CD and ACD spectroscopy on anisotropic samples: chirality of oriented molecules and anisotropic phases--a critical analysis.

Abstract

Chiral anisotropic phases exhibit an elliptical birefringence and dichroism instead of the circular effects of isotropic dissymmetric and the linear effects of achiral anisotropic phases. Therefore, techniques to determine chiral effects of chiral anisotropic phases yield not always results of chirality measurements, namely, pseudoscalar quantities. From definitions for a chirality measurement with non-oriented and oriented macroscopic anisotropic phases procedures are derived by which the pseudoscalar contribution included in the elliptical birefringence and dichroism can be obtained. Within this description the CD of chiral microcrystallites, isotropically distributed in pressed KBr plates or in Nujol, is a chirality measurement. With an excess of anisotropic distributed material in pressed KBr plates or, in general, with oriented macroscopic anisotropic phases, the ACD is no longer a pseudoscalar. An achiral information is included in the ACD which is not an artifact obtained by the mixing of "circular and linear effects" via nonideal optical elements in a CD spectrometer.