Polarimetric measurement

Measurement of the optical rotation of optically active compounds. Polarimetric measurements can likewise be used as a method of identifying pure substances, and can also be employed for quantitative purposes. 

Exchange reactions can be sometimes investigated by the techniques of polari-metry, nuclear magnetic resonance and electron spin resonance. The optical activity method requires polarimetric measurements on the rate of racemization in mixtures of d-X (or /-X) and /-Y (or d-Y). 

Usually polarimetric measurements are carried out to find out a given optical antipode and the result is reliable, but the sign of rotation, is not always a direct expression of configuration because rotation, as already stated, depends on nature of solvent, concentration of solution and temperature etc. as the following example shows -... 

This method is based on the polarimetric measurement of the optical activity induced by the KIE in a reaction mixture containing an isotopic quasiracemate, i.e. an approximately 50/50 mixture of the (+)-H and (-)-D substrate or vice versa, as one of the reactants. Variants of the method were independently reported by Bergson et al. (1977), Nadvi and Robinson (1978) and Tencer and Stein (1978). Later the method was successfully applied, particularly by Matsson and co-workers (Matsson, 1985 Hussenius etal., 1989 Hussenius and Matsson, 1990) to determine both primary and secondary KIEs in proton transfer reactions, and by Sinnott and co-workers (Bennet et al., 1985 Ashwell et al., 1992 Zhang et al., 1994) to determine both primary and secondary as well as heavy-atom KIEs for reactions of carbohydrate derivatives. 

As in the case of other chiral compounds, the optical and enantiomeric purity of chiral organosulfur compounds can be determined by various methods (241). The simplest and most common method for the determination of optical purity of a mixture of enantiomers is based on polarimetric measurements. However, this method requires a knowledge of the specific rotation of the pure enantiomer. In the... 

The sensitivity of the polarimeter is low. Therefore, large sample sizes of high chemical purity are required for polarimetric measurements. In addition, the chiral sample must be isolated and purified without accidental enantiomeric enrichment due to fractionation. 

Tozer, W. F., and D. E. Beeson, 1974. Optical model of noctilucent cloud based on polarimetric measurements from two sounding rocket campaigns, J. Geophys. Res., 79, 5607-5612. 

Highly levorotatory polymers were obtained when (58) was polymerized at either —28° or —90°, but no polymerization occurred at —97°. By comparison of the rotations (C ]d —70 to —95° in chloroform) of the polymers formed from (58) with those of the four unmethylated, anomeric methyl o-galactosides, it was concluded that /S-D-galactofuranosyl residues were the preponderant structural units of these polymers. The results of polarimetric measurements during acid hydrolysis of the polymer indicated that pyranosyl residues, also, were probably present. Isolation and characterization of the products from a graded hydrolysis with acid would confirm this contention. 

Routine polarimetric determinations are simple enough. First the polarimeter is balanced to zero degrees with the solvent. Then the solution is placed into the instrument, the instrument is rebalanced, and the angle a read off the scale. Nevertheless, when many measurements are taken, this becomes somewhat tedious. For the assessment of the half-shade field, the operator s eyes must be dark-adapted. Extended work in a darkened room peering through the eyepiece at an almost black field is tiring, The precision of visual polarimetric measurements will tend to increase rapidly at first, as die observer s eyes become adapted, but then it will decrease gradually... 

The optical activities of the chiral oligopropylenes were determined at various wavelengths. Polarimetric measurements were only made for product mixtures from oligomerizations performed with various monomer concentrations and at various reaction temperatures and also for individual fractions of dimers, trimers, and tetramers. The product mixtures were fractionated by distillation over a split tube column (Table XIII). 

The overall consumption rate of the covalent precursor (ks) is determined by HPLC and/or titration measurements this correlates with monomer consumption in propagation. The rate of racemization of optically active 1-phenylethyl chloride (ka) is determined by polarimetric measurements, Racemization is usually faster than solvolysis, confirming that activation is reversible and that internal return may occur before the carbenium ion reacts with an external nucleophile, Racemization requires not only that the C—Cl bond of the covalent precursor is broken, but that the lifetime of the ion pair is long enough for the flat carbenium ion to rotate, such that both sides of the carbenium ion are completely equivalent as shown in Eq. 18. 

A high-intensity white light coupled with a Klett no. 66 filter (640-700 nm) was used by the authors for rotations measured for the Fraunhofer C line. The polarimeter half-shade angle was maintained at 15° for all these polarimetric measurements. 

The equilibrium concentrations of the components of some sugar solutions may be affected by temperature, and the shifts, called thermomutarotations, may be followed by polarimetric measurements. For measurement of thermomutarotations, solutions are equilibrated at room temperature, or above, in a metal-jacketed tube, preferably a silver tube (because of its high heat-conductivity and minimal catalytic effect). A large volume of aqueous alcohol at a low, accurately controlled temperature is pumped through the water jacket. The solution should reach the desired temperature in 3 to 5 minutes. Consecutive readings of optical rotation can then be taken. 

Platinum catalyst (Adams), in oxidation of carbohydrate sulfonic esters, 258 Poisson distribution, 304 Polarimetric measurement, of mutarota-tion, 47,52... 

Sargeson and Searle (39) have followed the spatial changes in substitution reactions of the two pairs of dl isomers in complexes of trien and Co(III), such as [Cotrien (u2, uv, or GG)], where U2 = CU, (H20)2, (N02)2, uv = Cl and H2O and GG = C03. Optical methods such as circular dichroism, and rotary dispersion and absorption were used in addition to polarimetric measurements. Regrettably their paper did not present the synthesis of the enantiomers employed in the work. 

The monosaccharide carriers of Saccharomyces cerevisiae, and also of Kluyveromyces (Saccharomyces) fragilis, appear from polarimetric measurements to have a higher affinity for the a-D anomers of D-glucose, D-mannose, and D-xylose than for the corresponding /3 anomers.178 Intracellularly, a-D-glucopyranose is a slightly better substrate of hexokinase than the /3 anomer.179... 

The principles of polarimetry, which are fundamental for the detection and characterization of chiral molecules, were outlined for molecules that contain a single stereogenic centre. Polarimetric measurements are most commonly carried out at 589.1 nm, the wavelength of the sodium D line. Specific rotation was defined. 

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