Chiroptical properties instrumentation

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. 

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. 

Electronic chiroptical phenomena are among the oldest to be noted in chemistry. Understanding of the phenomena required first the introduction of quantum mechanics, and then the advent of computers sufficiently powerful to predict chiroptical properties for real molecules. The availability of excellent commercial instrumentation and pervasive applications in the sphere of biochemistry have maintained the field of ECD research as a mature but healthy one. 

The fundamental requirement for the existence of molecular dissymmetry is that the molecule cannot possess any improper axes of rofation, the minimal interpretation of which implies additional interaction with light whose electric vectors are circularly polarized. This property manifests itself in an apparent rotation of the plane of linearly polarized light (polarimetry and optical rotatory dispersion) [1-5], or in a preferential absorption of either left- or right-circularly polarized light (circular dichroism) that can be observed in spectroscopy associated with either transitions among electronic [3-7] or vibrational states [6-8]. Optical activity has also been studied in the excited state of chiral compounds [9,10]. An overview of the instrumentation associated with these various chiroptical techniques is available [11]. 

There are good reasons to remain optimistic that recent momentum in technological progress in chiroptical instrumentation and in development of new more sophisticated ab initio computational methodologies will continue in the future at faster pace. This will make the calculations of optical activity properties, including CD, a truly indispensable and widely affordable approach in the stereochemical analysis of natural products and their interactions on molecular and supramolecular levels. 

UV detection is used in most chiral analysis by HPLC and other liquid chromatographic modalities. However, some other detectors, such as conductivity, fluorescent and refractive index types, are also used. The choice of detector depends on the properties of the racemic compound to be resolved [41, 144]. Chiroptical detectors, which are based on the principle of polarimetry [145] or circular dichroism [146, 147], are also available. The enantiomer (+)- or (—)-notation is determined by these detectors. Some organochlorine pesticides are not UV-sensitive, and hence they are difficult to detect in liquid chromatography. The detection of these types of pollutant can be achieved by using a mass spectrometry (MS) detector, and therefore LC-MS instruments are now being put on the market for routine use [148, 149]. 

---