Electric circular dichroism

It may be worthwhile to compare briefly the PECD phenomenon discussed here, which relates to randomly oriented chiral molecular targets, with the likely more familiar Circular Dichroism in the Angular Distribution (CDAD) that is observed with oriented, achiral species [44 7]. Both approaches measure a photoemission circular dichroism brought about by an asymmetry in the lab frame electron angular distribution. Both phenomena arise in the electric dipole approximation and so create exceptionally large asymmetries, but these similarities are perhaps a little superficial. 

Linear dichroism data with DNA oriented by an electric field [53, 54] or a linear flow [55, 56], under linearly polarised light, lead to determinations of the angle between the absorbing transition dipole moment of the chromophore in the molecule and the DNA helix axis conclusions concerning intercalation may thus be drawn from this technique. Finally, with chiral compounds, circular dichroism is also an attractive method to determine the enantioselectivity in the binding of the molecule [48, 57,58]. 

Vibrational circular dichroism arises from the interference of the electric dipole transition moment (p joi and the magnetic dipole transition moment (m )io and is proportional to the rotational strength, / ,o, where... 

Quadrupole splitting (A q) correlates to electric field gradient and, based on model compounds, can identify some ligand types Can observe changes in ligand field induced by sample perturbation Can only detect iron sites Magnetic circular dichroism (MCD) spectra Require low temperature to observe (—2-70 K)... 

This result demonstrates the tendency of an optically active material to rotate the electric vector as it propagates through the sample. Materials possessing this property are normally composed of molecules having chiral symmetry. This effect leads to circular birefringence and circular dichroism, two optical properties that are frequently used in the characterization of biomaterials. 

Positive Cotton effects (c.d.) are reported for both 3a- and 3/8-trimethylstannyl-5a-cholestanes, at 203 nm and 210 nm, respectively.35 The compounds were studied in connection with an evaluation of the effects of /3 -trimethylstannyl substituents in cyclohexanone analogues, which provide evidence of through-bond coupling to augment that already recognized for electronegative substituents.36 The circular dichroism associated with the enone systems of cholest-4-en-3-one and 3/3-acetoxycholest-5-en-7-one has been recorded for samples oriented by an electrical field in a nematic phase composed of cholesteryl chloride and cholesteryl laurate.37 New rules are proposed for the correlation of D-Iine molecular rotations with structures of steroid derivatives.38 This work extends an earlier analysis39 and in the present case relates mainly to data for substituents at C-3, C-5, and C-6. 

The cholesteric mesophase formed by cholesteryl p-nitrobenzoate at 200 °C has been used as the solvent to effect an asymmetric synthesis lrans-but-2-enyl p-tolyl ether gave the product of an ortho-Claisen rearrangement, 2-(but-1 -en-3 -yl)-4-methylphenol. This material exhibited circular dichroism, although neither the optical yield nor the configuration of the product is yet known.262 Decarboxylation of ethylphenylmalonic acid in cholesteryl benzoate at 160 °C (cholesteric liquid-crystalline phase) also proceeded with asymmetric induction to give (R)-(—)-2-phenylbutyric acid, with 18% optical yield.263 Electric dipole moments are reported for some esters of 5a-cholest-8(14)-en-3j8-ol there is some slight correlation with melting points.264... 

Hunt KLC, Harris RA (1991) Vibrational circular-dichroism and electric-field shielding tensors - a new physical interpretation based on nonlocal susceptibility densities. J Chem Phys 94 6995-7002... 

Just as electric dipole selection rules ( ) govern the absorption spectrum, magnetic dipole selection rules (R ) govern the circular dichroism (CD)14 spectrum, in which Ae = eleft - ght is plotted as a function of wavelength. The intensity of a CD band143 3a is proportional to the rotational strength, R this is related to the projection of the electric and magnetic dipole moments of a transition ... 

LFnder the influence of a static electric field liquid crystalline solutions of PBLG become uniaxially oriented and show both linear and circular birefringence and linear and circular dichroism. Accordingly, the measured CD shows a dependence of the microscopic angle, a, that the fast optical axis of the oriented solution makes with respect to the plane of polarization of the polarizer in the CD instrument. When the film is reasonably thin (0.01-0.1 mm thick) or/and only partially oriented (less than 5% orientation), the apparent CD of oriented film of deoxyribonucleic acid is expressed by the equation (50) ... 

Coupled oscillator models are extensions to the simple models developed for electronic circular dichroism. They are well known under the name exciton theory (see e.g. Harada and Nakanishi, 1972). These models, extended to vibrational transitions, describe the coupling of pairs of electric dipole transition moments. They predict equal amounts of positive and negative VCD intensity ... 

The fundamental requirement for the existence of molecular dissymmetry is that the molecule cannot possess any improper axes of rofation, the minimal interpretation of which implies additional interaction with light whose electric vectors are circularly polarized. This property manifests itself in an apparent rotation of the plane of linearly polarized light (polarimetry and optical rotatory dispersion) [1-5], or in a preferential absorption of either left- or right-circularly polarized light (circular dichroism) that can be observed in spectroscopy associated with either transitions among electronic [3-7] or vibrational states [6-8]. Optical activity has also been studied in the excited state of chiral compounds [9,10]. An overview of the instrumentation associated with these various chiroptical techniques is available [11]. 

These new molecular tensors seem very promising, as they are related to other spectroscopic parameters. A periodic magnetic field induces an electric field at the nuclei, which can be described in terms of an electromagnetic shielding tensor [15]. This is related to the intensity of absorption bands in vibrational circular dichroism (VCD) [16, 20]. Also, in the presence of a periodic electric field, the nuclei are acted upon by an induced magnetic field. This leads to the definition of magnetoelectric shielding [15]. 

Some of the nuclear shielding tensors are related to spectroscopic parameters. The static electric shielding can be experimentally obtained via infrared (IR) intensities [12], and the static electromagnetic shielding is available from the intensities of vibrational circular dichroism spectra [16-20]. 

A variety of spectroscopic and physical techniques have been used to investigate the nature of these redox centers. EPR, Fe Mossbauer spectroscopy, and Mo and Fe X-ray absorption spectroscopy Mo, Fe, and H electron-nuclear double resonance (ENDOR) linear electric field effect and magnetic circular dichroism (MCD) have provided information about the environment of the Mo and Fe nuclei and their interaction with the unpaired spin of electrons in paramagnetic species of the MoFe proteins. 

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