Some Effects of Solvation upon Optically Active Molecules.

Solute-solvent interactions having chemically interesting consequences manifest themselves in many spectroscopic measurements. A study of such interactions requires the ability to perform dilute gas phase measurements that can serve as a reference standard for the optical properties of the "isolated" solute molecule. However, vapor phase measurements are not always feasible; hence, the concept of the "inert" solvent is widely employed. Briefly, one hopes to find a solvent in which, aside from unavoidable changes such as a variation in bulk dielectric constant, the molecular species of interest is in an environment that does not differ too markedly from that in the gas phase. For example, in spectroscopic studies of hydrogen bonding with alcohols, a hydrocarbon solvent such as hexane has been used' as such a reference medium. In connection with our recent work2' 3on the temperature dependence of circular dichroism (C.D.), we have observed a number of solvent effects' that have forced us to reappraise our estimations as to what might constitute an "inert" solvent. Our understanding of these effects is far from complete. Hence, our interpretative remarks can only be qualitative, but we feel that the observations themselves are of sufficient interest to merit mention at this time, especially since they also bear on recent optical rotatory dispersion (O.R.D.)4 and C.D.' results for which no explanation could be offered. The Composition of the Observed C.D. Curve.-The observed rotational strength RoT associated with a particular absorption band at temperature T is conveniently gauged by the wavelength-weighted area under the corresponding C.D. curve according to the equation6