Low temperature circular dichroism: conformational effects

Abstract

Circular dichroism (c. d.) is a very powerful tool for the determination of the geometry of a molecule in solution; the measured values are, however, often strongly dependent upon the experimental conditions used, such as, for example, solvent, concentration and temperature. In this paper only the latter effect and its applica­tion to conformational analysis is discussed. The two main reasons for a change of ∆ ϵ with temperature are solvation and conformational mobility (see, for example, Moscowitz et al . 1963 b ). Very often the investigated substance has some interactions with the solvent, leading to a solute-solvent complex, which might be as strong as a hemiketal, if a ketone is measured Table 1. Effect of temperature variation on C. D. (A) Solute + solvent ⇌[solute — solvent] (B) Conformational equilibria (I) Rigid structure ( R = const.) (II) Compounds with conformational mobility (1) One energy minimum during complete (pseudo) rotation ( I T 2 T 1 measure for steric hindrance) (2) Two energy minima ( R T = ( f ( R a , R b , T , ∆ G )) (3) Three or more energy minima in an alcoholic solution, or as weak as the interaction between, say, the same ketone and a saturated hydrocarbon, due to the induced dipole moment of the latter. In general, such an equilibrium between the components and the complex will be temperature dependent and as the complex may have a different c. d. from the non-solvated molecule, a change of ∆ ϵ may result. Frequently a blue-shift of the maxima is found at low temperatures as well as a more clearly resolved fine structure. These effects are ascribed to such solute-solvent interactions (see table 1).