Magnetic induced optical activity

The signs of optical activity or of the Faraday effect (magnetically induced optical activity see Appendix) used by physicists are frequently opposite to the chemically defined ones. Furthermore, the handness in liquid crystals, such as cholestric or chiral smectic ones, often has been defined erroneously and thus confused. 

Although the phenomenon of magnetically induced optical activity was discovered by Faraday in 1845, it has only been during the last... 

Optical activity arises from the coupling of given electric-allowed transitions with a chiral orientation (coupled oscillator mechanism or two-electron mechanism) or from the electric or magnetic moments of a transition being pertubed by a chiral static field (asymmetrically perturbed field mechanism or one-electron mechanism) in the given one molecule. A similar mechanism of the optical activity can be expected for molecular assemblies which are composed of chiral and achiral ones. This type of optical activity is called induced optical activity and depends on types of inter-molecular interaction modes. 

Referred to as the induced optical activity, the resultant CD spectrum has measurable peaks only when the transition is either electrically or magnetically allowed. Thus, this IC-D method is useful for discriminating the two types of transition 162). 

Aside from the intrinsic circular dichroism originating from the molecular structure, a so-called induced optical activity may result when a molecule is situated in an asymmetric environment. Magnetic circular dichroism, for instance, maybe produced by applying an external magnetic field to an absorbing sample. Adsorption of pigment molecules on nucleic acids or protein molecules may also induce circular dichroism. 

MCD spectroscopy combines the CD experiment with a longitudinal magnetic field, where the application of the magnetic field induces optical activity in any material so that all substances exhibit MCD activity. MCD probes the Zeeman splittings in the ground and excited states and the field-induced mixing between states. 

Chiral-at-metal cations can themselves serve as chirality inducers. For example, optically pure Ru[(bipy)3] proved to be an excellent chiral auxihary for the stereoselective preparation of optically active 3D anionic networks [M(II)Cr(III)(oxalate)3]- n (with M = Mn, Ni), which display interesting magnetic properties. In these networks all of the metalhc centers have the same configuration, z or yl, as the template cation, as shown by CD spectroscopy and X-ray crystallography [43]. 

For the optical activity of achiral chromophores with a dissymmetric environment, two types of theoretical treatments have been proposed coupled oscillator treatment and one-electron treatment. The charge distribution of the magnetic dipole transition correlates Coulombically with an electric dipole induced in the substituents, and the colinear component of the induced dipole provides, with the zero-th order magnetic moment, a non-vanishing rotational strength. 

The structure of this contribution is as follows. After a brief summary of the theory of optical activity, with particular emphasis on the computational challenges induced by the presence of the magnetic dipole operator, we will focus on theoretical studies of solvent effects on these properties, which to a large extent has been done using various polarizable dielectric continuum models. Our purpose is not to give an exhaustive review of all theoretical studies of solvent effects on natural optical activity but rather to focus on a few representative studies in order to illustrate the importance of the solvent effects and the accuracy that can be expected from different theoretical methods. 

The curve a v) or a(k) is referred to as natural optical rotatory dispersion (ORD) and the instrument used for its measurement is a spectropolarimeter. If the optical activity has been induced by an external magnetic field, the... 

If the optical activity is induced by an external magnetic field it is proportional to the magnetic field strength, and one measures magnetic circular dichroism (MCD), commonly expressed as magnetic molar ellipticity per gauss. 

In an external magnetic field all matter becomes optically active. This observation was first made by Faraday in 1845 the magnetically induced rotation of the plane of polarized light is therefore referred to as the Faraday effect. In recent years, the common mode of study of this phenomenon has been the measurement of magnetic circular dichroism (MCD). Similarly to natural circular dichroism, magnetic circular dichroism is defined as the difference Af = fi, - of the extinction coefficients for left-handed and right-... 

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