EPR
Spectroscopy. Introduction.
Objective: You will learn what kind of
information ESR can provide both, spectroscopic and kinetic, and investigate
some organic and inorganic radicals and ion radicals and gather information anot
only bout their hyperfine structure but also what may affect it (e.g.
concentration, oxygen etc.)
Electron Paramagnetic Resonance (EPR),
often called Electron Spin Resonance (ESR), is a branch of spectroscopy in which
electromagnetic radiation (usually of microwave frequency) is absorbed by
molecules, ions, or atoms possessing electrons with unpaired spins, i.e.
electronic spin
S > 0. EPR is similar to
Nuclear Magnetic Resonance (NMR)
which you will get acquainted with in the Lab 8. The NMR technique deals with
nonzero nuclear spins,
I > 0.
In both EPR and NMR, the sample material is immersed in a strong static
magnetic field and exposed to an orthogonal low- amplitude
high-frequency field. ESR usually requires microwave-frequency radiation (GHz),
while NMR is observed at lower radio frequencies (MHz). With ESR, energy is
absorbed by the sample when the frequency of the radiation is appropriate to the
energy difference between two states of the electrons in the
sample, but only if the transition satisfies the appropriate selection rules.
Most of materials in a bulk form at normal conditions have net zero
electronic spin and, thus, are EPR silent, but some are can provide an EPR
signal. In this lab will analyse some representative species from the three
groups below:
- Transition-metal and rare-earth species which contain unpaired nd
and/or mf electrons. This experiment will offer you to examine
liquid solutions of Mn2+, Fe3+ and Cu2+
ions with some ligands, as well as Cr3+ as a dilute impurity in a
solid Al2O3 host (ruby crystal).
- A small number of organic molecules are called free radicals because
they contain a single unpaired electron,
1,1'-Diphenyl-2-picryl-hydrazyl (DPPH) and
2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) are the examples; the
latter will be used in a first part of this experiment. All but one of the
electrons of these molecules are paired so there is only the orbital and
spin motion of one electron present per molecule.
- Organic ion-radicals, very reactive species which are created along a
course of redox reactions. They can be stabilized at certain conditions. You
will make and analyze perylene+ cation-radical and
benzosemiquinone anion-radical.
- Triplet-state organic molecules and biradicals.
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