Oriented molecules isolation

A question which may sometimes be asked is this If an enantio-morphous crystal- -that is, one possessing neither planes, nor inversion axes, nor a centre of symmetry—is dissolved in a solvent, does the solution necessarily rotate the plane of polarization of light The answer to this question is, Not necessarily . If the molecules or ions of which the crystal is composed are themselves enantiomorphous, then the solution will be optically active. But it must be remembered that enantiomorphous crystals may be built from non-centrosymmetric molecules which in isolation possess planes of symmetry—these planes of symmetry being ignored in the crystal structure such molecules in solution would not rotate the plane of polarization of light. (A molecule of this type, in isolation, may rotate the plane of polarization of light (see p. 91), but the mass of randomly oriented molecules in a solution would show no net rotation.) An example is sodium chlorate NaC103 the crystals are enantiomorphous and optically active, but the solution of the salt is inactive because the pyramidal chlorate ions (see Fig. 131) possess planes of symmetry. 

Along the path from isolated, oriented molecules to excitons in non-doped crystals, we next take up the triplet states of oriented dimers, which we called mini-excitons in Sect. 6.4. Mini-triplet-excitons are excitations of triplet states which are spatially distributed over exactly two molecules, for example the two A and B molecules in the unit cell of Fig. 7.11, and they are localised there. In principle, mini-excitons can also be localised on a pair of molecules with the same orientation in crystals with a dimeric structure, e.g. in the a-perylene crystal (Fig. 2.12). We will however restrict ourselves in this section to the treatment of A - B mini-excitons. 

For isolated atoms, the polarisability is isotropic - it does not depend on the orientation of fhe atom with respect to the applied field, and the induced dipole is in the direction of the electric field, as in Equation (4.51). However, the polarisability of a molecule is often anisotropic. This means that the orientation of the induced dipole is not necessarily in the same direction as the electric field. The polarisability of a molecule is often modelled as a collection of isotropically polarisable atoms. A small molecule may alternatively be modelled as a single isotropic polarisable centre. 

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. 

The calculations so far have been concerned with the permanent charge distribution which the molecule would have when isolated, and which would be observed in its dipole moment. The factor that actually determines the orientation of the entering group, however, is the charge distribution at the instant of attack, which differs from the permanent charge distribution in consequence of polarization by the entering group. In most cases it is not necessary to take this latter effect into account explicitly, since it represents only a small correction. However, in certain cases it is apparently of decisive importance.7... 

A mass spectrometer provides an example of a molecular beam, in this case a beam of molecular ions. Molecular beams are used in many studies of fundamental chemical interactions. In a high vacuum, a molecular beam allows chemists to study the reactions that take place through specifically designed types of collisions. For example, a crossed-beam experiment involves the intersection of two molecular beams of two different substances. The types of substances, molecular speeds, and orientations of the beams can be changed systematically to give detailed information about how chemical reactions occur at the molecular level. Chemists also have learned how to create molecular beams in which the molecules have very little energy of motion. These isolated, low-energy molecules are ideal for studies of fundamental molecular properties. 

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