Optical rotation chromophore model

The helical conductor model is useful for the prediction of optical rotations and configurations of aliphatic helices which do not contain chromophoric skeletons, and for twisted chains of atoms. In studies of the chiroptical properties of isotactic... 

The explicit form of the interaction Hamiltonian // ,(() consists of a series of multipolar terms, but for most purposes the electric-dipole (El) approximation is sufficient. Although the results are calculated within this approximation for each molecular center detailed analysis of the coupling provides results equivalent to the inclusion of higher-order multipole terms for the pair. The same assumption underlies the well-known coupled-chromophore model of optical rotation (Kuhn 1930 Boys 1934 Kirkwood 1937). The Hamiltonian for the system may thus be written as... 

The optical activity in valence excitations of chiral metal complexes has been effectively treated using the model of an achiral chromophore (metal ion) in a chiral environment (ligands) and this model appears also appropriate for XAS in view of the core nature of the initial orbital state. The zero-order electric and magnetic transition moments arise from different transitions and must be mixed by some chiral environmental potential (V ). Considering the case of a lanthanide ion, and taking tihe electric dipole transitions for the Z.2,3 edge as Ip—Kj), a first-order perturbation expression for the rotational strength looks like ... 

The chromophores are closely related to those in peptides. Dissymetric perturbations of the amide chromophores of the main chain arise solely from side chain effects the aliphatic polyisocyanate has enhanced rotational strength as compared to its model compound (5 )-(-)-A,A -diacetyl-2-methylbutylamine. In the aliphatic polymer, the main chain has inherently symmetric chromophores which acquire optical activity from dissymetric perturbation of their environment by the side chain. In the aromatic polymer an additional Cotton effect also arises from interactions among the aromatic side chains. This enhancement may be explained by a conformational preference resulting from favored spatial arrangements of the asymmetric side chain but the study was complicated by the fact that the polymer is insoluble in most organic solvents except in chloroforms and by the specific interactions between this solvent and the urea-like main chain (XIXc). 

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