Introduction to Electron Paramagnetic Resonance
Electron Paramagnetic Resonance (EPR) – also known as Electron Spin Resonance (ESR) and, less commonly, Electron Magnetic Resonance (EMR) – is a powerful spectroscopic method for studying paramagnetic materials, ie. those that contain (or can be induced to contain) unpaired electrons.
Crudely, it can be considered as the electron spin equivalent of NMR spectroscopy. Like NMR, "EPR" covers a wide range of techniques including continuous wave, pulsed, double resonance and multi-dimensional methods.
Because the technique is sensitive and selective for the electron spin and its environment, EPR can be used to probe, for example:
- Open-shell transition metal ions
- Free radicals (intrinsic, induced or labelled)
- Defects in materials
- Radical identity
- Spin-labelled materials
- Optically excited triplets
- Transient radicals (eg. by spin trapping)
- Geometric and electronic structure
- Radical environment/surroundings
- Reaction mechanisms/kinetics
- Radical quantitation
- Electron transfer kinetics
- Electron spin dynamics
Case studies
Below we highlight a few key papers which show the breadth of applications within EPR Spectroscopy.
2019-2020
Continuous wave and pulsed EPR of supramolecular assemblies (JACS)
Title
Conformational Flexibility of Hybrid [3]- and [4]-Rotaxanes
Abstract
The synthesis, structures, and properties of [4]- and [3]-rotaxane complexes are reported where [2]-rotaxanes, formed from heterometallic {Cr7Ni} rings, are bound to a fluoride-centered {CrNi2} triangle. The compounds have been characterized by single-crystal X-ray diffraction and have the formulas [CrNi2(F)(O2Ct Bu)6]{(BH)[Cr7NiF8(O2Ct Bu)16]}3 (3) and [CrNi2(F)(O2Ct Bu)6(THF)]{(BH)[Cr7NiF8(O2Ct Bu)16]}2 (4), where B = pyCH2CH2NHCH2C6H4SCH3. The [4]-rotaxane 3 is an isosceles triangle of three [2]- rotaxanes bound to the central triangle while the [3]-rotaxane 4 contains only two [2]- rotaxanes bound to the central triangle. Studies of the behaviour of 3 and 4 in solution by small-angle X-ray scattering and atomistic molecular dynamic simulations show that the structure of 3 is similar to that found in the crystal but that 4 has a different conformation to the crystal. Continuous wave and pulsed electron paramagnetic resonance spectroscopy was used to study the structures present and demonstrate that in frozen solutions (at 5 K) 4 forms more extended molecules than 3 and with a wider range of conformations
