Spectra circularly polarized

Figure 9.29 Two-photon fluorescence excitation spectrum of 1,4-difluorobenzene. The upper and lower traces are obtained with plane and circularly polarized radiation, respectively, but the differences are not considered here. (Reproduced, with permission, Ifom Robey, M. J. and Schlag, E. W., Chem. Phys., 30, 9, 1978)...

Circular dichroism (c.d.) spectroscopy measures the difference in absorption between left- and right-circularly polarized light by an asymmetric molecule. The spectrum results from the interaction between neighboring groups, and is thus extremely sensitive to the conformation of a molecule. Because the method may be applied to molecules in solution, it has become popular for monitoring the structure of biological molecules as a function of solvent conditions. 

Figure 15. Circular dichroism of the C=0 C li peak (BE = 292.7 eV) in fenchone at three different photon energies, indicated, (a) Photoelectron spectrum of the carbonyl peak of the (1S,4R) enantiomer, recorded with right (solid line) and left (broken line) circularly polarized radiation at the magic angle, 54.7° to the beam direction, (b) The circular dichroism signal for fenchone for (1R,4A)-fenchone (x) and the (lS,41 )-fenchone (+) plotted as the raw difference / p — /rep of the 54.7° spectra, for example, as in the row above, (c) The asymmetry factor, F, obtained by normalizing the raw difference. In the lower rows, error bars are included, but are often comparable to size of plotting symbol (l/ ,4S)-fenchone (x), (lS,4R)-fenchone (+). Data are taken from Ref. [38],...

Of the visible spectroscopic techniques, CD spectroscopy has seen the most rapid and dramatic growth. The far-UV circular dichroism spectrum of a protein is a direct reflection of its secondary structure [71]. An asymmetrical molecule, such as a protein macromolecule, exhibits circular dichroism because it absorbs circularly polarized light of one rotation differently from circularly polarized light of the other rotation. Therefore, the technique is useful in determining changes in secondary structure as a function of stability, thermal treatment, or freeze-thaw. 

Fig. 23a, b. ENDOR with circularly polarized rf fields (CP-ENDOR). Single crystal ENDOR spectra of Cu(bipyam)2 (C104)2 diluted into Zn(bipyam)2(C104)2 arbitrary orientation, temperature 20 K. a) Conventional ENDOR spectrum (linearly polarized rf field), b) CP-ENDOR spectrum applied rf field right hand rotating. The spectrum is dominated by the eight nitrogen transitions with msAN > 0. (From Ref. 104)... 

Fig. 21.7 Photoexcitation spectrum of the 6sl5d D2 —> (6p3/2 d) 7 = 3 transition in Ba as a function of the frequency of the third laser. All three lasers are circularly polarized in the same sense. The broken line is the spectrum calculated from Eqs. (21.15) and (21.16). The energy level inset is not to scale (from ref. 9).

As discussed, optical rotatory dispersion (ORD) is determined by the unequal indices of refraction for left- and right-circularly polarized light in a chiral medium, within an absorption band, the ORD spectrum exhibits anomalous dispersion, which is referred to as a Cotton effect. Full understanding of ORD and anomalous dispersion requires a more detailed examination of the properties associated with refractive indices. 

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