Term: magnetic circular dichroism (MCD)
https://doi.org/10.1351/goldbook.MT06778

Definition:
Magnetic circular dichroism is observed when a sample differentially absorbs left- and right- circularly polarized light in a magnetic field parallel to the light beam.

Notes:
0) The MCD signal, Δ, is calculated as \[\Delta  = \frac{\alpha(\lambda)^{-}\,-\,\alpha(\lambda)^{+}}{\alpha(\lambda)^{-}\,+\,\alpha(\lambda)^{+}}\] with α(λ)- and α(λ)+ the absorption coefficients for right and left circularly polarized light, respectively. The spectra are a representation of Δ vs @W06659@. Often, Δ is recorded as a function of the applied field (up to 10 T) and the temperature.
1) Phenomenon related to 'magnetically induced @O04303@ (Faraday effect)' by the 'Kramers-Kronig transformations', which connect optical refraction and absorption, i.e., MCD is observed in optically active materials at wavelengths with non-vanishing absorption. It occurs for @D01668@, @P04404@ and (@A00381@)-ferromagnetic material and has been observed from IR (@IT07399@) to X-ray regions. MCD optical transitions in molecular species arise if (i) degenerate electronic states are split in the presence of a magnetic field (first-order-@Z06739@) or (ii) states are mixed together by the applied magnetic field (second-order-@Z06739@). This may occur in the initial or the final states.
2) MCD is used as a probe of paramagnetism that permits the identification of the electronic and magnetic properties of the ground states of transition metal ion centres. The @W06659@ dependence of MCD can be used also to identify and assign optical transitions from metal ion sites.
3) Technique complementary to both EPR and electronic absorption spectroscopies in facilitating assignment of the ground-state spin and electronic transitions of a molecular entity.

Related Terms:
1) wavelength (http://doi.org/10.1351/goldbook.W06659).
2) optical activity (http://doi.org/10.1351/goldbook.O04303).
3) diamagnetic (http://doi.org/10.1351/goldbook.D01668).
4) paramagnetic (http://doi.org/10.1351/goldbook.P04404).
5) anti (http://doi.org/10.1351/goldbook.A00381).
6) infrared (http://doi.org/10.1351/goldbook.IT07399).
7) zeeman effect (http://doi.org/10.1351/goldbook.Z06739).
8) dichroism (http://doi.org/10.1351/goldbook.DT07357).

Source: PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 368 (https://doi.org/10.1351/pac200779030293)

Citation: 'magnetic circular dichroism (MCD)' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.MT06778

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