W12 Electron Paramagnetic Resonance Flashcards
what is EPR
technique used to study materials with unpaired electrons
non destructive technology
involves absorbance of microwave energy
most commonly used microwave source
X band: 8-12GHz
purpose of microwave source
to emit electromagnetic radiation at specific frequency > supply the energy needed to flip electron spins from lower to higher energy state
purpose of electromagnet
generates strong magnetic field that splits the spin states of unpaired electrons into two distinct energy levels
properties of sample cavity
cavity needs to resonate with microwaves > size of cavity is designed specifically for microwaves used in EPR > EPR uses a fixed microwave frequency
what is Q factor
measures how well the cavity resonates with the microwaves
high Q factor means cavity efficiently stores the microwave energy > stronger signal and improved sensitivity
what is filling factor
refers to how much of the cavity is filled with the sample
good filling factor ensures sample interacts with as much microwave energy as possible > improve signal quality
what is field modulation
a way to detect weak EPR signals more effectively by creating wave-like variation in the magnetic field
special coil called Helmholtz coils generate a small, oscillating magnetic field (Bm) that oscillates at high frequency of 100kHz
increasing modulation makes signal stronger but too much can distort the spectrum
what is Bres
the EPR spectrum occurs when the microwave energy matches the energy difference between these two spin states
this occurs when magnetic field is at a specific value called the resonance field (Bres) > peak observed because the spin states “flip” between their two possible orientations
what is g factor
describes how strongly an electron interacts with a magnetic field
acts as “fingerprint” for electron > can learn about local environment of electron (eg whether its near metal atom or in a certain molecule)
what is effective g factor
g factor can change a little due to the environment around the electron > change in g factor due to environment is called effective g factor
what is hyperfine coupling
electron’s spin can interact with magnetic fields of nearby nuclei that also have magnetic moment > interaction causes hyperfine splitting in EPR spectrum > appearance of additional peaks > make spectrum more complex
what is hyperfine splitting constant
the size of hyperfine splitting, which depends on the type of nucleus and how strongly it interacts with the electron
distance between the two peaks depends on A > larger A > larger splitting > further apart the peaks
what is anisotropy
a structural property where the way an electron interacts with the magnetic field is different depending on direction of the field applied
g-factor and hyperfine coupling constant, A are anisotropic > change depending on direction
definition of dynamics in EPR
refer to how the motion of electron changes in response to different temperatures
lower temp > molecules move slower > lower dynamics > broader peaks
