Polarimetry

In the laboratory, the technique of polarimetry is used to distinguish between enantiomers and to measure the extent to which each enantiomer rotates the plane of plane-polarised light. 

Values of [a] are quoted in British Pharmacopoeia (BP) monographs for chiral drugs and reagents, and limits are set within which drugs of BP quality must comply. The specific optical rotation of a solid is always expressed with reference to a given solvent and concentration. 

The specific optical rotation of a liquid is obtained from equation (4.2), where d = relative density of the liquid. 

Compounds that rotate the plane of polarised light towards the right (clockwise) are called dextrorotatory, while compounds that rotate the plane to the left, or anticlockwise, are called laevorotatory. The direction of rotation is often specified by the symbols (+) for dextrorotatory and (—) for laevorotatory and the direction is considered with the operator facing the light source. 

The specific rotation, [a] , of lactose in solution at equilibrium is -I- 55.4° expressed on an anhydrous basis (+52.6° on a monohydrate basis). The specific rotation is defined as the optical rotation of a solution containing 1 g ml Mn a 1 dm polarimeter tube it is affected by temperature (20°C is usually used indicated by superscript) and wavelength (usually the sodium D line (589.3 nm) is used indicated by subscript). 

The milk sample must first be defatted and deproteinated, usually by treatment with mercuric nitrate (Hg(N03)2). In calculating the concentration of lactose, a correction should be used for the concentration of fat and protein in the precipitate. 

Many complex fluids contain orientable molecules, particles, and microstmctures that rotate underflow, and under electric and magnetic fields. If these molecules or microstructures have anisotropic polarizabilities, then the index of refraction of the sample will be orientation-dependent, and thus the sample will be birefringent. In general, the anisotropic part of the index of refraction is a tensor n that is related to the polarizability a of the sample. The polarizability is the tendency of the sample to become polarized when an electric field is applied thus P = a E, where P is the polarization and E is the imposed electric field. When the anisotropic part of the index of refraction is much smaller than the isotropic part (the usual case), the index-of-refraction tensor n can be related to a by the Lorentz-Lorenz formula  

For simple polymeric liquids, the birefringence tensor n is often proportional to the stress tensor a, a relationship called the stress-optic law (Janeschitz-Kriegl 1983)  

If reactants and products have different optical rotation properties, it is possible to study the transformation by monitoring the optical rotation using a polarimeter. When more than one optically active substance is present in the reaction, their combined optical rotation effect upon the plane of polarised light is observed. The angle of rotation (a) caused by a solution of a single pure compound is given by  

It is difficult to take more than about three readings a minute by visual monitoring using a standard polarimeter, which limits rates to be followed to relatively slow ones. Nowadays, however, a digital polarimeter can be interfaced to a computer, allowing continuous recording and monitoring of faster reactions. 

Polarimetry is extremely useful for monitoring reactions of optically active natural products such as carbohydrates which do not have a useful UV chromophore, and samples for study do not need to be enantiomerically pure. Nevertheless, compared with spectrophotometry, the technique has been applied to relatively few reactions. It was, however, the first technique used for monitoring a chemical reaction by measuring a physical property when Wilhemy investigated the mutarotation of sucrose in acidic solution and established the proportionality between the rate of reaction and the amount of remaining reactant [50]. The study of a similar process, the mutarotation of glucose, served to establish the well-known Bronsted relationship, a fundamental catalysis law in mechanistic organic chemistry. 

The polarimetric method is sometimes useful for the study of exchange reactions in which large rotation changes but no net chemical change is involved 

This technique has been used mainly in solution studies, e.g. of sugar hydro- 

and Kq are the rotations at a time t and zero respectively, then 2 determined from equation (O ) 

The distance through which the analyzer must be turned de- 

In most instruments monochromatic light, corresponding to the D line of the solar spectrum, is used, and the specific rotary power for that ray is expressed by the sign [ ]d. The fact that the rotation is right-handed is expressed by the sign -t-, and that, it is left-handed by the sign —. 

It will be seen from the above formula that, knowing the value of [ ]d for any given substance, we can determine the weight of that substance in a solution by the formula 

The polarimeter or saccharometer is simply a peculiarly constructed polariscope used to determine the value of a. 

The chemical activity of the different colored rays of which the solar spectrum is composed is not the same. Those which are the most refrangible possess the greatest chemical activity—the greatest actinic power. The visible solar spectrum represents only about one-third of the rays actually emitted from the sun. Two-thirds of the spectrum are invisible as light. 

A solution of chiral molecules exhibits optical activity (Sec. 7.1) because there is a net rotation of the plane of polarized light as the light passes through the sample. An explanation of the physical basis for this phenomenon is beyond the scope of this discussion, but it is important to understand that achiral molecules do not exhibit this same property. 

Of primary interest to organic chemists is the fact that each member of a pair of enantiomers rotates plane-polarized light by exactly the same amount 

The sign of a is defined by convention. When you view through the sample toward the source of the plane-polarized light, a clockwise rotation of the plane of the light is taken as positive or dextrorotatory, whereas a counterclockwise rotation is negative or levorotatory. 

Optical rotation represents a physical constant of a chiral compound /the variables listed above are considered. By specifying the temperature and wavelength at which the measurement is taken, and dividing the observed rotation by the factors that define the average number of molecules in the light path, a constant called the specific rotation, [a], is obtained. This is expressed mathematically by Equation 7.7. 

I = pathlength of cell (decimeters, dm) c = concentration (g/mL of solution) d = density (g/mL, neat sample) 

The variations in the optical rotations of aqueous solutions of D-glucose, sucrose, and maltose, etc., on addition of calcium chloride have been investigated. Measurement of the Cotton effects induced by the addition of germanic acid to aqueous solutions of D-fructose in the pH range 6—10 indicated that a 

The optical rotations of a-lactose, jS-lactose, and equilibrated solutions thereof have been measured at 546 and 589 nm between 10 and 35 C. Although the measured optical rotation of an equilibrated solution of lactose showed good agreement with literature values, revised values were reported for the pure a- and j8-forms. 

In a study of the chiroptical properties of some substituted-phenyl glycosides, it was noted that steric and electronic effects associated with orrAo-substituents may change the sign of some of the c.d. bands of the aromatic chromophore.  

Boruch, Zesz. Probl. Postepow Nauk Roln., 1974,159, 205 Chem. Abs., 1975, 83, 10 644r). P. J. Antikainen, Finn. Chem. Letters, 1974, 159 Chem. Abs., 1975, 82, 43 669k). 

Investigations of the o.r.d. and c.d. spectra of laevoglucosenone (457) have attributed the unusually large rotation [ajc - 524°(EtOH) to the Cotton band at 357 nm,  

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It is worth noting that quantitative data on the rate of enolization could be obtained by polarimetry measurements since optically active thiazoline-5-ones are now available. 

Polarimetry. Polarimetry, or polarization, is defined as the measure of the optical rotation of the plane of polarized light as it passes through a solution. Specific rotation [ a] is expressed as [cr] = OcjIc where (X is the direct or observed rotation, /is the length in dm of the tube containing the solution, and c is the concentration in g/mL. Specific rotation depends on temperature and wavelength of measurement, and is a characteristic of each sugar it may be used for identification (7). 

Polarization is the most common method for the determination of sugar in sugar-containing commodities as well as many foodstuffs. Polarimetry is apphed in sugar analysis based on the fact that the optical rotation of pure sucrose solutions is a linear function of the sucrose concentration of the solution. Saccharimeters are polarimeters in which the scales have been modified to read directiy in percent sucrose based on the normal sugar solution reading 100%. 

The specific rotation ia water is [0 ] ° — +66.529° (26 g pure sucrose made to 100 cm with water). This property is the basis for measurement of sucrose concentration ia aqueous solution by polarimetry. 100°Z iadicates 100% sucrose on soHds. 

Many experimental approaches have been appHed to the deterrnination of stabihty constants. Techniques include pH titrations, ion exchange, spectrophotometry, measurement of redox potentials, polarimetry, conductometric titrations, solubiUty deterrninations, and biological assay. Details of these methods can be found in the Hterature (9,10). 

Composition The law of mass aclion is expressed as a rate in terms of chemical compositions of the participants, so ultimately the variation of composition with time must be found. The composition is determined in terms of a property that is measured by some instrument and cahbrated in terms of composition. Among the measures that have been used are titration, pressure, refractive index, density, chromatography, spectrometry, polarimetry, conduclimetry, absorbance, and magnetic resonance. In some cases the composition may vary linearly with the observed property, but in every case a calibration is needed. Before kinetic analysis is undertaken, the data are converted to composition as a function of time (C, t), or to composition and temperature as functions of time (C, T, t). In a steady CSTR the rate is observed as a function of residence time. 

The analytical capability of these matrices has been demonstrated for chiral amines [12, 13]. The procedure is illustrated in Fig. 8-4 for the separation of NapEtNH " CIO . Concentrated methanol/dichloromethane solutions of the racemic mixture were placed on a column containing the chiral macrocycle host. The enantiomers of the ammonium salts were resolved chromatographically with mixtures of methanol and dichloromethane as the mobile phase. The amounts of R and S salts in each fraction were determined by polarimetry. Because the chiral supported macrocycle interacts more strongly with S salts, the R salt passes through the column first and the S salt last, as seen in Fig. 8-4. 

The amount of rotation observed in a polarimetry experiment depends on the number of optically active molecules encountered by the light beam. This number, in turn, depends on sample concentration and sample pathlength. If the concentration of sample is doubled, the observed rotation doubles. If the concentration is kept constant but the length of the sample tube is doubled, the observed rotation is doubled. It also happens that the amount of rotation depends on the wavelength of the light used. 

The widely used technique of light spectroscopy has also been applied to the qual and quant detn of bound N in energetic materials. There are five distinct systems used colorimetry, infrared spectroscopy, polarimetry, Raman spec troscopy and ultraviolet spectroscopy... 

Polarimetry, in which a beam of polarized light is rotated by passage thru an optically active substance, has been applied to the quant detn of sucrose octanitrate (Vol 5, D1643-R Fef 61)... 

Racemization constitutes a special case of opposing first-order reactions. The equilibrium constant is unity, and the opposing rate constants are equal to one another. Racemization can be followed by polarimetry (monitoring the angle of optical rotation) or by circular dichroism (monitoring the ellipticity). The kinetic analysis can be done by either Eq. (3-15) or (3-16). The rate constant for racemization is krac = ke/2. 

The determination of the angle of rotation is called polarimetry. In some cases, it can help a chemist follow a reaction. For example, if a reaction destroys the chirality of a complex, then the angle of optical rotation decreases with time as the concentration of the complex falls. 

The d5Tiamic stereochemistries of M(dtc)3 and [M(dtc)3] (M = Fe, Co, or Rh) complexes have been studied (315). The cobalt complex is non-rigid, but the mechanism of optical inversion could not be determined. The Rh complex is stereochemically rigid up to 200°. The optical inversion of (-l-)546 [Colpyr-dtcla] in chloroform has been studied, by loss of optical activity, by polarimetry (316). 

Quenching and Analyzing. A series of reactions can be set up and each stopped in some way (perhaps by suddenly lowering the temperature or adding an inhibitor) after a different amount of time has elapsed. The materials are then analyzed by spectral readings, titrations, chromatography, polarimetry, or any other method. 

By far the most parsimonious, but nonstatistical, explanation for the observed pattern is that the titrations differ in selectivity, especially as regards basic and acidic impurities. Because of this, the only conclusion that can be drawn is that the true values probably lie near the lowest value for each batch, and everything in excess of this is due to interference from impurities. A more selective method should be applied, e.g., polarimetry or ion chromatography. Parsimony" is a scientific principle make as few assumptions as possible to explain an observation it is in the realm of wishful thinking and fringe science that combinations of improbable and implausible factors are routinely taken for granted. 

In situ polarimetry could be performed using an eight axis SXR polarimeter [87] with multilayer optics mounted immediately behind the experimental chamber. Full determinations of the four Stokes parameters are time consuming. 

Unfortunately, in the VUV region no polarimetry data are available, but calculations indicate the degree of circular polarization achieved by the wiggler may be 80%, estimated to be no worse than 70% delivered at the experimental chamber [95, 96]. In PECD experiments, we have calibrated the polarization state by deduction from cross-comparison of results at a few fixed energies previously studied on the SU5 beamline where accurate polarimetry data was available [36]. Because the horizontal magnetic field array in the insertion device is electromagnetic, fast current reversal to switch left- and right-handed elliptical polarizations is possible, with the usual potential benefit for dichroism measurements. 

The enantiomeric excess (ee) of the hydrogenated products was determined either by polarimetry, GLC equipped with a chiral column or H-NMR with a chiral shift reagent. Methyl lactate and methyl 3-hydroxybutanoate, obtained from 1 and 2, respectively, were analized polarimetry using a Perkin-Elmer 243B instrument. The reference values of [a]o(neat) were +8.4° for (R)-methyl pyruvate and -22.95° for methyl 3-hydroxybutcinoate. Before GLC analysis, i-butyl 5-hydroxyhexanoate, methyl 5-hydroxyhexanoate, and n-butyl 5-hydroxyhexanoate, obtained from 1, 5, and 6, respectively, were converted to the pentanoyl esters, methyl 3-hydroxybutanoate was converted to the acetyl ester, and methyl 4-methyl-3-hydroxybutanoate obtained from 2 was converted the ester of (+)-a-methyl-a-(trifluoromethyl)phenyl acetic acid (MTPA). 

Other diagnostic tests scanning laser polarimetry, confo-cal scanning laser ophthalmoscopy, and optical coherence tomography. 

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