Chiroptical measurements

Circular birefringence , - and circular dichroism e, - Cr can both be measured as a function of the wavelength A or the wave number v. The two quantities are not mutually independent. If /Il(A) - /Ir(A) is known for A = 0 up to A = , values of f,.(A) - Er(A) for A = 0 up to A = can be derived through a mathematical transformation, and vice versa. In spite of the fact that in practice it is not necessary to have values for the complete infinite interval of A, measurements over a sufficiently large range of values are in general quite difficult, so the transformation is not always possible. 

The sense of rotation is defined in the way it appears to a spectator when viewed against the light source. It is taken as positive if the polarization direction is rotated clockwise and negative if it is rotated counterclockwise. 

The curve a v) or a(k) is referred to as natural optical rotatory dispersion (ORD) and the instrument used for its measurement is a spectropolarimeter. If the optical activity has been induced by an external magnetic field, the 

When used as a means of characterization of a chiral compound, optical rotation is often measured at a single wavelength only (commonly the Na D line at A = 589 nm). Specific rotation [a] gives the rotation (in deg dm ) that is measured for a solution of a concentration of I g/100 cm The molar rotation [a]u is then obtained by multiplying with the relative molecular mass M and dividing by 100 it is commonly expressed in units of deg cm L mol . Then, 

Circular dichroism is measured using a dichrograph, which modulates a monochromatic beam of light between left-handed and right-handed circular polarization. A difference between and k produces an intensity modulation that can be amplified and displayed. A separate measurement of Ct nd r would be much more exacting because the differences between Sicl and rc7 are usually very small. (The optical densities differ by I0 -10 units.) 

CD spectra are often plotted not as ei,(A) — r(A), but rather as a so-called molar ellipticity [d]M(A) which is given by 

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Unfortunately, in most of the previous examples, the extent of the asymmetry-induction was determined by chiroptical measurements (ORD, CD) that gave qualitative and not quantitative information. The NMR chiral shift efficiency of TRISPHAT 8 and other hexacoordinated phosphate anions was therefore considered as an excellent analytical tool to provide accurate measurement of the induced selectivity by NMR spectroscopy. 

Among the spectroscopic methods applicable to polysaccharides, u.v. spectrophotometry is of little value for characterizing heparin, whose main, electronic chromophore (the C02 group) displays a band at 220 nm, that is, in a region where all glycosaminoglycans absorb (also through their N-acetyl chromophores), and where minor proportions of unsaturated or aromatic contaminants cause serious interference.77 With pure heparin preparations, the carboxylate chromophore is most useful for chiroptical measurements, and a semi-quantitative evaluation of the extent of N-acetylation of 2-amino-2-deoxy-D-glucose residues is also possible.78... 

It should be mentioned that in a number of VCD related studies published, including some examples discussed in this chapter, other chiroptical measurements such as optical rotation (OR) and ECD have also been utilized together with VCD. For example, both OR and VCD have been used to uncover the dominant species responsible for the chiroptical responses of glycidol in CDC13 solution in... 

Chiroptical measurements in the chrysanthemic acid series have shown that 1,2,2,3-tetra-alkylcyclopropanes have the same chiralities as, yet Cotton effects opposite to, those previously reported for corresponding 1,3-dialkylcyclopropanes. Magnetic circular dichroism of cyclopropane and c.d. studies of optically active derivatives indicate that configuration interaction is of considerable importance in the low-energy excited states. ... 

The absolute configuration of (-f )-2,3-dihydrotriquinacen-2-one, and of its tetra-hydro-derivative, have been established as 15 by chiroptical measurements of the long wavelength n-n transitions. The Py-double bond is a dominant chiroptical feature of this triquinacene derivative, and the positive Cotton effect of the enhanced n-Ti transition is consistent with the absolute configurational assignment indicated by (9 RR = O). These are the first members of the triquinacene family for which... 

Even if there is some influence both from the protective group and the employed thiol, in essence it is only the a-carbon side chain that really makes the difference in the stabihzation of the corresponding enolate as expected. Indeed, only with stronger organic bases (e.g., l,8-diazabicyclo[5.4.0]undec-7-ene (DBU)), it is possible to racemize the thioesters of N-protected alkyl amino adds in organic solvents. For instance, chiroptical measurements in isopropanol have shown that the ethylth-ioester of N-Boc-phenylglycine racemizes 60 times faster than the corresponding... 

During 1972-3 great concern was felt regarding the correctness of results based on the Bijvoet method of anomalous dispersion of X-rays. Calculations by Tanaka and his co-workers (Chimia, 1972, 26, 271 Chem. Comm., 1973, 21, 22) appeared to indicate a contradiction between Bijvoet determinations for some compounds and ab initio calculations of the absolute configurations of the same compounds based on chiroptical measurements. This was taken as an indication that the Bijvoet treatment was giving the wrong answer. 

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