Optically active rotational strength

Natural circular dichroism (optical activity). Although circular dichroism spectra are most difficult to interpret in terms of electronic structure and stereochemistry, they are so very sensitive to perturbations from the environment that they have provided useful ways of detecting changes in biopolymers and in complexes particularly those remote from the first co-ordination sphere of metal complexes, that are not readily apparent in the absorption spectrum (22). It is useful to distinguish between two origins of the rotational strength of absorption bands. 

The planar C2h and C2V geometries of the 1,3-butadiene moiety are achiral structures and obviously they cannot show optical activity (i.e. ORD and CD). This has, of course, a spectroscopic origin. The optical activity of a transition Pq — Pi is determined by its Rotational Strength (R)1 defined as the scalar product... 

Dienes in quasi-s-fraws conformation are found only in cyclic structures where perfect planarity is hindered. The DR also holds valid for this kind of conformation, as demonstrated by the considerations of Section II.D.l.a and also confirmed by all the reported calculations. Indeed, contrary to what is sometimes found for cisoid systems, the rotational strength evaluated by many types of calculation is invariably found to follow the diene rule for transoid systems. However, very small skew angles are usually found in real molecules and this implies that the main contribution to the observed optical activity cannot come from the weak intrinsic distortion, but is more likely to stem from the dissymmetric perturbations, notably of the allylic axial substituents. 

An optically active transition is one that causes the plane of polarization of a light wave to rotate upon passage through a sample. The rotational strength of a transihon will be zero if the following quanhty is zero ... 

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. 

Now lets consider the rotational strength of the n-n transition for one example generating the optical activity through the mechanism. Thus the rotational strength is given by ... 

Chromophores which are asymmetric by nature are characterized by the absence of a center and plane of symmetry in the group of atoms participating in the optical transition. The rotational strength of these are usually larger when compared with chromophores that become optically active due to substitution. This is demonstrated in Mason and Schnepp s8 study of trans-cyclooctene, a-pinene and /1-pi none. They pointed out that the g (anisotropy factor, g = Ae/e) value of the major bands in trans -cyclooctene is relatively high as expected for an intrinsically asymmetric chromophore when compared with the other two olefins. 

M(A-A)3 complexes are optically active, and the problem can be remedied if a circular dichroism spectrum of one enantiomer can be measured and the sharp electronic lines identified. The bite and twist angles have very different effects on the rotational strengths. The twist angle has a marked effect that is quite plausible when one considers that a 60° twist causes conversion to the opposite enantiomer. Sign changes in the rotational strength can also occur at twist angles near / = 0°. 

In the "degenerate extended coupled oscillator" (DECO) description of the optical activity of n interacting dipoles, the rotational strength R, and hence the VCD intensities, is given by [18] ... 

The CD of bilirubin-complexes with serum albumin from a variety of species are very different under identical conditions. In several complexes, the observed CD spectrum is far more complicated than what is observed in the case of HSA. Other rotational strength mechanisms, such as coupling with protein groups, were suggested as additional sources of optical activity [239]. 

This is evident in a comparison of the expression for the magnitudes of optical activity, the rotator strength J ,, which is for an electronic transition 0 — fc,... 

The conventional view is that the molecular basis of optical activity relates to an assumed non-zero scalar product of electric and magnetic transition moments that defines a rotational strength... 

Maradudin AA, Montroll EW, Weiss GH, Ipatava P (1971) Theory of lattice dynamics in the harmonic approximation. Academic Press, New York March N (1992) Electro Density Theory of atoms and molecules. Academic Press Marcott C, Havel HA, Hedlund B, Overend J, Moscowitz A (1979) A vibrational rotational strength of extraordinary intensity. Azidomethemoglobin A. In Mason SF (cd) Optical activity and chiral discrimination, Reidel, Dordrecht, p 289... 

The theory of quadratic variations in optical activity with respect to the electric field strength was first formulated for macromolecules by Tinoco and Hammerle," and then developed by others." The earliest e qperi ments are due to Tinoco in solutions of poly-y-L-sJutamate in ethjdene dichloride of late, this experiment has been extended to transient optical rotation changes by Jennings and Baily." Also, electric field effects on the optical rotatory power of a compensated cholesteric liquid crystal have been stuped." ... 

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