Magnetic circular dichroism
Magnetic circular dichroism (MCD) is the differential absorption of left and right circularly polarized (LCP and RCP) light, induced in a sample by a strong magnetic field oriented parallel to the direction of light propagation.
Paramagnetic systems are common analytes, as their near-degenerate magnetic sublevels provide strong MCD intensity that varies with both field strength and sample temperature.
The MCD signal also provides insight into the symmetry of the electronic levels of the studied systems, such as metal ion sites.
[2] The development of MCD really began in the 1930s when a quantum mechanical theory of MOR (magnetic optical rotatory dispersion) in regions outside absorption bands was formulated.
The expansion of the theory to include MCD and MOR effects in the region of absorptions, which were referred to as "anomalous dispersions" was developed soon thereafter.
Since that time there have been numerous studies of MCD spectra for a very large variety of samples, including stable molecules in solutions, in isotropic solids, and in the gas phase, as well as unstable molecules entrapped in noble gas matrices.
More recently, MCD has found useful application in the study of biologically important systems including metalloenzymes and proteins containing metal centers.
However, in MCD in the presence of a magnetic field, LCP and RCP no longer interact equivalently with the absorbing medium.
So, natural CD is much more rare than MCD which does not strictly require the target molecule to be chiral.
However, like CD, it is dependent on the differential absorption of left and right hand circularly polarized light.
[1] This is distinctly different from the related phenomenon of optical rotatory dispersion (ORD), which can be observed at wavelengths far from any absorption band.
The MCD signal ΔA is derived via the absorption of the LCP and RCP light as This signal is often presented as a function of wavelength λ, temperature T or magnetic field H.[1] MCD spectrometers can simultaneously measure absorbance and ΔA along the same light path.
[6] This eliminates error introduced through multiple measurements or different instruments that previously occurred before this advent.
The PEM is adjusted to cause an alternating plus and minus 1/4 wavelength shift of one of the two orthogonal components of the ordinary beam.
The departing circularly polarized light oscillates between RCP and LCP in a sinusoidal time-dependence as depicted below:
If there is a ΔA, then a small AC voltage will be present that corresponds to the modulation frequency, ω.
This voltage is detected by the lock in amplifier, which receives its reference frequency, ω, directly from the PEM.
Light from the source enters one side, interacts with the sample (usually also temperature controlled) in the magnetic field, and exits through the opposite window to the detector.
Optical relay systems that allow the source and detector each to be about a meter from the sample are typically employed.
In the case of ferric heme proteins,[8] MCD is capable of determining both oxidation and spin state to a remarkably exquisite degree.
In addition, the application of MCD spectroscopy greatly improved the level of understanding in the ferrous non-heme systems because of the direct observation of the d–d transitions, which generally can not be obtained in optical absorption spectroscopy owing to the weak extinction coefficients and are often electron paramagnetic resonance silent due to relatively large ground-state sublevel splittings and fast relaxation times.
In the presence of a static, uniform external magnetic field applied parallel to the direction of propagation of light,[2] the Hamiltonian for the absorbing center takes the form
term is a frequency-independent correction factor allowing for the effect of the medium on the light wave electric field, composed of the permittivity
The slight offsets result in incomplete cancellation of the positive and negative features, giving a net derivative shape in the spectrum.
[9] The relative contributions of A, B and C terms to the MCD spectrum are proportional to the inverse line width, energy splitting, and temperature: where
[12] In the visible and near-ultraviolet regions, the hexacyanoferrate(III) ion (Fe(CN)63−) exhibits three strong absorptions at 24500, 32700, and 40500 cm−1, which have been ascribed to ligand to metal charge transfer (LMCT) transitions.
Additionally, only A terms, which are temperature independent, should be involved in MCD structure for closed-shell species.
[14] An MCD study of Fe(CN)63− embedded in a thin polyvinyl alcohol (PVA) film revealed a temperature dependence of the C term.
The room-temperature C0/D0 values for the three bands in the Fe(CN)63− spectrum are 1.2, −0.6, and 0.6, respectively, and their signs (positive, negative, and positive) establish the energy ordering as 2t2g→2t1u2<2t2g→2t2u<2t2g→2t1u1 To have an A- and B-term in the MCD spectrum, a molecule must contain degenerate excited states (A-term) and excited states close enough in energy to allow mixing (B-term).
In addition to containing A- and B-terms, this example demonstrates the effects of spin-orbit coupling in metal to ligand charge transfer (MLCT) transitions.
