Instrumentation chiroptical measurements

The availability of commercial instrumentation in the 1960s led to numerous investigations aimed at the determination of chiral absolute structures through the measurement of ECD as well as associated advances in the chiroptical (chiral optical) theory for metal complexes. The relationships between chiral structure and ECD for inert cobalt(III) and chromium(III) complexes with d6 and d3 configurations have... 

Conformational changes are easily followed by optical rotation (Hui and Neukom, 1964). Circular dichroism spectroscopy (CD) of polysaccharides (Morris, 1994) exploits optical anisotropy. In a CD instrumental design, the clockwise and counterclockwise rotation of two polarized beams of equal intensity, traversing a 180° path through a chiroptical medium, display a molar ellipticity maximum and minimum. CD is the differential measurement as a function of X. By CD spectroscopy, mixed interchain association rather than nonspecific incompatibility or exclusion was identified as the molecular basis of alginate-polyguluronate interaction (Thom et al., 1982). 

Current instruments allow CD measurements not only to be performed in the vacuum-ultraviolet (vacuum-UV) region X < 190 nm), but also in the infrared (IR) spectral region. This means that not only chiral absorption effects related to excitations of molecular electronic subsystems are amenable to experimental observations, but also effects involving excitations of the nuclear subsystems of molecules ( vibrational circular dichroism VCD) Recently, results of VCD experiments with cyclopropanes were published. Therefore, in the present chapter the discussion of chiroptical properties of cyclopropanes can include vibrational circular dichroism. Hence, the discussions of chiroptical properties of cyclopropanes will cover the spectral range extending from the vacuum-ultraviolet to the infrared region. 

The basic instrumental needs for chiroptical methods are virtually the same as for other spectroscopic methods, namely, a stable unpolarized illuminating source of sufficient intensity, a wavelength-selection device, sample holder, and detector polarizing elements are essential. Because the only parameter measured in polarimetry and ORD is rotation, the polarizing elements are common to both. A monochromatic source, such as an Na or Hg lamp, is all that is required for polarimetry. Deuterium or halogen lamps are of sufficient intensity for ORD, but highly intense (150 50 W) Xe arc lamps are needed for CD. 

Commercial instrumentation is available for both the ECD and VCD experiments and the commercial ROA spectrometer was introduced last year. To measure chiroptical spectra is relatively easy, provided that a good laboratory routine is worked out and consistently maintained. Electronic CD is easily measurable from 175 nm to more than 1 pm (see Section 8.2.1), and custom-built instruments can go even further. At present, VCD can be measured from 4000 to about 900 cm, routinely from 2000 cnr to about 900 cm (see Section 8.2.2) measurements were reported below 600 cnr [39] and in the near IR up to 6150 cm [40, 41]. The ROA spectra can be reasonably obtained from 2000 cm up to a region close to the Rayleigh line [42]. 

Although the Cotton effects of a-amino acids can be measured directly with up-to-date ORD/CD equipment, more often it is advantageous to form chromophoric derivatives or complexes whose chiroptical properties reflect the absolute configuration of the a-carbon atom. This is especially important when adequate instrumentation is not available or chromophoric substituents other than COOH or NH2 are present. For instance, in the case of aromatic amino acids, more complex chirospectral patterns have to be considered which might prevent the unequivocal assignment of absolute stereochemistry. Also, minor optically active impurities might interfere with spectral data in the 200-250-nm area. 

The very first systematic chiroptical analyses have been obtained with ORD measurements in the 1950s and 1960s. The dominance of the ORD analyses has been broken by commercial CD instruments in the 1960s when CD and ORD were measurable with an equivalent accuracy. Since that time ORD is just of interest for selected problems. It should be mentioned here that a new development based on the invention of new numerical quantum mechanical methods, which allows calculating [ajS with a sufficient accuracy, may lead to a situation in which even CD, VCD, and ORD measurements can be avoided. Here, in order to avoid experimental artifacts by association, etc., [a]5 has to be extrapolated with c- 0 section on Quantum me-... 

VCD instruments, based also on the principles depicted in Figure 13, are in comparison to the ECD instruments, relatively new products on the market. Thus, there have been further developments. Two of them should be mentioned, namely, the use of the Fourier transform technology and the use of a second PEM with and without a second polarizing element. This dual polarization modulation (DPM) method increases the quality of the VCD spectra. Also, the Raman spectrometers for ROA, with which the difference of scattered left and right circularly polarized Raman light is measured, have been developed to a standard that allows systematic chiroptical analyses, nowadays. 

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