Optical active center

In the second method, which can be applied to compounds with an optically active center near the potentially tautomeric portion of the molecule, the effect of the isomerization on the optical activity is measured. In favorable cases both the rate of racemization and the equilibrium position can be determined. This method has been used extensively to study the isomerization of sugars and their derivatives (cf. reference 75). It has not been used much to study heteroaromatic compounds, although the very fact that certain compounds have been obtained optically active determines their tautomeric form. For example, oxazol-5-ones have thus been shown to exist in the CH form (see Volume 2, Section II,D,1, of article IV by Katritzky and Lagowski). 

Up to this example we have given relatively little consideration to the performance of the catalyst. However, in order to obtain a product with high enantiomeric excess, the ligand used to modify rhodium must be selected with particular care. At a minimum it must contain an optically active center to have any hope of achieving enantiomeric excess [18] It must also show high selectivity towards the branched product, although for styrenes and vinyl naphthalenes this isomer is somewhat favoured on thermodynamic grounds. 

Throughout, an asterisk ( ) denotes a chiral, optically active center. 

Inhomogeneous broadening in solids typically occurs as a result of nonequivalent static distortions in the crystalline environment of an optically active center. As can happen with the paving stones in a floor, the crystal reticules are not perfectly equal there is a distribution of crystalline environments for the absorbing atom, and consequently a distribution of resonance frequencies. 

Solid state lasers are those whose active medium consists of an insulating material activated by an optically active center. Three different types of active center have usually been used as active laser centers rare earth ions, transition metal ions, and color centers (see Chapter 6). 

When designing a new solid state laser system, an appropriate choice of the matrix - active center combination is needed. On the one hand, the active center should display optical transitions in the transparency region of the solid, which consequently requires the use of wide-gap materials. Additionally, the transitions involved in the laser action should show large cross sections in order to produce efficient laser systems. This aspect, which is directly related to the transition probability, is treated in depth in Chapters 5 and 6, where the physical basis of the behavior of an optically active center in a solid is studied. 

We will briefly describe the basis of two spectroscopic methods that are commonly used in the investigation of optically active centers in solids. As we will see, both of them constitute good examples of the usefulness of laser properties in the held of spectroscopy. 

In this chapter, we will treat those centers that produce the appearance of optical bands. This type of center is called an optically active center. We will try to understand how these centers give rise to the appearance of new optical bands (which are not present in the undoped crystal) and to predict their main features (spectral location, intensity, shape, etc.). 

In the previous section we have seen how to determine the energy levels of an optically active center. Optical spectra result from transitions among these energy levels. For instance, an optical absorption spectrum is due to different transitions between the ground energy level and the different excited energy levels. The absorption coefficient at each wavelength is proportional to the transition probability of the related transition. 

Optically active centers may also occur as a result of stractural defects. These defects are usually called colour centers, and they produce optical bands in the colorless perfect crystal. We will also discuss the main features of color centers in this chapter (Section 6.5). From the practical viewpoint, color centers are used to develop solid state lasers. Moreover, the interpretation of their optical bands is also interesting from a fundamental point of view, as these centers can be formed unintentionally during crystal growth and so may give rise to unexpected optical bands. 

So far, we have dealt with optically active centers based on dopant ions, which are generally introduced during crystal growth. Other typical optically active centers are associated with inhinsic lattice defects. These defects may be electrons or holes associated with vacancies or interstitials in ionic crystals, such as the alkali halide matrices. These centers are nsually called color centers, as they prodnce coloration in the perfect colorless crystals. 

Chapters 5 and 6 deal with the spectra of optically active centers. The term optically active center corresponds to a dopant ion and its environment (or to a color center), which produces absorption and/or emission bands that are different to those of the pure crystalline host. This is the case for a large variety of optical materials, such as phosphors, solid state lasers, and amplifiers. 

In Chapter 5, we discuss in a simple way static (crystalline field) and dynamic (coordinate configuration model) effects on the optically active centers and how they affect their spectra (the peak position, and the shape and intensity of optical bands). We also introduce nonradiative depopulation mechanisms (multiphonon emission and energy transfer) in order to understand the ability of a particular center to emit light in other words, the competition between the mechanisms of radiative de-excitation and nonradiative de-excitation. 

Fine and super-fine structure of luminescence lines of Cr and TR are narrower in artificial gems. The reason is that in natural precious stones many optically active centers are usually present, while in artificial ones only one or two occur. 

On the other hand, the simple complexes (hemins I and II) have a reverse relation between the ICD magnitudes in the Soret region and the ligand field strengths of the axial ligands bound to the heme for peptides and proteins. This difference between our models 199) and Urry s findings 196 198> may be partly due to the existence of the ring(s) with optically active center(s). 

Molecules of inheient structural asymmetry aie anisotropic they are optically active and exhibit optical rotation in solution. The typical optically active center is a carbon atom with four different substituents. In addition, any structural dissymmetry that results in a spatial left- and right-handedness will cause optical activity. Compounds of these types of come in a right-hand l R) and left-hand (L) form. When equal amounts of these two forms are mixed (racemic mixtures) there is no optical rotation because the activity of the two forms exactly cancel. Internal compensation of optically active centers m complex molecules is also found. Left- and right-handed optical isomers were first studied by Pasteur well over 100 years ago. and extensive surveys are found in most organic chemical texts. 

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