Circular Dichroism Signals: Qualitative Description of Origins

Synonyms

Optical activity; Spectroscopy of chiral molecules

Definition

One can use circular dichroism (CD) spectroscopy as an empirical technique without understanding its underlying theory. However, it is helpful to have at least a pictorial understanding of the origin of CD before attempting to interpret data. This entry contains a description of why the handedness of a molecule (also called its absolute configuration) relates to the sign, or sign pattern, of its CD spectrum.

An electronic transition occurs because either the electric or magnetic field (or both) of the electromagnetic radiation “pushes” the electrons to a new stationary state. The effect of the electric field is to cause a linear rearrangement of the electrons called the electric dipole transition moment of the transition, which we denote by the vector μ. The direction in which μpoints is the same as the transition polarization—it is the direction in which the electrons are pushed during the transition. In contrast...