Flashcards in EPR Deck (18):
What does EPR stand for
Electron Paramagnetic Resonance
What is EPR used to measure
Species that have an unpaired electron.
What types of atomic species contain unpaired electrons
Defects in materials
What is the difference between dimagnetic and paramagnetic species?
Dimagnetic - all electrons are paired, so no overall spin
Paramagnetic - at least one electron is unpaired, so has overall spin (used in EPR)
How does EPR work
Paramagnetic species placed in a magnetic field. Spin is either parallel to the field (low energy), or antiparallel to the field (high energy).
EPR measures the difference between the electron with high and low energy
What does delta E represent?
The change in energy needed to go from the low energy state to the high energy state
What does delta E equal
= Planck's constant (h)(Js) X frequency (v)(s^-1)
= g factor (g)(no unit) X Bohr magneton (muB)(J T^-1) X magnetic field strength (Bo)(T)
What is proportional to delta E
Magnetic field strength (Bo)
How is the state of resonance achieved
For an electron to move between energy levels (low/high) it needs to be able to absorb or release (emit) the correct amount of electromagnetic radiation (hv).
When the correct level of radiation is achieved, it is known as resonance energy, and the electron is in a state of resonance
What are the two ways that an electron is brought into a state of resonance
Fixed frequency; varying magnetic field (continuous wave technique)
Fixed magnetic field; varying frequency (time domain/pulsed technique)
What are the key features of the continuous wave technique
When y=0 -> absorbance peak maximum
Maxima and minima are at the midpoint of the absorbance peak
Information recorded as the 1st derivation of the absorption peak graph.
What are the key features of the pulsed technique
Resonance only occurs when correct frequency is obtained. NEEDS FINISHING!
How is EPR different to NMR
Uses higher frequencies (gigahertz, instead of mega)
Weaker magnets needed
Broader lines (graph broader)
Must be carried out at a lower temperature
What are the general components of an EPR instrument
Source (microwave bridge)
Sample (put in EPR tube in magnetic field)
Detector (also in microwave bridge)
Spectrum (output to computer screen)
Name some uses for EPR
Structural biology (nucleic acids, proteins etc)
Find primary sequences, and 3D shape in solution
Helps to work out electron transport processes
May be used in quantum computing
Name some advantages of EPR (vs other methods)
Easier to interpret
Introduction of 'spin labels' - easy to identify
Name some disadvantages of EPR (vs other methods)
Fewer accessible systems (because unpaired electron needs to be introduced