Vibrational circular dichroism spectra

Chem., 60, 1918 (1995). Determining Absolute Configuration by Spectroscopic Means The Vibrational Circular Dichroism Spectrum of (+)-(lS,5S,6S)- and -)- iR,SR,6R)-Spiro[4.4]nonane l,6-diol. 

The vibrational circular dichroism(VCD) spectroscopy can be used to elucidate the stereochemistries of chiral molecules, including the accurate estimation of enantiomeric excess and their absolute configrations[20]. Optically pure samples as well as a racemic sample(c) were used as a reference to compare the VCD spectra. Three VCD spectra are shown in Fig. 7 a spectrum of 99 % ee R(-)-1-phenyl 1,2-ethanediol(a) and that of 99 % ee S(+ )-1-phenyl l,2-ethanediol(b) obtained from Aldrich Co., and the other is that of the product obtained on the Ti-MCM-41/chiral Co(HI) salen catalyst(d). 

FIGURE 1. Gas-phase vibrational circular dichroism (VCD) spectra for (2S, 3S)-1-l3C-l, 2,3,-dj-cyclopropane (more intense spectrum) and for a 360 min, 407 °C thermal reaction product mixture of l3C, d3-cyclopropanes (less intense spectrum)165166... 

To confirm the importance of ester linkage for chiral formation, we conducted vibrational circular dichroism (VCD) measurements [64]. FT-IR spectrum of this molecule shows many C=0,0-C-O related peaks in addition to peaks of N=N and phenyl and CH2 vibrational modes, as shown in Fig. 22a. It is noted that strong VCD signals are observed in C=0 and O-C-O vibrations, as shown in Fig. 22b. This implies that ester linkage is strongly related to the chiral structure. 

Chemical compounds absorb infrared radiation when there is a dipole moment change (in direction and/or magnitude) during a molecular vibration, molecular rotation, or molecular rotation-vibration. Absorptions are also observed with combinations, differences or overtones of molecular vibrations. A specific type of molecule is limited in the number of vibrations and rotations it is allowed to undergo. Therefore, each chemical compound has its own specific set of absorption frequencies and thus exhibits its own characteristic IR spectrum. This unique property of a compound allows the organic chemist to identify and quantify an unknown sample. (A special infrared technique called vibrational circular dichroism (VCD) is required to distinguish optical isomers). 

The degree of absorption polarization and the ratio of the extinction coefficient and exhibits a sharp increase starting at about 29,000 cm . Tliis indicates that a weak transition, namely the L j-transition, is buried below the much stronger L -band at wavelengths above 29,000 cm". A comparison of the absorption polarization spectrum, the fluorescence polarization spectrum and the circular dichroism spectrum shows that the 0-0-vibrational band of the L transition is located at about 29,000 cm [7]. This example demonstrates the usefulness of the liquid crystal method to localize weak hidden optical transitions. 

Yet another spectroscopic method that can be used to help determine molecular structure is vibrational circular dichroism (VCD). This technique detects differences in attenuation of left and right circularly polarized hght passing through the sample. VCD is sensitive to the mutual orientation of groups of atoms in a molecule and provides three-dimensional structural information. It is especially important in the study of chirality and molecular conformation. Only chiral molecules have a VCD spectrum. In particular, molecules that have either a plane of symmetry or a center of symmetry are VCD inactive. 

Vibrational circular dichroism (VCD) is defined as circular dichroism (CD) in vibrational transitions in molecules. These transitions typically occur in the infrared (IR) region of the spectrum and hence a VCD spectrometer is an infrared spectrometer that can measure the circular dichroism associated with infrared vibrational absorption bands. CD is defined as the difference in the absorption of a sample for left versus right circularly polarized radiation. This difference is zero unless the sample possesses molecular chirality, either through its constituent chiral molecules or through a chiral spatial arrangement of non-chiral molecules. 

Circular dichroism (CD) can be observed in the vibrational transitions of chiral molecules vibrational circular dichroism (VCD). An example of a VCD spectrum is shown in Figure 1, together with the corresponding unpolarized absorption spectrum. The sample is a 0.6 M solution of (lR,4jR)-(-i-)-cam-phor in CCI4. Here, we discuss the theoretical analysis of VCD spectra. The current state-of-the-art is illustrated in Figure 1, where VCD and absorption spectra of camphor, predicted within the harmonic approximation (HA) using ab initio density functional theory (DFT), are shown. 

As the time scale of the Raman scattering event ( 3.3 x 10 14 s for a vibration with a Stokes wave number shift of 1000 cm 1 excited in the visible) is much shorter than that of the fastest conformational fluctuations, an ROA spectrum is a superposition of snapshot spectra from all the distinct conformations present in a sample at equilibrium. Since ROA observables depend on absolute chirality, there is a cancellation of contributions from enantiomeric structures arising as a mobile structure explores the range of accessible conformations. Therefore, ROA exhibits an enhanced sensitivity to the dynamic aspects of biomolecular structure. In contrast, conventional Raman band intensities are blind to chirality and so are generally additive and therefore less sensitive to conformational mobility. Ultraviolet circular dichroism (UVCD) also demonstrates an enhanced sensitivity to the dynamics of chiral structures ... 

The significance of vibrational optical activity becomes apparent when it is compared with conventional electronic optical activity in the form of optical rotatory dispersion (ORD) and circular dichroism (CD) of visible and near-ultraviolet radiation. These conventional techniques have proved most valuable in stereochemistry, but since the electronic transition frequencies of most structural units in a molecule occur in inaccessible regions of the far-ultraviolet, they are restricted to probing chromophores and their immediate intramolecular environments. On the other hand, a vibrational spectrum contains bands from most parts of a molecule, so the measurement of vibrational optical activity should provide much more information. 

Although Lowry iu his classical treatise in 1935 discussed the possibility of detection of circular dichroism arising from molecular vibration transitions, only in the past two decades has it been possible to measure optical activity associated with infrared absorption transitions, CD maxima first being detected in the VCD spectrum for the C—H stretching modes of the enantiomers of 2,2,2-trifluoro-l-phenylethanol as the neat liqnid . This work was initiated with the view that such measurements would eventually yield information concerning absolute configurations and molecular conformations of... 

The solvent-induced stereochemical behaviour of a bile acid-based biphenyl phosphite has been studied experimentally using circular dichroism (CD) spectroscopy, and theoretically using DFT quantum mechanical methods. " The FTIR, Raman and surface-enhanced Raman scattering (SERS) spectra of phenyl phosphate disodium salt have been recorded and its vibrational wavenumbers, calculated using the Hartree-Fock/6-31G basis set, compared with experimental values. From SERS spectra study, the molecule is adsorbed on the silver surface with the benzene ring in a tilted orientation. The presence of the phenyl ring and the phosphate group vibrations in the SERS spectrum reveal the interactions between the phenyl... 

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