EPR Notes Flashcards
Paramagnetic
One or more unpaired electron(s)
ms
(in presence of a magnetic field) Electron spin magnetic quantum number
- S, S-1, … , -S
Spin quantum number (ms) only has values of +/- 1/2
How does EPR differ from NMR?
NMR - interactions of nuclear spins forming in an atom with a magnetic field applied to them
EPR - interaction of electron spins with the applied magnetic field
Since the electron magnetic moment is about 1000 times greater than the proton nuclear magnetic moment, there are significant differences in characteristic frequencies and sensitivities
Magnetic moment is inversely proportional to mass
Materials studied by EPR
- Organic molecules with one unpaired electron (doublet state)
- Molecules with two unpaired electrons (triplet state, like O2)
- Transition metal complexes with incomplete 3d, 4d, or 5d orbitals
Low spin complex
Minimum number of unpaired electrons
delta is greater than pairing energy (P)
High spin complex
Maximum number of unpaired electrons
delta is less than the pairing energy (P)
Geometry for coordination number 6 and 4
6 - octahedral
4 - weak field/high spin = tetrahedral
4 - strong field/low spin = square planar
Spin multiplicity
n + 1 (where n = unpaired electrons)
n = 0, multiplicity = 1, singlet state
n = 1, multiplicity = 2, doublet state
n = 2, multiplicity = 3, triplet state
Resonance condition: u = -gBJ
u = magnetic moment (passed by electron)
g = spectroscopic splitting factor, reflects nature of orbital
B = electron Bohr magneton, 9.274 x 10^-24 J/T
J = total angular momentum vector = L + S
- we neglect L in transition metal complexes, then J = S = (n/2) for n unpaired electrons
- equation becomes u = -gBS
- measurable components are g, B, ms
Presence of a magnetic field (Bo or H)
The electron magnetic moments assume orientations with respect to the applied magnetic field, giving rise to 2S+1 energy states
What does plotting the first derivative curve (dP/dBo) allow?
Allows us to improve resolution and decrease background/signal noise
What causes hyperfine coupling in isotropic systems in EPR?
Interaction between electron spin magnetic moment and nuclear spin magnetic moment
Mechanism of couping is Fermi contact interaction which results from unpaired electron density at the nucleus in question
Unpaired electron density at the nucleus is only present when the electron is in an orbital having some s character
(Coupling provides indication of extent of electron delocalization over nuclei of nuclear spin I > 0)
Ms
Total electron spin magnetic quantum number
- given by addition of electron spin magnetic quantum numbers (for a greater than doublet system)
mi
Nuclear spin magnetic quantum number
(for one nucleus of nuclear spin one)
Mi
Total nuclear magnetic spin magnetic quantum number
- given by addition of nuclear spin magnetic quantum numbers (for more than one nucleus of nuclear spin one)
Summary of coupling
- hyperfine splitting patterns provide information about the numbers and types of nuclei couples to the unpaired electron
- the magnitude of the coupling constants (a) indicate the extent to which the unpaired electron is associated with a given nucleus (spin density)
- anisotropic g values may show whether the unpaired electron is transition metal based or ligand based
What does coupling to a nuclei of nuclear spin (I) greater than 1/2 cause?
Produces a symmetrical multiplet whose intensities are different than a binomial multiplet (Pascal’s triangle)
Nature of a nonbinomial multiplet is deduced by a coupling tree
Isotropy/anisotropy
In solution, molecules are tumbling at rates greater than the EPR spectrometer frequency
- anisotropic effects due to differences in molecular orientation with respect to magnetic field are averaged to 0 by rapid tumbling
Consequence of isotropy
Any anisotropic effects due to differences in molecular orientation with respect to the magnetic field are averaged to 0 by rapid tumbling
Consequence of anisotropy (solid, crystal, powder, frozen solution)
No symmetry - 3g, 3a values
Axial symmetry (symmetry about one axis) - 2g, 2a values (parallel and perpendicular)
Molecular axis in parallel
larger a value
lower intensity
Molecular axis in perpendicular
smaller a value
larger intensity
