Induced circular dichroism. Part III. Chiroptical properties and ionpair equilibria in 2-benzoylbenzoic acid–amphetamine

Abstract

The molecular ellipticity of the induced circular dichroism (i.c.d.) of the 2-benzoylbenzoic acid-amphetamine system is highly dependent not only on the polarity of solvent but also on the concentration of the salt and the change in temperature. The i.c.d. effect at various concentrations of the salt indicates that the ion-pair dissociates at concentrations below 10^–3M, and aggregates to form triple and/or quadruple ions at concentrations over 10^–2M in non-polar solvents. The magnitude of the i.c.d. effect is affected mainly by two factors, the intramolecular rotation and the ion-pair equilibrium. The enthalpy of chiroptical induction, ΔH, due to the intramolecular rotation is estimated as 1 ± 0.5 kcal mol^–1 which is independent of the polarity of the solvent. The ΔH value due to ion-pair equilibrium is very large and strongly dependent on the solvents polarity. A plot of the ΔH versus E_T for various solvents resulted in two straight lines, one for benzenoid solvents and the other for aliphatic solvents. The significance of the ΔH values is discussed.