Optical activity, defined

Quantitative measurements of optical activity are usually expressed in terms of the specific rotation, defined as... 

A molecule that contains just one chiral carbon atom (defined as a carbon atom connected to four different groups also called an asymmetric or stereogenic carbon atom) is always chiral, and hence optically active. As seen in Figure 4.1, such a molecule cannot have a plane of symmetry, whatever the identity of W, X, Y, and Z, as long as they are all different. However, the presence of a chiral carbon is neither a necessary nor a sufficient condition for optical activity, since optical activity may be present in molecules with no chiral atom and since some molecules with two or more chiral carbon atoms are superimposable on their mirror images, and hence inactive. Examples of such compounds will be discussed subsequently. 

Point defects were mentioned in a prior chapter. We now need to determine how they aiffect the structure auid chemical reactivity of the solid state. We will begin by identifying the various defects which can arise in solids and later will show how they can be manipulated to obtain desirable properties not found in naturally formed solids. Since we have already defined solids as either homogeneous and heterogeneous, let us look first at the homogeneous t5 e of solid. We will first restrict our discussion to solids which are stoichiometric, and later will examine solids which can be classified as "non-stoichiometric", or having an excess of one or another of one of the building blocks of the solid. These occur in semi-conductors as well as other types of electronically or optically active solids. 

A second kind of electronic defect involves the electron. Let us suppose that the second plane of the cubic lattice has a vacancy instead of a substitutional impurity of differing valency. This makes it possible for the lattice to capture and localize an extraneous electron at the vacancy site. This is shown in the following diagram. The captured electron then endows the solid structure with special optical properties since it ean absorb photon energy. The strueture thus becomes optically active. That is, it absorbs light within a well-defined band and is called a "color-center" since it imparts a specific color to the crystal. 

Enantiomer A single form of an optically active compound. Optically active compounds usually (but not exclusively) contain one or more chiral centres. Enantiomers are defined by their ability to rotate the plane of beam of polarised light one way or the other and these are referred to as either D or L , or alternatively + or, depending on whether the polarised light is rotated to the right (Dextro) or the left (Levo). 

The planar C2h and C2V geometries of the 1,3-butadiene moiety are achiral structures and obviously they cannot show optical activity (i.e. ORD and CD). This has, of course, a spectroscopic origin. The optical activity of a transition Pq — Pi is determined by its Rotational Strength (R)1 defined as the scalar product... 

In the case of synthetic optically active polymers, the intuitive meaning of a CD signal intensity is very similar to that of UV spectroscopy, with the additional dimension of the subtracted absorption between left and right circularly polarized light.37 Absorption of light obeys the Beer-Lambert law, and thus CD intensity is defined as Ae = eL - eR = (AL - AR)/cl, where Ae is the... 

Fischer s Convention. Initially, the absolute configurations of optical isomers were unknown to chemists working with optically active compounds. Emil Fischer, the father of carbohydrate chemistry, decided to relate the possible configurations of compounds to that of glyceraldehyde of which the absolute configuration was yet unknown but was defined arbitrarily. 

The method although simple is veiy limited in its application because it is difficult to find conditions under which well defined and separate crystals are formed. Further the method is too tedious and time consuming and is now only of historical importance because it was first employed by Pasteur for resolution while he was still 26 years of age. Therefore, it was the first experiment in the history of science that gave optically active compound from an inactive one. 

Define plane-polarized light, optical rotation, optical activity, asymmetric carbon atom, enantiomers, racemic mixture, polarimeter, and specific rotation. 

Lord Kelvin lla> recognized that the term asymmetry does not reflect the essential features, and he introduced the concept of chiralty. He defined a geometrical object as chiral, if it is not superimposable onto its mirror image by rigid motions (rotation and translation). Chirality requires the absence of symmetry elements of the second kind (a- and Sn-operations) lu>>. In the gaseous or liquid state an optically active compound has always chiral molecules, but the reverse is not necessarily true. 

The (I ,S)-nomenclature still reminds the user of the right and left handed helical pattern arising from Fresnel s 29> interpretation of optical activity. These patterns are characterized by the combination of a translational and a rotational direction. The Ta skeletal symmetry of tetracoordinate systems submits itself to the pictorial models not applicable to other configurational types. The CIP rules may as well be used to define a configurational nomenclature on the basis of the Fischer projection. If one specified that in such a projection of an (R)-... 

The enzyme activity was assayed by measuring the production of optically active mandelonitrile synthesized from benzaldehyde and cyanide. The standard assay solution contained 300 gmo citrate buffer (pH 3.5-6.0), 50 /rmol of benzaldehyde, 100 /rmol potassium cyanide and 100 jA of the enzyme in a final volume of 1.0 mL. The reaction was started by an addition of 100 fx of the enzyme solution and incubated at 25 °C for 1-120 min. Aliquots (100 jiY) were withdrawn at various reaction times and the reaction was stopped by the addition of 0.9 mL of organic solvent (9 1 hexane iso-propanol by volume). The mandelonitrile formed was extracted and the supernatant, obtained by centrifugation (15,000 x g, 1.0 min at 4 °C), was assayed by HPLC. A blank reaction was also performed without enzyme and the amount of mandelonitrile obtained was deducted from the biocatalyzed reaction product. One unit of the enzyme is defined as the amount of the enzyme that produces 1 /imol of (R)-mandelonitrile under reaction conditions in 1 min. 

We can see why a compound with chiral centres should end up optically inactive by looking again at the eclipsed conformer. The molecule itself has a plane of symmetry, and because of this symmetry the optical activity conferred by one chiral centre is equal and opposite to that conferred by the other and, therefore, is cancelled out. It has the characteristics of a racemic mixture, but as an intramolecular phenomenon. A meso compound is defined as one that has chiral centres but is itself achiral. Note that numbering is a problem in tartaric acid because of the symmetry, and that positions 2 and 3 depend on which carboxyl is numbered as C-1. It can be seen that (2R,3S) could easily have been (3R,2S) if we had numbered from the other end, a warning sign that there is something unusual about this isomer. 

Certain alkaloids are able to effect asymmetric induction during a reduction process at a mercury cathode even when present in low concentration in an aqueous alcohol acetate buffer. Asymmetric induction under these conditions was first observed [39] during the conversion of 4-methylcoumarin to 4-methyl-3,4-dihydro-coumariit (sec page 60). Induction results because a layer of alkaloid is strongly adsorbed on the electrode surface thus permitting transfer of a proton to a carban-ion intermediate m an asymmetric environment. Up to 16% asymmetric induction has been achieved in 1-phenylethanol recovered from reduction of acetophenone in a buffer of pH 4.8 containing a low concentration of quinidine. lire pinacol formed simultaneously shows no optical activity. However quinidine is itself reduced at the potential employed so that the actual catalyst for the asymmetric process is not defined [34,40],... 

With respect to an enzyme, the rate of substrate-to-product conversion catalyzed by an enzyme under a given set of conditions, either measured by the amount of substance (e.g., micromoles) converted per unit time or by concentration change (e.g., millimolarity) per unit time. See Specific Activity Turnover Number. 2. Referring to the measure of a property of a biomolecule, pharmaceutical, procedure, eta, with respect to the response that substance or procedure produces. 3. See Optical Activity. 4. The amount of radioactive substance (or number of atoms) that disintegrates per unit time. See Specific Activity. 5. A unitless thermodynamic parameter which is used in place of concentration to correct for nonideality of gases or of solutions. The absolute activity of a substance B, symbolized by Ab, is related to the chemical potential of B (symbolized by /jlb) by the relationship yu,B = RTln Ab where R is the universal gas constant and Tis the absolute temperature. The ratio of the absolute activity of some substance B to some absolute activity for some reference state, A , is referred to as the relative activity (usually simply called activity ). The relative activity is symbolized by a and is defined by the relationship b = Ab/A = If... 

For an organic material to show electro-optical activity it must exhibit high second order non-linear behaviour. This is one of the terms in the equation that defines the polarisation (u) of an organic molecule as shown below, where E is the perturbing electric field. 

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