays exactly the same role as an electric field [86]. Much as with the reaction field, the amplitude of the oscillations self-consistently depends on the molecular polarity. 

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