Chromophore rotational strength

The rotational strength calculated for I is as large as that of a butadiene twisted by 20°. In II, with an out-of-plane methyl, R increases by a factor of about 2. This shows that the contributions to R of dissymmetric substituents of chiral cisoid dienes may be comparable to and even outweigh the contributions arising from the intrinsic dissymmetry of the chromophore. 

In the skewed form, instead, the transition is allowed both electrically and magnetically, with parallel transition moments. The product in equation 1 is hence non-vanishing, implying that this transition has finite rotational strength. This observation leads to the conclusion that skewed 1,3-butadiene is an intrinsically dissymmetric chromophore. 

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. 

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. 

A variety of studies have been performed in which the chirality induced in ketone chromophores has been characterized by CD spectroscopy. CD has been induced in the n-+ r transition of cyclohexanone upon its dissolution into several alcoholic and ester-functionalized solvent systems [5]. In this work it was established that hydrogen bonding between solute and solvent was not required for the generation of solvent-induced CD. In a more comprehensive work, the CD induced in 20 compounds containing a ketone chromophore was studied in approximately 15 chiral solvent systems [6]. It proved difficult to develop general rules for the observation (or lack thereof) of solvent-induced CD, with the rotational strengths being found to be solvent, temperature, and... 

In Ref 105 the (n, n ) circular dichroism of the tricyclic ketones 152-155 was analyzed with the aim to see whether the systems must be viewed as inherently chiral chromophores or rather inherently symmetric chromophores, respectively. If they fall more nearly into the inherently symmetric classification the octant rule should be applicable. As a basis for the classification the order of magnitude of the rotational strengths was used together... 

Referring to the very similar rotational strengths of a-fenchocampherone (159) and spiro(cyclopropane-l,7 -norbornanone) (155) (Figure 13 Table 9) it was suggested that 155 has an inherently symmetric chromophore in which the normal octant perturbers dominate the contributions to the chirality of the (n, k ) transition. 

Dienes are inherently chiral if they are twisted around the central CC bond. This produces a rotational strength that is positive if the twisted butadiene chromophore forms a right-handed helix. But since this inherent effect is not always predominant, contributions of other substituents have to be taken into account as well in order to be able to predict the CD spectrum. Substituents in allylic positions are frequently most important. 

In the ligand polarization mechanism for optical activity, the potential of the electric hexadecapole component, Hxy(x>-y>), produces a determinate correlation of the induced electric dipole moment in each ligand group which does not lie in an octahedral symmetry plane of the [Co Ng] chromophore (Fig. 8). The resultant first-order electric dipole transition moment has a non-vanishing component collinear with the zero-order magnetic moment of the dxy dxj yj transition in chiral complexes, and the scalar product of these two moments affords the z-component of the rotational strength, RJg, of the Aj -> Ti octahedral excitation. 

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