Circular dichroism signals: qualitative description of origins

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 μ points is the same as the transition polarization—it is the direction in which the electrons are pushed during the transition. In contrast to the linear effect of the electric field, the magnetic field induces a circular rearrangement of electron density. The net circulation of charge is the magnetic dipole transition moment, m. The vector μ is known as a polar vector: It has magnitude and direction. m describes rotation about an axis and is known as an axial vector. Its direction is such that when viewed from its tip the rotation is anticlockwise. The key feature that gives rise to a nonzero CD spectrum is that the electron redistribution that happens during a transition is helical. This is mathematically expressed by the transition having μ ≠ 0 and m ≠ 0.