EPR

Question 1

What does EPR stand for

Answer 1

Electron Paramagnetic Resonance

Question 2

What is EPR used to measure

Answer 2

Species that have an unpaired electron.

Question 3

What types of atomic species contain unpaired electrons

Answer 3

Free radicalsTransition metalsDefects in materials

Question 4

What is the difference between dimagnetic and paramagnetic species?

Answer 4

Dimagnetic - all electrons are paired, so no overall spinParamagnetic - at least one electron is unpaired, so has overall spin (used in EPR)

Question 5

How does EPR work

Answer 5

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

Question 6

What does delta E represent?

Answer 6

The change in energy needed to go from the low energy state to the high energy state

Question 7

What does delta E equal

Answer 7

= Planck’s constant (h)(Js) X frequency (v)(s^-1) ^= hv= g factor (g)(no unit) X Bohr magneton (muB)(J T^-1) X magnetic field strength (Bo)(T)^= g[mu]bBo

Question 8

What is proportional to delta E

Answer 8

Magnetic field strength (Bo)

Question 9

How is the state of resonance achieved

Answer 9

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

Question 10

What are the two ways that an electron is brought into a state of resonance

Answer 10

Fixed frequency; varying magnetic field (continuous wave technique)Fixed magnetic field; varying frequency (time domain/pulsed technique)

Question 11

What are the key features of the continuous wave technique

Answer 11

When y=0 -> absorbance peak maximumMaxima and minima are at the midpoint of the absorbance peakInformation recorded as the 1st derivation of the absorption peak graph.

Question 12

What are the key features of the pulsed technique

Answer 12

Resonance only occurs when correct frequency is obtained. NEEDS FINISHING!

Question 13

How is EPR different to NMR

Answer 13

Uses higher frequencies (gigahertz, instead of mega)More sensitiveWeaker magnets neededBroader lines (graph broader)Must be carried out at a lower temperature

Question 14

What are the general components of an EPR instrument

Answer 14

Source (microwave bridge)Sample (put in EPR tube in magnetic field)Detector (also in microwave bridge)Spectrum (output to computer screen)

Question 15

Name some uses for EPR

Answer 15

Structural biology (nucleic acids, proteins etc)Find primary sequences, and 3D shape in solutionHelps to work out electron transport processesMay be used in quantum computing

Question 16

Name some advantages of EPR (vs other methods)

Answer 16

Easier to interpretSimple systemsIntroduction of ‘spin labels’ - easy to identify

Question 17

Name some disadvantages of EPR (vs other methods)

Answer 17

Fewer accessible systems (because unpaired electron needs to be introduced

Question 18

What are the two names for the process of multiple EPR experiments (to measure electron distance)

Answer 18

PELDOR (Pulsed ELectron DOuble Resonance)DEER (Double Electron Electron Resonance)