Electron paramagnetic resonance

Using the electron spin resonance ( ESR or short english electron paramagnetic resonance, EPR), the resonant microwave absorption of a sample is measured in an external magnetic field. This makes it an excellent suitable method for the investigation of samples that have a permanent magnetic moment ( unpaired electrons).

History

The first ESR experiment was conducted by Yevgeny Konstantinovich Sawoiski in Kazan in 1944. Regardless of the development was carried out in the UK by brebis Bleaney. Sawoiski observed with paramagnetic salts such as copper sulfate, and manganese ( II) chloride resonance absorption of the incident energy at defined ratios of the magnitude of the static magnetic field to the frequency. In order to build this very first ESR spectrometer, Sawoiski had used the klystron from the radar of a captured German tank.

Background

Putting a sample with a permanent magnetic moment in a magnetic field, then the degenerate energy levels split on ( Zeeman effect). Often, the Zeeman effect is investigated on the basis of transitions between levels of different principal quantum number; in contrast, the ESR transitions between levels of the same principal quantum number are observed: By irradiation with a microwave, whose energy corresponds to the splitting of the levels, it comes to absorption.

In practice, the sample to be tested is irradiated in a variable magnetic field with a fixed frequency microwave. The recorded absorption spectrum allows conclusions on the magnetic environment of the magnetic moments (see g -factor).

In ESR spectroscopy, only substances can be analyzed with one or more unpaired electrons. Typical examples are:

• Paramagnetic transition metal ions in solution and in the solid state
• Generated by ionizing radiation radicals in crystals
• Organic molecules in the triplet state
• Organic radicals in solution
• Detection of food irradiation
• Nitric oxide ( in tissue)

Application

ESR spectroscopy is applied for example in biophysics and semiconductor physics. ESR can be carried out on natural biological samples. Thus, diseased tissue are examined and means of the radical concentration statements are made on irradiated material. ESR is also a method of investigation in materials research. So paramagnetic defects can be limited in the photovoltaic impurities of the material through characterization.

For structural elucidation of macromolecules, such as proteins, lipids, DNA or RNA, the ESR is suitable if you coupled the macromolecules previously with spin labels. A spin label for proteins nitroxides are used which contain more than one unpaired electron, the NO bond delocalized and enter into a bond with a cysteine ​​in the amino acid sequence. From the ESR spectra, distances between two bound spin labels can be determined and thus draw conclusions such as the folding of protein strands. An established method for this is the double - electron-electron resonance ( DEER ), a variant of the pulsed ESR are at the non-overlapping pulses irradiated at different frequencies. The method is also suitable for studies of protein - membrane interactions.

Resonance absorption

A static (usually homogeneous ) magnetic field in which atoms or molecules are located with incomplete electronic shells, lifts the energetic degeneracy of the states on ( Zeeman effect). This splitting is proportional to the applied magnetic field in a first approximation:

Is the magnetic quantum number. Therefore, every magnetic energy level has the distance from the nearest neighboring state ( equidistant splitting ). Between the states are magnetic dipolar transitions between adjacent levels possible (). If you put perpendicular to the static magnetic field, a high-frequency alternating field (eg at X- band ESR 9 to 10 GHz ), so can these transitions stimulated specifically. For this, the magnetic energy of the microwave energy sector. In this case, the frequency of microwave radiation emitted or absorbed - observing resonance absorption.

In the paramagnetic resonance, this results in the resonance condition (also called " fundamental equation of EPR spectroscopy " called )

The so-called g - factor ( or Landé factor ) relates the size of the magnetic moment of an atom with its total angular momentum. For pure orbital magnetism () for pure spin magnetism () holds (more precisely, 2.002322 ). General can specify the following relationship:

This formula can be derived from the vector model of atomic physics.