Raman optical activity observables

Hecht and Barron (1993) discuss the time reversal and Hermiticity characteristics of optical activity operators. They formulate the Raman optical activity observables for the four different forms of ROA in terms of matrix elements of the absorptive and dispersive parts of these operators. Rupprecht (1989) applied a matrix formalism for Raman optical activity to intensity sum rules. 

In addition to a review of the recent developments in the preparation of chiral amino compounds, developments concerning the interpretation of their ORD and CD in the visible and ultraviolet spectral regions will be reviewed, together with the emerging impact of vibrational (infrared) optical activity (VOA) observations, including vibrational circular dichroism (VCD) and Raman optical activity (ROA) measurements23, on important stereochemical problems concerning chiral amino compounds. 

Computational efforts to describe the conformational preferences of (R,R)-tartaric acid and its derivatives - mainly for isolated molecules - were made recently [18-25]. The conformations of these molecules also attracted attention from experimental chemists [22-40]. (/ ,/ [-tartaric acid and its dimethyl diester were observed in crystals, in conformations with extended carbon chain and planar a-hydroxy-carboxylic moieties (T.v.v and Tas for the acid and the ester, respectively) [25-28] (see Figure 2). The predominance ofthe T-structure was also shown by studies of optical rotation [31], vibrational circular dichroism (VCD) [23], Raman optical activity [32, 35], and nuclear magnetic resonance (NMR) [22, 33, 34]. The results of ab-initio and semiempirical calculations indicated that for the isolated molecules the Tsv and T as conformers were those of lowest energy [22, 21, 23, 25]. It should be noted, however, that early interpretations of NMR and VCD studies indicated that for the dimethyl diester of (/ ,/ [-tartaric acid the G+ conformation is favored [36-38]. 

Nafie (1992) has given a review about the latest VOA instrumentation. Until 1988, the only measured form of ROA was incident circular polarisation (ICP) ROA, but as the process observed in Raman spectroscopy is a two-photon process, there are four possibilities for measuring Raman optical activity. ICP ROA is the unpolarized measurement of the Raman radiation emitted upon excitation with alternating right and left circularly polarized light. It is shown in Fig. 6.3-12, following the sketches of Nafie. As the first of the other possibilities scattered circular polarisation (SCP) ROA was measured. This... 

Bell et al. (1993) showed vibrational Raman optical activity of the carbohydrates D-galactose, D-maltose and laminarin at low wavenumbers. The authors observed a ROA couplet at 427 10 cm which, according to their opinion, reflects the configuration of the glycosidic links in complex carbohydrates. 

This article reviews all the published work concerned with the study of vibrational optical activity in chiral molecules from measurements of a small difference in the intensity of Raman scattering in right and left circularly polarized incident light. The history and basic theory are described briefly, followed by an account of the instrumentation and the precautions that must be observed in order to suppress spurious signals. The various theories that have been proposed in order to relate stereochemical features to the observations are then outlined, this being followed by a survey of all reported Raman optical activity spectra. 

Nonlinear optical activity phenomena arise at third-order and include intensity dependent contributions to optical rotation and circular dichroism, as well as a coherent form of Raman optical activity. The third-order observables are - like their linear analogs - pseudoscalars (scalars which change sign under parity) and require electric-dipole as well as magnetic-dipole transitions. Nonlinear optical activity is circular differential. 

In the case of non-degenerate frequencies, the nonlocal third-order effects may give rise to chiral pump-probe spectroscopies. The only observation of a coherent Raman optical activity process to date is also due to a third-order pseudoscalar. Spiegel and Schneider have observed Raman optical activity in coherent anti-Stokes Raman scattering in a liquid of (-l-)-trans-pinane and report chiral signals that are 10 of the conventional electric-dipolar CARS intensity [23],... 

In addition to the magnetic Raman circular intensity difference, outlined above, that all molecules can show in a magnetic field, chiral molecules can show a circular intensity difference without a magnetic field. This natural Raman optical activity was first observed by Barron, Bogaard and Buckingham in 1973 Z27j, and provides detailed stereochemical information by measuring vibrational optical activity. 

CD), vibrational circular dichroism (VCD), Raman optical activity (ROA)), (5) XRD data for fiber samples of polymers, (6) direct miaoscopic observation, or (7) single-crystal X-ray analysis. 

Back-scattered Raman optical activity spectra were measured in the 7(X)-15(X) cm region for a range of carbohydrates in aqueous solution. Features of the spectra were identified that appear to be characteristic of the stereochemistry of the sugars. The i.r. spectra of methyl 2,3-di-, 2,3,6-tri- and 2,3,4,6-tetra-O-nitro-p-D-glucosides were examined in the 30-7(X) cm range, and interpretation of the bands observed was given in terms of the conformation and location of the nitro-groups and of crystal lattice vibrations. ... 

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. 

Methyl torsion Raman optical activity of trans-2,3-dimethyloxirane has been compared with that of rraws-2,3-dimethylthiirane [5]. Torsion of the two methyl groups in these molecules can occur as in-phase and out-of-phase combinations. The former can be observed as a very weak and broad Raman band at -200 cm" in the dimethyloxirane with positive ROA of medium size, whereas it is observed as a shoulder at -220 cm i in the spectrum of the dimethylthiirane with large positive ROA. Contrary to the dimethyloxirane spectra, the dimethylthiirane spectra also contain the out-of-phase torsion it shows up as a medium size Raman band at -245 cm i with zero or small negative ROA. This is in good agreement with ab initio calculations and even with the old inertial model for methyl torsions. 

The possibility of observing Raman optical activity from chiral surfaces has been mentioned and a theory for the generation of second-harmonic optical activity (SHOA) from chiral surfaces and interfaces has been derived. 

Raman optical activity (ROA) or, more precisely, spontaneous vibrational Raman optical activity scattering is, like vibrational circular dichroism (VCD), a spectroscopic method that directly probes the chirality, or handedness, of molecular vibrations. ROA and VCD therefore have an obvious stereochemical potential. That such phenomena could yield structural information not otherwise available was realized long before their measurement became feasible and the first observations of ROA and VCD date back only a quarter of a century. For ROA, measurement was preceded by a detailed theoretical analysis and, perhaps inevitably so in view of the experimental difficulties, some false claims of its observation. 

Activity R = Raman-active, ir = infrared-active, S = optically silent, observed in neutron scattering. tCarbon atom displacement il or J. to C. 

It should be noted that the Raman-inactive soft mode is observed in the temperature region above Tc. A spectral shape completely different from that of the Lorentz-type peak function indicates the defect-induced Raman scattering (DIRS) in the paraelectric phase of ST018. When centrosymmetry is locally broken in the paraelectric phase, the nominally Raman-inactive soft mode is optically activated locally to induce DIRS in the soft mode. 

First measurements for identifying optical activity in Raman scattering were undertaken soon after the Raman effect itself was discovered but proved unsuccessful [1,2]. The fact that the measurement of solutions of biomolecules is at present the most important application of ROA is remarkable in the face of the experimental difficulties which haunted these early attempts to observe it, even for pure, chiral liquids, the most favorable experimental situation. 

However, problems frequently arise in the comparison of calculated frequencies with optical spectra. Since infrared and Raman measurements are subject to optical selection rules, only frequencies associated with certain active phases of a phase-frequency curve are observed. In certain cases, a mode may have frequencies that lie outside the range of optical measurements, or it may have no optically active phases. For exainple, the skeletal deformation and torsional modes for an infinite and isolated polyethylene chain in the trans-configuration have optically active phases that correspond to zero frequency (36). 

Optical activity is an old subject dating back to the early years of the last century. But it is far from exhausted. Recent developments in optical and electronic technology have led to large increases in the sensitivity of conventional optical activity measurements, and have enabled completely new optical activity phenomena to be observed. Optical activity has been traditionally associated almost exclusively with electronic transitions, but one important advance over the past decade has been the extension of optical activity measurements into the vibrational spectrum using both infrared and Raman techniques. It is now apparent that the advent of vibrational optical activity has opened up a new world of fundamental studies and practical applications. 

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