Magnetic Circular Dichroism (MCD)

Wavelength Range: 300 - 2000 nm
Purpose: Measures the differential absorption of right circularly polarized (RCP) and left circularly polarized (LCP) light in a magnetic field.
Primary Applications:
- Transition metal complexes
- Overlapping electronic transitions
- Analysis of porphyrins and bio-inorganic proteins
Key Formula:
- $Δ A = \frac{A_{-} - A_{+}}{A_{-} + A_{+}}$ , where $A_{-}$ is the absorption of LCP light, and $A_{+}$ is the absorption of RCP light.

Magnetic Circular Dichroism (MCD) Overview

Selection Rules:
- MCD is sensitive to both spin-allowed and spin-forbidden transitions, depending on the specific components of the MCD signal.
Typical Uses:
- Investigating the electronic structure of transition metal complexes
- Studying metalloproteins
- Determining the absolute configuration of chiral compounds

Procedure:
- A sample is placed in a magnetic field and irradiated with both left- and right-handed circularly polarized light.
- The absorption of the LCP and RCP light is measured separately using a spectrophotometer.
- The difference in absorption between the two polarizations is the MCD signal, which provides insights into the electronic structure, symmetry, and magnetic properties of the molecule.
Applications:
- Electronic Structure & Symmetry: MCD can reveal detailed information about the electronic structure and symmetry of a molecule.
- Magnetic & Electronic Properties: It is particularly useful for studying the magnetic properties of transition metal complexes and metalloproteins.
- Chiral Compounds: MCD can determine the absolute configuration of chiral compounds, which is critical in chemical synthesis.

Magnetic Circular Dichroism (MCD):
- MCD signals arise due to the imbalance in absorption caused by a magnetic field.
- Does not require the molecule to be chiral.
Circular Dichroism (CD):
- CD signals are due to chirality, causing an imbalance in the absorption of LCP and RCP light.
- Requires the molecule to be chiral.
Comparison Table:

Key Equation:
- $C D = A + B + \frac{C}{k_{B} T}$ , where $A$ , $B$ , and $C$ are components of the MCD signal.

A-Term:
- Represents the sum of the absorption of LCP and RCP light, independent of handedness.
- Arises from transitions between degenerate excited states.
- Non-zero for both spin-allowed and spin-forbidden transitions.
- Temperature Independence: The A-term is independent of temperature.
B-Term:
- Reflects the difference in absorption of LCP and RCP light when a degenerate excited state is mixed.
- Associated with zero-field splitting and the interaction between the electron's angular momentum and the photon's angular momentum.
- Non-zero primarily for spin-allowed transitions.
- Weak Temperature Dependence: Due to thermal population of different electronic states, though often negligible.
C-Term:
- Corresponds to the difference in absorption between degenerate ground states.
- Arises from spin-orbit coupling.
- Non-zero for spin-forbidden transitions.
- Strong Temperature Dependence: The C-term is strongly dependent on temperature.

MCD Signal Interpretation:
- Switching the poles of the magnetic field can switch between the Cotton effect and the anti-Cotton effect, providing detailed information on the electronic transitions.
- Diagram:

General Principles:
- Differentiating Transitions: MCD can reveal transitions that are buried under stronger transitions in absorption spectra if the weaker transition has a much larger first derivative or an opposite sign.
- Detection Sensitivity: MCD can detect transitions where no absorption is observed, especially in paramagnetic systems at low temperatures or with sharp spectral lines.
- Influence of Degenerate Levels: The presence of metals with degenerate energy levels often leads to strong MCD signals.
- Oxidation & Spin State Determination: MCD is capable of determining both the oxidation and spin state with high precision.
- Direct Observation of d–d Transitions: MCD allows for the direct observation of d–d transitions, which are typically weak in optical absorption spectroscopy and often silent in electron paramagnetic resonance (EPR) due to large ground-state sublevel splittings and fast relaxation times.