Solvent Effects on ORD and CD Spectra

Optical rotatory dispersion (ORD) involves the measurement of the rotation of plane-polarized light by a chiral compound as a function of the wavenumber. Circular dichroism (CD) is the unequal absorption of right and left circularly-polarized light as a function of its wavenumber. CD and anomalous ORD curves observed for chiral solute 

The CD spectrum of the thiolactam (R)-5-methylpyrrolidine-2-thione reveals a pronounced solvent dependence [490], Its long-wavelength n k thiocarbonyl CD band at Imax = 326 nm (in cyclohexane) undergoes a large solvent- and concentration-dependent blue shift of Av = +4880 cm (A1 = —54 nm) on going from cyclohexane to water. This corresponds to the blue shifts usually observed for n n bands in UV absorption spectra (see Section 6.2.3) and is best explained in terms of a monomer dimer equilibrium of the thiolactam involving the formation of intermolecular solute/ solute hydrogen bonds in nonpolar solvents. In HBA solvents e.g. DMSO) and HBD solvents [e.g. EtOH), this equilibrium is disturbed by competitive solute/solvent H- 

For some carbonyl compounds, a reverse in optical rotation sign has been observed when the ORD spectra were measured in solvents of different polarity. For example, (5)-5-hydroxy-l,7-diphenyl-3-heptanone is dextrorotatory in trichloromethane (molar optical rotation [ I)]p = +39.6) and levorotatory in methanol ([ F]d = 7.6), whereas its acetate is dextrorotatory in both solvents [363], This observation suggests that the interaction between the y9-ketol moiety and the solvent is responsible for the change and reversal in optical rotation with increasing solvent polarity. 

Structurally symmetric achiral compounds may show optical activity in the presence of chiral solvent molecules owing to asymmetry induced by the chiral solvent. For example, the achiral carbonyl compounds benzil and benzophenone surprisingly show optical activity in the region of the n absorption in the CD spectrum when dis- 

Conversely, it has been shown that a chiral solute, dissolved in an achiral solvent, can induce a chiral distribution of the solvent molecules around the solute even though the solvent molecules themselves are achiral. This allows the achiral solvent to contribute significantly to the CD spectrum, as has been shown for solutions of (-l-)-camphor. Measurements and calculations have shown that 5-10% of the intensity of the carbonyl CD band of camphor are contributed by its solvation shell [491]. 

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