Solvent effects electronic circular dichroism

Wamke I, Ay S, Brase S, Furche F (2009) Chiral cooperativity and solvent-induced tautom-erism effects in electronic circular dichroism spectra of [2.2]paracyclophane ketimines. J Phys Chem A 113 6987-6993... 

There have been numerous applications of continuum models to equilibria and reactions in solution surveys of these and extensive listings are provided by Cramer and Truhlar.16 Other studies have focused upon the effects of solvents upon solute molecular properties, such as electronic and vibrational spectra,16 dipole moments, nuclear quadrupole and spin-spin coupling constants and circular dichroism.12... 

There are many more solvent effects on spectroscopic quantities, that cannot be even briefly discussed here, and more specialized works on solvent effects should be consulted. These solvent effects include effects on the line shape and particularly line width of the nuclear magnetic resonance signals and their spin-spin coupling constants, solvent effects on electron spin resonance (ESR) spectra, on circular dichroism (CD) and optical rotatory dispersion (ORD), on vibrational line shapes in both the infrared and the UV/visible spectral ranges, among others. 

It should be recalled that the calculation of solvent effects on optical activity presents some unique problems. A chiral solute induces a chiral structure of the surrounding solvent, even when the individual solvent molecules are achiral. This means that the solvent participates in the observed optical effect not only by a modification of the geometric structure and electronic density of the solute, but that part of the observed OR or circular dichroism arises from the chiral solvent shell rather than from the solute molecule as such. This is not accounted for by the PCM, and can be rendered only by an explicit quantum mechanical treatment of at least the first solvent shell, or preferably by molecular dynamics simulations. 

The electronic structure of barbiturates has also been investigated by spectropolarimetric methods.73-77 Circular dichroism (CD) spectra of a series of (S)-5-alkyl-5-(2 -pentyl)barbituric acids show three Cotton effects centered around 212, 240, and 260 nm, and the short-wavelength band is positive and has an opposite sign in relation to the longer wavelengths. However, in the case of (S)-5-(2 -pentyl)barbituric acid, the signs of all three Cotton effects are opposite to those of all the other derivatives. The solvent studies indicate that the 212- and 260-nm bands arise from the n - n and n - n transitions, respectively, but the 240-nm band is due to n - a or the second n - n transitions.74 An influence of concentration and ionization mode has not been observed. Similar assignment of three Cotton effects has been reported for (S)-5-alkyl-5-(2 -pentyl)-2-thiobarbituric acids.75... 

Because all substances, including solvents, cell windows, etc., exhibit the Faraday effect, it is, in general, only possible to get interpretable results by measuring the magnetic circular dichroism (MCD) through the electronic absorption bands of the complex of interest. This can be done with apparatus normally used for conventional CD measurements by surrounding the sample cell with a small superconducting solenoid. 

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