Vibrational optical activity spectroscopy

The second chapter ends with two overviews by Stephens Devlin and by Hug on the theoretical and the physical aspects of two vibrational optical activity spectroscopies (VCD and VROA, respectively). In both overviews the emphasis is more on their basic formalism and the gas-phase quantum chemical calculations than on the analysis of solvent effects. For these spectroscopies, in fact, both the formulation of continuum solvation models and their applications to realistic solvated systems are still in their infancy. 

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. 

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. 

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. 

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. 

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. 

Nafie LA (1984) Vibrational Optical Activity. In Clark RJH, Hester RE (eds) Advances in Infrared and Raman Spectroscopy, vol 11. Wiley-Heyden, Chichester, pp 49 Nafie LA (1992) SPIE 1681 29... 

VIBRATIONAL SPECTROSCOPY USING TUNABLE LASERS, Robin S. McDowell INFRARED AND RAMAN VIBRATIONAL OPTICAL ACTIVITY, L. A. Nafie RAMAN MICROPROBE SPECTROSCOPIC ANALYSIS, John J. Blaha THE LOCAL MODE MODEL, Bryan R. Henry... 

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.)...

L. A. Nafie and T. B. Freedman, Biological and pharmaceutical applications of vibrational optical activity, in Infrared and Raman Spectroscopy of Biological Materials, H.-U. Gremlich and B. Yan, Eds., Marcel Dekker, New York, 2001, p. 15. 

Hoffmann GG (1995) Vibrational optical activity (VOA). In Schrader B (ed) Infrared and Raman Spectroscopy -Methods and Applications, pp 543-572. Weinheim VCH. 

Nafie LA (1996) Vibrational optical activity. Applied Spectroscopy 50 14A-26A. 

Vibrational optical activity may also be measured by the technique of Raman spectroscopy. The theory is well... 

TOCSY total correlation spectroscopy VGA vibrational optical activity... 

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. 

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). 

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). 

P. L. Polavarapu, Vibrational Spectroscopy Principles and Applications with Emphasis on Optical Activity, Elsevier, New York 1998. 

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). 

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. 

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