Overview
Source: Joshua Wofford, Tamara M. Powers, Department of Chemistry, Texas A&M University
Mössbauer spectroscopy is a bulk characterization technique that examines the nuclear excitation of an atom by gamma rays in the solid state. The resulting Mössbauer spectrum provides information about the oxidation state, spin state, and electronic environment around the target atom, which, in combination, gives evidence about the electronic structure and ligand arrangement (geometry) of the molecule. In this video, we will learn about the basic principles of Mössbauer spectroscopy and collect a zero field 57Fe Mössbauer spectrum of ferrocene.
Principles
Nuclear Spin Angular Momentum (I):
The nuclear spin (I) of an atom is defined as the total angular momentum of the nucleus. The ground state nuclear spin for a given atom is dependent on the number of protons and neutrons in the nucleus and can be a half integer value (1/2, 3/2, 5/2, etc.) or an integer value (1, 2, 3, etc.). Nuclear spin excited states, I + 1n, where n is an integer value, exist and can be accessed if enough energy is applied to the nucleus.
Instrument Setup:
The general instrument setup is shown in Figure 1. The source that generates gamma rays is connected to a driver, which continuously moves the source with respect to the sample (the necessity of this will be explained below). The gamma rays then hit the solid sample, which is frequently suspended in an oil. Upon passing through the sample, the resulting transmitted radiation hits the detector, which measures the intensity of the beam upon interaction with the sample.
Figure 1. General instrumentation setup.
Gamma Ray Generation:
The source used to generate the gamma rays for the experiment needs to be of the same isotope as the atoms in the sample that are absorbing the radiation. For example, for 57Fe Mössbauer spectroscopy, a radioactive 57Co source is utilized. 57Co decays (half life = 272 days) to the excited state of 57Fe, I = 5/2. The resulting excited state further decays to either the I = 3/2 excited state or to the I = 1/2 ground state. Relaxation from the I = 3/2 excited state of 57Fe to the ground state produces a gamma ray of desired energy for the experiment. However, the energy of the generated gamma ray does not exactly match the energy required for a nuclear excitation of the atom in a molecule. Returning to the example of 57Fe, the energy levels of the I = 1/2 and I = 3/2 states change upon putting Fe within a molecule, where the oxidation and spin state of the metal as well as the ligand environment influence the electron field gradient at Fe. Therefore, to tune the energy of the resulting gamma ray, the source is moved with respect to the sample during the experiment using a driver (Figure 1). The conventional "energy" unit in Mössbauer spectroscopy is mm/s.
What Does a Typical Mössbauer Spectrum Look Like?:
In a Mössbauer spectrum, the percent transmission (dips in % transmission or the location of a peak that indicate gamma rays were absorbed at that energy) is plotted against the energy of the transition (mm/s). A typical spectrum is shown in Figure 2. The two peaks together are considered a single quadruple doublet, which is the result of two types of observable nuclear interactions:
1. The isomer shift (or chemical shift, δ, mm/s) is a measure of nuclear resonance energy and is related to the oxidation state of the atom. In Figure 2, the isomer shift is the energy value half way between the peaks in the spectrum. Table 1 includes the typical ranges of isomer shifts for given oxidation states and spin states of Fe.
Table 1. Some typical ranges of isomer shifts for Fe-containing compounds.1
| Oxidation state | Spin state (S) | Isomer shift range (mm/s) |
| Fe(II) | 0 | –0.3 to 0.5 |
| Fe(II) | 2 | 0.75 to 1.5 |
| Fe(III) | 1/2 | –0.2 to 0.4 |
| Fe(III) | 5/2 | 0.2 to 0.55 |
2. The quadrupole splitting (ΔEQ, mm/s) is a measure of how the electric field gradient around the atom affects the nuclear energy levels of the atom. Like the isomer shift, ΔEQ provides information about the oxidation state. The spin state and the symmetry of the electron density around the atom (placement of ligands around a metal) will also affect the observed ΔEQ. In Figure 2, the quadrupole splitting is the energy difference in mm/s between the two peaks in the spectrum.
Figure 2. A typical Mössbauer spectrum is plotted with velocity (energy) along the x axis and % transmission along the y axis. Here we see a single quadrupole doublet, with isomer shift, δ, and quadrupole splitting, ΔEQ.
Isomer Shift and Quadrupole Splitting - What Nuclear Transitions Do These Values Represent?:
Here we will consider the nuclear spin transitions for an Fe atom (I = 1/2 ground state). The isomer shift is directly related to the electron transition in the s orbital of the atom from the I = 1/2 to an excited state (Figure 3). If the surrounding electric field gradient is non-spherical, due to either a non-spherical electronic charge or asymmetric ligand arrangement, the nuclear energy level splits (Figure 3), i.e., the I = 3/2 excited state splits into two mI states ± 1/2 and ± 3/2. As a result, both nuclear transitions are observed in the Mössbauer spectrum and the distance between the two resulting peaks is called the quadrupole splitting. The quadrupole splitting value is therefore a measure of the effect on the nuclear energy levels with the electric field gradient around the atom.
Figure 3. Observable nuclear interactions in a 57Fe Mössbauer spectrum including isomer shift, quadrupole splitting, and hyperfine splitting in the presence of a magnetic field.
Hyperfine Splitting:
Hyperfine splitting (or Zeeman splitting) can also be observed in the presence of an internal or external magnetic field. In the presence of a magnetic field, each nuclear energy level, I, splits into 2I + 1 sub-states. For example, in an applied magnetic field the nuclear energy level I = 3/2 would split into 4 non-degenerate states including 3/2, 1/2, -1/2 and -3/2, with 6 allowed transitions (Figure 3).
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Procedure
1. Preparation of the Sample
- Weigh 100 mg of ferrocene in a delrine Mössbauer cup.
- Add several drops of paratone oil to the sample. Using a spatula, mix the sample and the oil into a uniform paste.
- Freeze the sample in liquid nitrogen.
2. Mounting the Sample
- Fill the sample chamber with He gas.
- Unscrew the sample rod from the instrument and remove the sample rod.
- While mounting the sample, close the sample chamber with a cap and secure with screws.
- Load the Mössbauer cup into the sample holder at the end of the rod.
- Tighten the screw to secure the cup in the sample holder.
- Dust off any ice that forms before freezing the end of the sample rod in liquid nitrogen.
- With He flowing through the sample chamber, unscrew and remove the cap, and insert the sample rod.
- Secure the rod to the instrument with the screws.
- Turn off the He and pull the vacuum on the sample chamber.
- Turn off the vacuum and refill the sample chamber slightly with He to enable thermal exchange between the sample and the cold head of the instrument via the He gas.
3. Data Collection and Workup
- Open the Mössbauer data collection software. Here we use W302 by Science Engineering & Education (SEE) Co.
- The first screen will show the total count of gamma rays hitting the detector at a range of energies. Select the peak that includes the energy value 14.4 keV and 2 keV escape peak.
- Hit the "Send Windows" button. This will send the data to the W302 software (SEE Co).
- Open the W302 program. Select the desired source velocity (0-12 mm/s). Hit the "clear channel" to begin a new data collection.
- After the desired resolution is reached, fit the data with a suitable program. Here we use WMOSS by SEE Co. The fit provides the values for isomer shift and quadrupole splitting (if a doublet is present).
Mössbauer spectroscopy is a method for evaluating the oxidation state, electronic spin state, and electronic environment of an atom.
The nuclear spin angular momentum of an atom, or nuclear spin for short, describes the discrete energetic states available to a nucleus. The energy levels are affected by the oxidation state, electronic spin state, and ligand environment.
Differences in nuclear energy levels are reflected in the nuclear excitation energy. Mössbauer spectroscopy takes advantage of this relationship by irradiating a solid sample with gamma rays over a narrow range of energies and comparing the energies absorbed by the sample to known values.
This video will discuss the underlying principles of Mössbauer spectroscopy, illustrate the procedure for determining the spin state and oxidation state of ferrocene, and introduce a few applications in chemistry.
When a nucleus absorbs or emits a gamma ray, some energy is lost to recoil. Thus, the gamma ray emitted by a relaxing nucleus cannot excite an identical nucleus.
However, a percentage of emission and absorption events in crystal structures have negligible recoil, allowing resonance to occur between identical nuclei in solids. This is called the Mössbauer effect.
A standard Mössbauer spectrometer consists of a moving gamma ray source and a sensitive radiation detector. Iron Mössbauer spectroscopy is performed with a 57Co source, which decays by electron capture to excited 57Fe.
The different chemical environments of the source and sample nuclei result in slightly different energy gaps between the ground and excited states. The source is therefore moved back and forth at various speeds to induce a Doppler shift in the gamma rays.
The radiation detector measures the gamma rays transmitted through the sample. When the received gamma rays are the precise energy needed to excite the sample, resonant absorption can occur between the source and the sample.
A Mössbauer spectrum typically plots % transmission vs. energy in terms of source velocity.
The isomer shift is the shift in resonance energy relative to the source, and is related to the oxidation state of the atom.
Nuclear energy levels split when the surrounding electric field gradient is non-spherical, resulting in two distinct absorption energies. This interaction, called quadrupole splitting, occurs in asymmetric ligand environments, and at nuclear spins greater than ½.
Quadrupole splitting results in a quadrupole doublet in the Mössbauer spectrum. In these cases, the isomer shift is halfway between the two peaks and the quadrupole splitting value is the difference between the peaks.
Hyperfine splitting occurs in an internal or external magnetic field. Each nuclear energy level splits into sub-states based on its nuclear spin state. 57Fe has six allowed transitions between those states, resulting in six peaks.
Now that you understand the principles of Mössbauer spectroscopy, let's go through a procedure for determining the oxidation state and electronic spin state of ferrocene with Mössbauer spectroscopy.
To begin the procedure, measure 100 mg of ferrocene into a polyoxymethylene Mössbauer sample cup.
Add to the sample several drops of a cryoprotectant oil composed of a blend of polyisobutylenes. Use a spatula to mix the sample and oil into a uniform paste. Using tweezers, place the filled Mössbauer cup into a 20 mL scintillation vial and cap it for transportation to the Mössbauer instrument room.
Once in the instrumentation room, freeze the sample in liquid N2.
Next, remove the temperature probe from the sample rod. Unscrew the sample rod and fill the Mössbauer chamber with He gas. Then, with the He gas flowing, withdraw the sample rod.
Close the sample chamber with a cap, and close the He valve.
Transfer the Mössbauer sample into a secondary container filled with liquid N2. Then, carefully load the Mössbauer sample cup into the rod-mounted sample holder, and tighten the set screw to secure the cup in the holder.
Brush away any ice on the sample holder and the rod. Then, immerse the sample holder in liquid N2, and open the He valve.
Insert the sample rod into the chamber and fix the rod in place with screws.
Then, stop the He flow and evacuate the sample chamber. Once the sample chamber is at the minimum pressure, stop the vacuum pump and allow a small amount of He gas into the sample chamber. Finally, re-connect the temperature probe to the sample rod.
Open the gamma ray spectrometer interface to see a plot of the detector readings. Select the 14.4-keV peak and the 2-keV escape peak and hit the "Send to Windows" button.
Open the data collection software and set the source velocity range to 0 to 12 mm/s. Acquire data until the spectrum has achieved the desired resolution. Save the acquired data. Use appropriate software to fit the data and apply it to determine the isomer shift and the quadrupole splitting.
The Mössbauer spectrum of ferrocene has a single quadrupole doublet with an isomer shift of 0.54 mm/s. When compared to typical ranges of isomer shifts for iron containing compounds, the isomer shift suggests either an Fe(II), S = 0 complex or an Fe(III), S = 5/2 complex.
From the proton NMR of ferrocene, it is known that the compound is a diamagnetic, neutral complex. Furthermore, its two cyclopentadienyl ligands each bear a charge of 1-, indicating that the iron center in ferrocene is in the 2+ oxidation state. Finally, based on the Mössbauer result, it is evident that ferrocene has a spin state of 0.
Mössbauer spectroscopy is widely used in inorganic chemistry. Let’s look at a few examples.
Iron-sulfur proteins contain Fe/S clusters of two or more iron atoms bridged by S atoms. In a ferredoxin iron-sulfur protein, the diiron 2+ cluster contains two high-spin Fe(III) centers. Exchange coupling between these Fe centers results in an overall diamagnetic state with a spin of 0. The individual Mössbauer spectra of each Fe center are indistinguishable from each other, so the spectrum of the ferredoxin shows only one quadrupole doublet.
Ferredoxins participate in electron transport by redox reactions at their Fe atoms. For example, a ferredoxin can accept an electron by a single-electron reduction at one of the Fe centers, resulting in a cluster with one high-spin Fe(III) center and one high-spin Fe(II) center. This appears as two superposed quadrupole doublets in the Mössbauer spectrum.
Lipoyl synthase, which contains two 4-Fe/4-S clusters, performs the final step of lipoyl cofactor synthesis. The proposed mechanism involves an intermediate with the substrate cross-linked to a degraded Fe/S cluster.
To investigate the properties of the reaction intermediate, Mössbauer spectra were acquired in the presence and absence of a weak magnetic field. The resulting difference spectrum showed only the effects of an external magnetic field on the chemical shifts. The difference spectrum was combined with a simulated spectrum, revealing a 2:1 ratio from a mixed-valent Fe pair and an Fe(III) site.
You've just watched JoVE's introduction to Mössbauer spectroscopy. You should now be familiar with the underlying principles of the Mössbauer effect, the procedure for performing 57Fe Mössbauer spectroscopy, and a few examples of how Mössbauer spectroscopy is used in inorganic chemistry. Thanks for watching!
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Results
Zero field 57Fe Mössbauer of ferrocene at 5 K.
δ = 0.54 mm/s
ΔEQ = 2.4 mm/s
Referring to Table 1, we see that the isomer shift at 0.54 mm/s falls into several possible oxidation state/spin state ranges (Table 1). In cases such as this, it is not possible to determine the oxidation state and spin state based on δ-values alone. Chemists must use other characterization methods to gather evidence to support oxidation state and spin state assignments. Based on the proton NMR of ferrocene, we know that ferrocene is diamagnetic, and therefore must have a spin state of S = 0. The structure of ferrocene allows us to determine that the Fe center is in the 2+ oxidation state. The isomer shift value of 0.54 mm/s is close to the typical range of Fe(II), S = 0 compounds and therefore the Mössbauer spectrum is consistent with other characterization data.
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Applications and Summary
Here, we learned about the basic principles of Mössbauer spectroscopy, including details on the experimental setup, the gamma ray source, and the information that can be gathered from a Mössbauer spectrum. We collected the zero field 57Fe Mössbauer spectrum of ferrocene.
Mössbauer spectroscopy is a powerful technique that provides information about the electronic field gradient around an atom. While there are numerous Mössbauer active atoms, only elements with a suitable gamma ray source (long-lived and low-lying excited nuclear energy state) can take advantage of this technique. The most commonly studied atom is 57Fe, which is used to characterize inorganic/organometallic molecular species, bioinorganic molecules, and minerals. For example, Mössbauer spectroscopy has been used extensively to study iron-sulfur (Fe/S) clusters found in metalloproteins.2 Fe/S clusters are involved in a variety of functions, ranging from electron transport to catalysis. 57Fe Mössbauer spectroscopy has helped elucidate valuable information about Fe/S clusters in proteins, including, but not limited to, the number of unique iron centers present in a Fe/S cluster as well as the oxidation state and spin state of those ions.
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References
- Fultz, B. “Mössbauer Spectrometry”, in Characterization of Materials. John Wiley. New York. (2011).
- Pandelia, M.-E., Lanz, N., Booker, S., Krebs, C. Mössbauer spectroscopy of Fe/S proteins Biochim Biophys Acta. 1853, 1395–1405 (2015).
