Dichroism origin

In the years to follow the key to the measurement of vibrational circular dichroism was the development of photoelastic modulators suitable for work in the infrared spectral region. The first successful measurements of circular dichroism originating from vibrational transitions in the infrared were done by Hsu and Holzwarth (1973) on thin slices of monocrystalline a-NiS04 6 H2O and a-ZnSe04 6 H2O. For this measurements the authors used a normal dispersive IR spectrometer supplemented by a linear polarizer and a photoelastic modulator made from Germanium. 

Aside from the intrinsic circular dichroism originating from the molecular structure, a so-called induced optical activity may result when a molecule is situated in an asymmetric environment. Magnetic circular dichroism, for instance, maybe produced by applying an external magnetic field to an absorbing sample. Adsorption of pigment molecules on nucleic acids or protein molecules may also induce circular dichroism. 

Circular dichroism (CD) is another interesting example of an optical property of the small Au SR clusters. Since the first observation of Schaaff et al. [23,24], several reports have appeared regarding the CD activities of gold clusters protected by chiral thiols such as penicillamine [25] and A-isobutyryl-cysteine [26]. Figure 11 shows the CD spectra of 1-9, which is a good reproduction of the original report by Whetten s group [23,24]. 

Natural circular dichroism (optical activity). Although circular dichroism spectra are most difficult to interpret in terms of electronic structure and stereochemistry, they are so very sensitive to perturbations from the environment that they have provided useful ways of detecting changes in biopolymers and in complexes particularly those remote from the first co-ordination sphere of metal complexes, that are not readily apparent in the absorption spectrum (22). It is useful to distinguish between two origins of the rotational strength of absorption bands. 

Not mentioned in Table 2 (and often not in the original papers ) is the optical form (chirality) of the amino acids used. All the amino acids, except for glycine (R = H), contain an asymmetric carbon atom (the C atom). In the majority of cases the optical form used, whether l, d or racemic dl, makes little difference to the stability constants, but there are some notable exceptions (vide infra). Examination of the data in Table 2 reveals (i) that the order of stability constants for the divalent transition metal ions follows the Irving-Williams series (ii) that for the divalent transition metal ions, with excess amino acid present at neutral pH, the predominant spedes is the neutral chelated M(aa)2 complex (iii) that the species formed reflect the stereochemical preferences of the metal ions, e.g. for Cu 1 a 2 1 complex readily forms but not a 3 1 ligand metal complex (see Volume 5, Chapter 53). Confirmation of the species proposed from analysis of potentiometric data and information on the mode of bonding in solution has involved the use of an impressive array of spectroscopic techniques, e.g. UV/visible, IR, ESR, NMR, CD and MCD (magnetic circular dichroism). 

Figure B3.5.3 The relation of ellipticity to the differential absorption of circularly polarized radiation. The oscillating radiation sine wave, 01, is proceeding out of the plane of the paper towards the viewer. (A) Plane-polarized radiation is made up of left- and right-handed circularly polarized components, OL and OR, respectively. Absorption by a chromophore in a nonchiral environment results in an equal reduction in intensity of each component, whose resultant is a vector oscillating only in the vertical plane—i.e., plane-polarized radiation. (B) Interaction of the radiation with achiral chromophore leads to unequal absorption, so that combination of the emerging vectors, OL and OR, leads to a resultant that describes an elliptical path as it progresses out of the plane of the paper. The ratio of the major and minor axes of the ellipse is expressed by tan 0, thus defining ellipticity. The major axis of the ellipse makes an angle (q) with the original plane, which defines the optical rotation. This figure thus demonstrates the close relation between optical rotation and circular dichroism.

Essentially, the origin of spontaneous chiral resolution is the same as the previous example. When molecules with the same chiral conformation form small chiral domains due to packing entropy effects, the same chiral conformation of molecules is stabilized when they approach the chiral domain. Thus both chiral domains with different chiral conformations grow, resulting in spontaneous chiral resolution [6-8]. Chirality enhancement occurs even in such chiral domains. For instance, chirality in both segregated chiral domains is enhanced by doping nonchiral bent-shaped molecules (BSMs) with nonchiral rod-shaped molecules (RSMs), as observed by circular dichroism (CD) or optical rotatory power (ORP) [9],... 

---