Optical rotations, solvent effects

The racemization of the phosphine (118) has been followed by optical rotation. The lack of a solvent effect indicates that there is little change in dipole moment in the formation of the planar transition state. Circular dichroism has been used to study the interactions of nucleotides with proteins and DNA with a histone. Faraday effects have been reviewed. Refraction studies on chloro-amino-phosphines, fluoro-amino-phosphines, and some chalcogenides are reported. 

Some aspects of the chemistry of helicenes require still more attention. Since the interpretation of the mass spectrum of hexahelicene by Dougherty 159) no further systematic work has been done on the mass spectroscopy of helicenes, to verify the concept of an intramolecular Diels-Alder reaction in the molecular ion. Though the optical rotation of a number of helicenes is known and the regular increase of the optical rotation with increasing number of benzene rings has been shown, the dependence of the rotation on the helicity is still unknown. The asymmetric induction in the synthesis of helicenes by chiral solvents, or in liquid crystals, though small, deserves still more attention because application to other organic compounds will be promoted when the explanation of observed effects is more improved. 

To avoid any complications caused by the intermolecular association, Goodman (78) reinvestigated the optical rotation of the peptides in dimethyl formamide, since in this medium the specific rotation is independent of concentration. From the latter study it was concluded that at 25° C the spontaneous helix formation of poly-y-methyl-L-glutamate in dimethyl formamide is occurring at the critical range of 7—9 units. Extension of these studies (73 b, 79) led to a better understanding of temperature and solvent effects upon the helix-coil transition of oligomeric polypeptides. 

The optically active square pyramidal complexes 39 and 41—49a and b epimerize in solution, as shown in Fig. 4 for 38a and b, some of them at lower, and some at higher temperatures5 f 63-65. The first-order decline in optical rotation yields overall rate constants k, which are the sum of the individual rate constants k and fc2 f°r the interconversion of both epimers into each other56,63-65 [Eq. (19)]. These first-order rate constants k for the approach to equilibrium were used for the calculation of the half-lives given in Table 1. A change in the solvent from DMF to toluene has only a small effect on the half-lives of the configurational stability63. ... 

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 are traditionally accounted for using the Lorentz effective field approximation [38], in which the optical rotation is multiplied by a local field factor... 

At a more detailed level, we note that the solvent effects on the optical rotation have the same origins as solvent effects on the energy itself, as described in detail in other contributions to this book. Most other studies of solvent effects on natural optical activity have focused on the electrostatic contributions. These contributions can be partitioned into direct effects arising from the influence of the dielectric environment on the electronic density of the solute, and into indirect effects arising from the relaxation of the nuclear structure in the solvent. For conformationally flexible molecules, we may also consider a third possible solvent effect due to the changes in the conformational equilibria when going from the gas phase to solution. 

Calculations of Solvent Effects on Natural Optical Activity Optical Rotation... 

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

Less optimistic conclusions about the performance of the DFT/PCM scheme were drawn in a study of solvent effect on the optical rotation of (.S )- -mclhy I benzyl amine [67]. The authors compared the optical rotation of this amine measured in 39 different solvents (whenever possible extrapolated to infinite dilusion) with the results obtained by means of IEF-PCM with the B3LYP functional and the aug-cc-pVDZ basis set. They observed substantial discrepancies for many of the hydrogen-bond forming solvents (which is not... 

The PCM/DFT model failed to predict the intrinsic rotation (i.e. the specific rotation extrapolated to infinite dilution) of (R)-3-methylcyclopentanone dissolved in carbon tetrachloride, methanol and acetonitrile [68], This molecule has been investigated because it exists in both an equatorial and an axial form, allowing researchers to investigate the interplay of solvent and conformational effects. The conformer populations used in the Boltzmann averaging were derived from IR absorption and VCD spectra. The deviation of the calculated optical rotation from experiment was found actually to be larger when IEF-PCM was used to account for direct solvent effects (and geometry relaxation) on the optical rotation than when the gas-phase values were used. 

The measurement of the optical rotatory power of chiral substances has been of major importance in the characterization of the enantiomeric purity. A number of computational techniques have been developed in the last year to evaluate this property. A recent review [142] shows in detail the advances in this field. Application of the new implementation of the evaluation of the optical rotatory power has allowed to the study of the conformational [143-146] and solvent effects [147,148] on the magnitude and sign of the optical rotation power. 

The solvent effects show almost the same tendencies as when hydrolysis is effected by optically active acids. The absolute value of the optical rotation of the recovered a-phenylpropionaldehyde is much larger in these examples. This indicates that the use of an optically active amine is more effective for asymmetric transformation than the use of an optically active acid. In addition, this method has been proved very effective for each type of carbonyl compound previously mentioned (3). 

J. Kongsted, K. Ruud, Solvent effects on zero-point vibrational corrections to optical rotations and nuclear magnetic resonance shielding constants, Chem. Phys. Lett. 451 (2008) 226. 

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