Optical rotatory dispersion spectroscopy

Table III. (Also continued on next page) Predicted and observed n - t Cotton effects for steroid ketones from circular dichroism and optical rotatory dispersion spectroscopy"... 

Especially for the application to biological relevant surfaces (e.g., adsorbed proteins) the sensitivity of polarized SHG to the chirality of the molecules (the "handedness" of their structures) is important (Verbiest et al. 1998). Recently, the second-order nonlinear optical analog to circular dichroism and optical rotatory dispersion spectroscopy has been successfully developed (Yee et al. 1994 Byers et al. 1994). 

Although the usual absorption and scattering spectroscopies caimot distinguish enantiomers, certain techniques are sensitive to optical activity in chiral molecules. These include optical rotatory dispersion (ORD), the rotation by the sample of the plane of linearly polari2ed light, used in simple polarimeters and circular dichroism (CD), the differential absorption of circularly polari2ed light. 

If two different three-dimensional arrangements in space of the atoms in a molecule are interconvertible merely by free rotation about bonds, they are called conformationsIf they are not interconvertible, they are called configurations Configurations represent isomers that can be separated, as previously discussed in this chapter. Conformations represent conformers, which are rapidly interconvertible and are thus nonseparable. The terms conformational isomer and rotamer are sometimes used instead of conformer . A number of methods have been used to determine conformations. These include X-ray and electron diffraction, IR, Raman, UV, NMR, and microwave spectra, photoelectron spectroscopy, supersonic molecular jet spectroscopy, and optical rotatory dispersion (ORD) and CD measurements. Some of these methods are useful only for solids. It must be kept in mind that the conformation of a molecule in the solid state is not necessarily the same as in solution. Conformations can be calculated by a method called molecular mechanics (p. 178). 

The primary motivation for the development and application of vibrational optical activity lies in the enhanced stereochemical sensitivity that it provides in relation to its two parent spectroscopies, electronic optical activity and ordinary vibrational spectroscopy. Over the past 25 years, optical rotatory dispersion and more recently electronic circular dichroism have provided useful stereochemical information regarding the structure of chiral molecules and polymers in solution however, the detail provided by these spectra has been limited by the broad and diffuse nature of the spectral bands and the difficulty of accurately modeling the spectra theoretically. 

D-a-hydroxyvaleric add, L-valine, L-lactic acid] 3, having a 36-atom ring with twelve carbonyl oxygen atoms. From i.r. (65) and n.m.r. (66) spectroscopy it was established that the cationic complex has three-fold symmetry with the peptide carbonyl oxygen atoms involved in N—H... O hydrogen bonding. According to optical rotatory dispersion (ORD) and... 

A fascinating category of experiments can be found in Table IV. These are the use of lasers to determine thermodynamic parameters. These include calorimetry (43), enthalpies of vaporization and vaporization rates (44, 45), and heat capacities (46). Other laser experiments that can be found in Table IV include the use of CW laser spectroscopy to determine the iodine binding-energy curve (47), the study of vibrational line profiles to determine intermolecular interactions (48), two photon ionization spectrometry (49), a study of optical activity and optical rotatory dispersion (50) and the development of several experiments using blue diode lasers (57). 

For blood cell membranes the agreement of optical rotatory dispersion and infrared spectroscopy is reassuring. About one-third to one-fourth of the protons in the peptide bonds do not exchange in D20. This non-exchangeable fraction can be equated with the helical content of... 

Like other phenomena involving interactions between electromagnetic radiation and organic molecules, as in infrared, ultraviolet, and nmr spectroscopy, optical rotatory dispersion curves often are quite sensitive to small changes in structure. As an example, the rotatory dispersion curves for enantiomers of cis- and trcMr-lO-methyl-2-decalones, 16 and 17, are reproduced in Figure 19-7 ... 

In spectroscopy the Kramer s Kronig relations are often used. For example the optical rotatory dispersion (ORD) is related to the circular dichroism (CD) through such a pair of transforms.36 Workers in the area usually measure the (CD) and determine the (ORD) through Kramer s Kronig inversion of the (CD). 

The elucidation and confirmation of structure should include physical and chemical information derived from applicable analyses, such as (a) elemental analysis (b) functional group analysis using spectroscopic methods (i.e., mass spectrometry, nuclear magnetic resonance) (c) molecular weight determinations (d) degradation studies (e) complex formation determinations (f) chromatographic studies methods using HPLC, GC, TLC, GLC (h) infrared spectroscopy (j) ultraviolet spectroscopy (k) stereochemistry and (1) others, such as optical rotatory dispersion (ORD) or X-ray diffraction. 

More recently the relationship between absolute stereochemistry and spectroscopy has given rise to a good deal of work (11), and the increased availability of instruments for measuring either circular dichroism or optical rotatory dispersion (or both) has led to many applications in studies of molecular shape, including a good deal of work on metallo-enzymes. 

The measurement of vibrational optical activity (VOA) lacks some of the severe disadvantages mentioned. Vibrational spectral bands are less likely to overlap and can be measured using two complementary techniques namely infrared and Raman spectroscopy. They can be measured as well in the crystalline as in the liquid or gaseous state, and the techniques are applicable to solutions while nearly reaching (complemented with the appropriate theoretical models) the accurateness of the X-ray method. VOA has drawbacks too the effects are quite small and tend to be obscured by artifacts. They are about 10 times weaker than the optical rotatory dispersion (ORD) and the circular dichroism (CD) in the UV-VIS range. However, this apparent disadvantage is more and more relieved by instrumental advances. 

Figure 3 The Faraday Effect within magnetic optical rotatory dispersion (MORD) spectroscopy. A plane polarized beam is rotated by a magnetic field applied parallel to the axis of propagation. The rotation is defined as positive if the beam is rotated to the right in a clockwise direction when viewed toward the source (a). The original definition of Icp and rep within CD spectroscopy—fight plane polarized in the yz-plane can be described in mathematical terms as being comprised of left or right circularly polarized components of equal phase and amplitude (b). The perspective is depicted schematically toward the source. (Reprinted from Mack, Stillman and Kobayashi, Elsevier 2007)...

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]. 

The highly interesting and now classical monograph of Born contains the theory of the optical properties of matter covering atomic and molecular structural considerations. The fundamentals of molecular optics, presented by Bom, have been developed by Volkenshteyn in tensorial form. Various monographs have been written on the laws of classical optics and the fundamentals of physical optics, including natural optical activity," optical rotatory dispersion and circular dichroism," " i.r. spectroscopy, and other related topics. 

FIGURE 9-20 The Cotton Effect in ORD and CD, (a) Idealized optical rotatory dispersion (ORD) and circular dichroism (CD) curves at an absorption peak, with a positive Cotton effect, (b) Structures of tris-(S-alaninato) cobalt(IIl) complexes, (c) Absorption and circular dichroism spectra of the compounds in (b). (Data and structures in (b) adapted with permission from R. G. Denning and T. S. Piper, Inorg. Chem., 1966, 5, 1056. 1966 American Chemical Society. Curves in (c) adapted with permission from J. Pujita and Y. Shimura, Optical Rotatory Dispersion and Circular Dichroism, in K. Nakamoto and P. J. McCarthy, eds.. Spectroscopy and Structure of Metal Chelate Compounds, John Wiley Sons Inc., New York, 1968, p. 193. 1968 John Wiley Sons, Inc. Reprinted by permission of John Wiley Sons, Inc.)... 

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