Editing Circular dichroism (section)

=== Magnetic circular dichroism (MCD) ===
[[Magnetic circular dichroism]] (MCD) is the differential absorption of left and right circularly polarized light in the presence of a magnetic field oriented parallel to the direction of light propagation. MCD differs from natural CD in that it does not require a chiral sample; the origins of CD and MCD are quite different.

There are three contributions to an MCD spectrum, known as the A-, B-, and C-terms. At room temperature, the A- and B-terms dominate, especially for non-magnetic chromophores, and form the majority of what can be observed in a standard MCD experiment.

The A-term is due to [[Zeeman splitting]] of degenerate excited states by a difference in spin. The result is a small shift apart in the wavelengths of the UV-Visible spectrum for each spin state, with each state preferentially absorbing left or right circularly polarized light. The A-term MCD spectrum is observed as a derivative of an absorbance transition with a sharp transition around the absorbance peak. Because of the A-term bands, the MCD spectrum is more structured than the absorbance or natural CD spectrum, multiple overlapping chromophores can be quantified much more easily, and changes in individual chromophores can be tracked.

The B-term is due to mixing of non-degenerate ground states. It is normally observed as a single absorption band type peak, which may be either positive or negative.

The C-term is due to changes in populations of molecules over the Zeeman sublevels due to the response to the field of a paramagnetic ground state. They become significant in the MCD spectrum only at very low temperatures, and the instrumentation requirements of high field strengths and cryogenic temperatures are very similar to those of the related technique of [[electron paramagnetic resonance]] (EPR). This is an expensive and dedicated system, and access to the C-terms is therefore beyond the scope of an easily interchangeable MCD accessory on a benchtop CD instrument.

Collection of MCD data is similar to that of natural CD, except that there are two possible orientations of the magnet, with either the north or south pole closer to the detector. Normally a spectrum is acquired with the magnet in each orientation; since the MCD contribution to the spectrum changes sign when the magnetic field is reversed, whereas the natural CD contribution does not, both contributions can be obtained by averaging the two spectra.

For further information on MCD see this book.<ref>{{Cite book |last=Mason |first=W. Roy |url=https://onlinelibrary.wiley.com/doi/book/10.1002/9780470139233 |title=A Practical Guide to Magnetic Circular Dichroism Spectroscopy |date=2006-11-08 |publisher=Wiley |isbn=978-0-470-06978-3 |language=en |doi=10.1002/9780470139233}}</ref>