Dissymmetry method

In Example 10.5 we extracted both the molecular weight and the radius of gyration from Ught-scattering data. There may be circumstances, however, when nothing more than the dimensions of the molecule are sought. In this case a simple alternative to the analysis discussed above can be followed. This technique is called the dissymmetry method and involves measuring the ratio of intensities scattered at 45° and 135°. The ratio of these intensities is called the dissymmetry ratio z ... 

The dissymmetry method is useful especially if the instrument does not afford facilities for a wide angular scan of scattered intensities. For large particles the scattering envelope is not symmetrical and, as already indicated in Fig. 1, the forward scatter is larger than that in the backward direction. Hence the dissymmetry Z is greater than unity, where... 

Figure 12. Changes in apparent particle size with pH of batch MMA-MAA copolymer latexes (determined by dissymmetry method of light scattering)...

A dissymmetry method (Z) has been used to characterize polymers by employing a ratio of /45 and /135o (with solvent and depolarization corrections) ... 

Measurement of Rg,z with the KMX-6 photometer using the dissymmetry method described previously (3) yielded values within 5% of those determined with the Brookhaven photogoniometer. For all measurements, Rg,z was independent of concentration in the range measured. The angular dependence of KdR%,c of sample 64-2 is shown in Figure 1. 

Problem 4.19 Using the light-scattering data given in Problem 4.17 for a polystyrene sample with benzene as the solvent determine by the dissymmetry method (a) the molecular weight of the polymer, (b) size of the polymer molecule in solution, and (c) second virial coeflScient for the polymer/solvent system. 

The agreement between the values of M, < r > / and A2 obtained by the dissymmetry method and the Zimm method are seen to be quite satisfactory. 

The dissymmetry method. For any model, the ratio of intensities at two angles symmetrical about the normal enables the particle scattering factor for any angle to be derived. Usually angles of 45° and 135° are taken the ratio of intensities at these angles is known as the dissymmetry ratio (Z). If Z is measured, Pe can be found from tabulated data if the appropriate model is knoton. This method has been used, for example, for hemicellu-loses and glycogen. ... 

LD dissymmetry method in a theta solvent. Less reliable for heterogeneous samples than the former method. 

If both the molecular architectme of a polymer in solution and its molecular weight dispersion are known, the scattering function Px q) is known (in principle) and can be combined with limited angular scattering data to deduce Afw and R. Thus, in the dissymmetry method (7) scattered intensities at 90° and the ratio of intensities at 45 and 135 (or another pair of complementary angles) are measured for each concentration. Then the extrapolated transverse scattering function Kc/R q9o)] and the extrapolated dissymmetry ratio... 

Except for routine characterization of systems of well known character exhibiting low dissymmetry, the dissymmetry method cannot be recommended if a photometer is available that is capable of a full complement of angular readings. Apart from the desirability of determining and R without assumptions, it is... 

This report presents various methods developed primarily at our laboratory for chromatographic resolution of racemates of several pharmaceuticals (e.g., -blockers, NSAIDS, anta-acids, DL-amino acids, Bupropion, Baclofen, Etodolac, Carnitine, Mexiletine). Recently, we developed methods for establishing molecular dissymmetry and determining absolute configuration of diastereomers (and thus the enantiomers) of (/< .S )-Baclofcn, (/d.SJ-Bctaxolol with complimentary application of TLC, HPLC, H NMR, LCMS this ensured the success of diastereomeric synthesis and the reliability of enantioseparation. 

The following discussion is divided into three subsections the ketone chromophore (Section 4.4,1.1.), for which configurational assignments are based on the effect of ring dissymmetry and substitution pattern on the rotatory power of the n-rt transition. For conjugated chro-mophores (Section 4.4.1.2.) it is both the helicity of the chromophore and the vicinal substituent effect that determines the rotatory power of the 71-71 transition. Finally, the versatile stereochemical method, exciton chirality method (Section 4.4.2.), is based on the chiral interaction between the electric dipoles of the allowed transitions in two or more chromophores. 

If the intensity of light scattered by a colloidal dispersion is measured as a function of c and 0, the Zimm method enables us to convert this information into several parameters that characterize the colloid M, B, and Rg. In some situations this is more information than is actually needed. If spatial extension is the only information sought, a simpler method for evaluating it employs the so-called dissymmetry ratio. 

For further discussion of these methods, see Mason, S. F. Molecular Optical Activity and the Clurd Discriminations Cambridge University New York, 1982 Saito, Y. Inorganic Molecular Dissymmetry-, Springer-Veriag Berlin, 1979. 

Compared to the colligative methods light scattering can yield information on a possible dissymmetry of the aggregates. For anisotropic particles the direction of the electric field associated with the incident light may not coincide with the shift of the electron cloud. The intensity of light scattered at (usually) 90° from anisotropic aggregates is increased over the value predicted on the basis of isotropy by the Cabannes factor. 

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