Circular intensity difference

The fundamental scattering mechanism responsible for ROA was discovered by Atkins and Barron (1969), who showed that interference between the waves scattered via the polarizability and optical activity tensors of the molecule yields a dependence of the scattered intensity on the degree of circular polarization of the incident light and to a circular component in the scattered light. Barron and Buckingham (1971) subsequently developed a more definitive version of the theory and introduced a definition of the dimensionless circular intensity difference (CID),... 

The measurement of optical activity in Raman spectra is most often done by measuring the circular intensity difference (CID) and the circular intensity sum (CIS). Using this technique the linearly polarized la.ser beam used for excitation is modulated with a KD P electrooptical modulator resulting in an excitating beam which changes its state of polarization from left to right circularly polarized at a frequency of about 1/3 Hz. The Raman intensities are then collected separately and the dimensionless Raman CID calculated using ... 

As there could be some confusion between the circular intensity difference and the dimensionless circular intensity difference (which is only occasionally given in the literature), the latter should be given a different name, e.g. DCID. 

It can be seen from Eq. (188) that the circular intensity difference is a complicated function of Aw that differs from a simple superposition of Lorentz functions. The temperature dependence is also rather complicated since in vibronic systems the matrix elements m jiK-K]m ) between the states m) and m ) of differing energy (wm + wm ) are nonzero. 

Raman optical activity has only been measured so far in pure liquids and strong solutions. Crystals and powders are harder to study crystals must be polished and oriented carefully to eliminate artefacts, whereas multiple scattering in powders depolarizes the incident light. It would be of great interest to measure pure rotational, and rotational-vibrational, ROA in gases, but insufficient scattered intensity has so far prevented this. An additional complication in resonance scattering is that circular dichroism of the incident beam can contribute to the measured circular intensity difference. 

To give an idea of the potential of this technique, figure 3.3 shows the depolarized magnetic resonance-Raman circular intensity difference spectrum of a very dilute aqueous solution of the low-spin d complex IrCl recorded recently on the Glasgow multichannel Raman instrument [ 2aJ. The three Raman-active fundamentals of the octahedral MXe structure are assigned to 3 M c aT )... 

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. 

Natural ROA originates in interference between waves scattered via the polarizability and optical activity tensors of the molecule. The relevant experimental quantity is a dimensionless circular intensity difference... 

Raman optical activity (ROA), which is most often measured as the circular intensity difference (CID, the difference between a Raman spectrum excited with right circularly polarized light and that excited with left circularly polarized light), is a very small effect which can be easily obscured by artifacts. It can also be reported as the dimensionless circular intensity difference (CID or better DCID,... 

Note, also, that the presence of a circular-difference effect is not a necessary requirement for chirality. The presence of an intensity difference effect depends on the exact phase relationships between the chiral and achiral expansion coefficients in-phase coefficients will produce no circular-difference effect (see Figures 9.9b and e), while out-of-phase components will... 

FIGURE 15. Raman circular intensity spectra for (R)-a-phenylethylamine [(R)-22] in photon counts (A) difference spectrum, (if — if) (B) sum spectrum, (if + if) (C) difference spectrum, (if — if). Reprinted with permission from Reference 160. Copyright (1975) American Chemical Society... 

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). 

The dependence of C(B) on two parameters uoj /T1 and ujj>/T2 is of course connected to the fact that, as can be seen from (2.37) and (2.39), alignment is always created along with orientation under circularly polarized excitation. Hence, for the purpose of determining the orientation relaxation rate Ti, one has to register the intensity difference Ir — h... 

Raman optical activity (RO A) Due to molecular chirality there is a difference in the intensity of Raman scattered right and left circularly polarized light. Raman optical activity (ROA) is a vibrational spectroscopic technique that is reliant on this difference and the spectrum of intensity differences recorded over a range of wavenumbers reveals information about chiral centers within a sample molecule. It is a useful probe to study biomolecular structures and their behavior in aqueous solution especially those of proteins, nucleic acids, carbohydrates, and viruses. The information obtained is in realistic conditions... 

It seems likely that the dominant artefacts are produced by linearly polarized contaminants in the incident circularly polarized beam, with different azimuths in the right and left phases. Hug noticed that, if the intensities of the linearly polarized contaminants in the two phases are identical, a twin-lens collection system comprising two identical collection optics with their axes in the xy plane and inclined at 45° to the y axis will eliminate the spurious intensity difference. For the collection optic with its axis at + 45° to the y axis, the incident azimuth is effectively 0 — 45°, whereas... 

Fig. 6. The depolarized Raman circular intensity sum (I + I ) and difference (Ij — Ij) spectra of (—) menthol in methanol solution...

Fig. 8. The depolarized Raman circular intensity sum and difference spectra of neat (R)—( + ) 3-methylcyclohexanone... 

Further important technical advances include the development of devices for the automatic scanning of the depolarisation ratio, for measuring Raman CID (circular intensity differentials), for measuring difference Raman spectra (i.e. the difference between the Raman signals from solutions and solvents), for studying optical-fibre Raman spectroscopy, for rapid (i.e. picosecond) Raman spectroscopy and Raman micrography 64). 

CP) Qjj-Quiaj-iy polarized radiation. The intensity difference means that a Circular Dichroism in the Angular Distribution of photoelectrons (CDAD) effect is present. 

In the upper part of Fig. 5.5 photoelectron spectra of a 7 ML thick iron film on W(llO) taken with left and right circularly polarized light are displayed for opposite magnetization direction (M" and M ). The required geometric arrangement was presented in Fig. 2.2. The data clearly show a huge intensity difference in the main iron Jid band directly at the Fermi level [26], the corresponding asymmetry A ... 

As already mentioned, the phenomenon of magnetic circular dichroism in photoemission originates from spin-orbit and exchange interactions in combination with the dipole selection rules. In the atomic model picture, the splitting of the 3p level (into sublevels with orbital momentum m) is caused by the electrostatic interaction of the core level with the magnetically polarized valence electrons [57]. The observed intensity differences and the respective asymmetry values in photoemission from the Fe 3p levels are small (typically 3%) compared to the large MCDAD and MLDAD asymmetries (up to about 12%) observed in valence band photoemission [27]. 

Circular dichroism Difference in intensity of absorption of right- and left-circularly polarized light by functional groups in asymmetric environment N, P, G Applications similar to but more powerful than ORD especially for functional groups such as 0=0. Conformational analysis of biomacromolecules in solutions. 

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