Chiral molecules optical rotation effects

Optical activity is the ability of a compound to rotate the plane of polarized light. This property arises from an interaction of the electromagnetic radiation of polarized light with the unsymmetric electric fields generated by the electrons in a chiral molecule. The rotation observed will clearly depend on the number of molecules exerting their effect, i.e. it depends upon the concentration. Observed rotations are thus converted into specific rotations that are a characteristic of the compound according to the formula below. 

Paul Pfeiffer discovered a very interesting stereochemical phenomenon, which now bears his name — the Pfeiffer effect this has received a good deal of attention.30 When an optically active substance which is stable in solution is added to a solution of a labile chiral substance, the optical rotation of the solution changes, reaching a new level in some hours. Several theories have been advanced to explain the phenomenon, the most satisfactory based on the supposition that the optically active ion or molecule forms an association with one isomer of the racemic pair of the labile substance and thus shifts the dextro—levo equilibrium. In general it is not possible to use this as a means of resolution, for when the added optically active substance is removed from the labile material, the latter immediately racemizes. 

B. Mennucci, J. Tomasi, R. Cammi, J. R. Cheeseman, M. J. Frisch, F. J. Devlin, S. Gabriel and P. J. Stephens, Polarizable Continuum Model (PCM) calculations of solvent effects on optical rotations of chiral molecules, J. Phys. Chem. A, 106 (2002) 6102-6113. 

Solvent effects on the optical rotation of several conformationally rigid chiral organic molecules (fenchone, camphor, a- and /3-pinene, camphorquinone, verbenone and methy-... 

Figure 4.6-9 Induced cholesteric solutions Schematic outline of experiment and evaluation of the optical rotation p(A) related to the selective reflection band (reflection Cotton effect, RCE, centred at the wavelength A/ ) in order to characterize the chirality of the solute molecules by the helical twisting power.

In an earlier work, the same group had used Hartree-Fock calculations in calculating An for the most stable isomer of some chiral organic molecules in vacuum.In the more recent work, ° they extended the study in several directions. At first, they applied the B3LYP density-functional method, whereby also correlation effects were included. Second, they compared gas-phase results with those obtained for a solution, whereby they applied the polarizable-continuum method for the treatment of the solvent. And third, for some of the larger molecules they included the effects of having a mixture of more different stable structures in an approach very similar to that we discussed above in section III I for the calculation of the optical rotation. 

So far, optical rotations of compounds have been discussed where the optical activity is associated with the particular structure of the cyclopropane ring. This means that the rotations are generated by a chiral arrangement of (achiral) ligands attached to the (achiral) molecular skeleton. For certain substituent patterns of I, in particular, the rotations are induced by atomic asymmetry. This is true for III and IV. The effect of the (achiral) cyclopropane moiety (viewed as a ligand) on open-chain molecules with an asymmetric carbon atom can be seen from the rotations of (S)-( —)-l-methyl-1-(1-ethoxyethyl) cyclopropane (80) and its counterpart 81 with only acyclic substituents. ... 

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