Helicity of Molecules Different Definitions and Application to Circular Dichroism

Abstract

Any object which is not superposable onto its mirror image was called “chiral” at the end of last century by Lord Kelvin [1], who derived the term from the Greek word χειϱ for hand. In particular, molecules are called chiral if they have this mentioned property, and it has been well known since the middle of last century [2] that in the non-ordered state (gases, liquids, amorphous solids) optical activity can be measured only if the molecules are chiral. Chirality is thus a molecular property, whereas optical activity is a bulk property of a substance, i.e. in all practical cases can be measured only for a large ensemble of molecules. It is thus wrong to speak of a “chiral substance”, as it is incorrect to speak of “optically active molecules”!