Vibrational spectroscopies Raman optical activity

N. A. Macleod, C. Johannessen, L. Hecht, L. D. Barron, and J. P. Simons, From the gas phase to aqueous solution Vibrational spectroscopy, Raman optical activity and conformational struc tore of carbohydrates. Int. J. Mass Spectrom. 253, 193 200 (2006). 

Keywords Vibrational spectroscopy Raman optical activity Ab initio calculation Helicene Localized mode method... 

Chiroptical properties related to molecular vibrations can be studied not only by vibrational circular dichroism, but also by using the chiral variant of the Raman spectroscopy - Raman optical activity (ROA) [14-16]. This method has been developed into practical use only recently, but it is very promising and similarly to the... 

The other form of optical activity in vibrational transitions is known as Raman optical activity (ROA). Here, also, one measures an intensity difference for left compared to right circularly polarized incident radiation however, optical activity in light scattering has no direct analog in electronic spectroscopy. ROA was first measured by Laurence Barron, A. D. Buckingham, and M. P. Bogaard in 1973 (9) and several reviews of the subject have since appeared (10-14). 

This chapter is not an update of a previous chapter and will therefore try to review the reported chiroptical data on the carbon-carbon double bond, starting from 1968. It will refer only to the available literature on the C=C chromophore itself. It will analyze the available data of molecules which contain only one chromophore, the carbon-carbon double bond. It will not dwell on molecules which have the C=C bond as one of the chromophores which are responsible for its optical activity. It will cover the literature in the field of electronic excitations and will not provide information on vibrational CD (VCD) or Raman optical activity. The chiroptical properties provide information regarding the spectroscopy of the chromophore, as well as its absolute configuration. The latter is usually done with the help of sector rules around the chromophore of interest. In this review both aspects will be discussed. 

W. Hug, Raman Optical Activity Spectroscopy, In Handbook of Vibrational Spectroscopy , J. M. Chalmers and P. R. Griffiths (eds), John Wiley sons, Ltd, Chichester, 2002 p. 745. 

Figure 11 ROA (1 - f) and backscattered Raman (/ -i- f) spectra of the a-helical (A), disordered (B), and j5-sheet (C) po-ly(L-lysine) at 20°C. (Reproduced with permission from Barron LD, Blanch EW, McColl IH, et al. (2003) Structure and behavior of proteins, nucleic acids and viruses from vibrational Raman optical activity. Spectroscopy 7 . 101-126.)...

While vibrational spectroscopy is not capable of the structural resolution of X-ray diffraction, it nevertheless has some important advantageous features. First, it is not generally limited by physical state samples can be in the form of powders, crystals, films, solutions, membranous aggregates, etc. Second, a number of different experimental methods probe the structure-dependent vibrational modes of the system infrared (IR), Raman (both visible and UV-exeited resonance), vibrational circular dichroism, and Raman optical activity, many of these with time-resolution capabilities. Finally, in addition to providing structural information, vibrational spectra are sensitive to intra- and intennolecular interaction forces, and thus they also give information about these properties of the system. 

When the line symmetry is broken down by the defects in the polymer chain, in principle, all vibrations will be optical active however, the intensity of the band depends strong on the local phase of the vibration. When the conformational order accompanying the ordering process of gelation and crystallization is studied by infiared and Raman spectroscopy, the relationship between the local phase lag and intensity is important. 

Optical measurements (/) such as Raman Scattering, Fluorescence techniques. Vibrational Circular Dichroism, (VCD), Optical Rotational Dispersion (ORD), Raman Optical Activity (ROA) and infrared absorption spectroscopy can overcome many of the obstacles mentioned above due to the fact that optical techniques are non-invasive and can monitor proteins in their native environment and with accurate time resolution. One disadvantage is the low sensitivity. However, the use of Surface Enhanced Raman Scattering (SERS), techniques (2-4) means that proteins can be observed down to the single molecule level. Thus, optical teclmiques hold great promise for the future investigation of protein dynamics processes provided that proteins can be maintained in a suitable and controllable sample cell. 

See also Fibre Optic Probes in Optical Spectroscopy, Clinical Applications Light Sources and Optics Raman Optical Activity, Applications Raman Optical Activity, Theory Raman Spectrometers Vibrational CD Spectrometers. 

See also Biochemical Applications of Raman Spectroscopy Chiroptical Spectroscopy, Emission Theory Chiroptical Spectroscopy, General Theory Chiroptical Spectroscopy, Oriented Molecules and Anisotropic Systems Electromagnetic Radiation ORD and Polarimetry Instruments Raman Optical Activity, Applications Raman Optical Activity, Spectrometers Raman Spectrometers Scattering Theory Vibrational CD Spectrometers Vibrational CD, Applications Vibrational CD, Theory. 

Nafie LA, Yu G-S and Freedman TB (1995) Raman optical-activity of biological molecules. Vibrational Spectroscopy 8 231-239. 

Vibrational spectroscopy is an important tool to obtain information about the secondary structure of proteins [827]. The ability to relate protein conformations to infrared vibrational bands was established very early in the pioneering work of Elhot and Ambrose before any detailed X-ray results were available [828]. Vibrational circular dichroism (VCD) provides sensitive data about the main chain conformation [829, 830]. The Raman optical activity (ROA) signal results from sampling of different modes but is especially sensitive to aromatic side chains [831, 832]. A theoretical prediction for the ROA phenomenon was developed by Barron and Buckingham [833, 834], and the first ROA spectra were measured by Barron, Bogaard and Buckingham [835, 836]. First ab initio predictions were provided by Polavarapu [837]. In 2003, Jalkanen et al. showed that DPT approaches in combination with explicit water molecules and a continuum model reproduce the experimental spectra much better [838]. DFT-based approaches to VCD spectra were, for example, pioneered by Stephens et al. [839]. To extract the local structural information provided by ROA, Hudecova et al. [721] developed multiscale QM/MM simulation techniques. 

An electric dipole operator, of importance in electronic (visible and uv) and in vibrational spectroscopy (infrared) has the same symmetry properties as Ta. Magnetic dipoles, of importance in rotational (microwave), nmr (radio frequency) and epr (microwave) spectroscopies, have an operator with symmetry properties of Ra. Raman (visible) spectra relate to polarizability and the operator has the same symmetry properties as terms such as x2, xy, etc. In the study of optically active species, that cause helical movement of charge density, the important symmetry property of a helix to note, is that it corresponds to simultaneous translation and rotation. Optically active molecules must therefore have a symmetry such that Ta and Ra (a = x, y, z) transform as the same i.r. It only occurs for molecules with an alternating or improper rotation axis, Sn. 

The measurement of vibrational optical activity requires the optimization of signal quality, since the experimental intensities are between three and six orders of magnitude smaller than the parent IR absorption or Raman scattering intensities. To date all successful measurements have employed the principles of modulation spectroscopy so as to overcome short-term instabilities and noise and thereby to measure VOA intensities accurately. In this approach, the polarization of the incident radiation is modulated between left and tight circular states and the difference intensity, averaged over many modulation cycles, is retained. In spite of this common basis, there are major differences in measurement technique and instrumentation between VCD and ROA consequently, the basic experimental methodology of these two techniques will be described separately. 

A general discussion regarding the instrumentation which can be used for the measurement of CD spectroscopy has been provided by Donald Bobbitt, while the more specialized requirements associated with vibrational optical activity have been discussed by Laurence Nafie. This latter chapter deals with both the methods of vibrational CD spectroscopy as well as raman vibrational optical activity. The use of CD as a detector in liquid chromatography represents an important area of heightened activity, and this has been covered by Andras Gergely. 

When my interest returned and we began researching the analytical applications of CD in the 70 s, I felt I had a head start. But there was so much that was new. A great deal had happened to CD over the years as it matured and expanded to include the far-UV the study of optical activity in excited state emissions, and in vibrational and Raman spectroscopy and the evolution of new empirical models applicable to the interpretation of the structural properties of macromolecules. Most important of all, perhaps, was the arrival of high tech electronics and materials which had brought CD instrumentation out of the dark ages. And now, ironically, almost 35 years after my introduction to CD, my special interest is the exploitation of chiral transition metal complexes as chirality induction reagents in chemical analysis. 

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