Electron Spin Resonance (ESR) Spectroscopy

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Electron Spin Resonance (ESR) Spectroscopy
applied to species having one or more unpaired
electrons free radicals, biradicals, other
triplet states, transition metal
compounds species having one unpaired electron
has two electron spin energy levels E
gmBBoMs selection rule DMs 1 gt DE
gmBBo  g proportionality constant,
2.00232 for free electron 1.99 2.01 for
radicals 1.4 3.0 for transition metal
compounds in isotropic systems (gas, liquid or
solution of low viscosity, solid sites with
spherical or cubic environment) , g is
independent of field direction mB Bohr
magneton 9.274 x 10-24 J T-1 for
electron MS electron spin quantum number
1/2 or 1/2
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• Bo external magnetic field
• commonly 0.34 1.24 T
• gt corresponding frequency
• 9.5 (X-band) 35 (Q-band) GHz
• the electron interacts with a neighboring
  nuclear
• magnetic dipole, the energy levels become
•   E gmBBoMS amBMSmI
• mI nuclear spin quantum number for the
• neighboring nucleus
• a hyperfine coupling constant
• energy levels and transitions for a single
• unpaired electron in an external magnetic field
• with no coupling coupling to one nucleus
  with spin 1/2

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• spin-lattice relaxation microwave radiation
• transferred from the spin system to its
• surroundings
• long relaxation time
• gt decrease in signal intensity
• short relaxation time
• gt resonance lines become wide
• typical ESR spectrometer
• a radiation source (klystron)
• a sample chamber between the poles of a magnet
• a detection and recorder system
• ESR spectrum
• (a) absorption curve
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• (b) first-derivative
• spectrum
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•    standard DPPH (diphenylpicrylhydrazyl
  radical)

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• hyperfine coupling in isotropic systems
• interactions between electron and nuclear
• spin magnetic moments
• gt fine structure in ESR spectrum
• couplings arise in two ways
• (i) direct dipole-dipole interaction
• (ii) Fermi contact interaction
• coupling patterns in ESR are determined by
  the same rules that apply to NMR
• coupling to nuclei with spin gt 1/2 are more
• frequently observed
• hyperfine coupling constant
• gmB MHz or cm-1
• hyperfine splitting constant
• A gauss or millitelsla
• depends on the unpaired electron spin
• density at the nucleus in question
• is related to the contribution to the atom of
• the molecular orbital containing the
• unpaired electron

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• Ex. 1 C6H6- coupling to all 6 H atoms
• the electron is delocalized
  over all
• 6 C atoms
• Ex. 2 pyrazine radical anion
• (a) coupling to 2 14N nuclei (12321
• quintet), and split by 4 H
  atoms
• further into 14641 quintet
• (b) Na salt, further splitting into
  1111
• quartet
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• Ex. 3 BH4- C(CH3)3
• 
  BH3- HC(CH3)3
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• Ex. 4 NBut
• S(NBut)2 Me2SiCl2 S
  SiMe
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  NBut

g 2.005
A(N) 0.45 mT
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• Ex. 5 S(NBut)2 - g 2.0071
• A(N) 0.515
  mT
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• Ex. 6 (MeO)3PBH2
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• Ex. 7 CrIII(porphyrin)Cl
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• the patterns of hyperfine splittings provide
• direct information about the numbers and
• types of spinning nuclei coupled to the
• electrons
• the magnitudes of the hyperfine couplings
• indicate the extent to which the unpaired
• electrons are delocalized, g values show

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• zero-field splitting
• in the absence of magnetic field, 2S 1
• energy states split depends on the structure of
  
• sample, spin-orbit coupling
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• anisotropic systems
• solids, frozen solutions, radicals prepared by
• irradiation of crystalline materials, radical
• trapped in host matrices, paramagnetic
• point defect in single crystals
• for systems with spherical or cubic symmetry
• g factors
• for systems with lower symmetry,
• g gt g? and g- gt gxx, gyy, gzz
• ESR absorption line shapes show distinctive
• envelope
• system with an axis of symmetry no symmetry

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• Ex. 8 Li 13CO2- in CO2 matrix
• large 13C and small 7Li (I 3/2) hyperfine
• splitting
• Ex. 9 HMn(CO)5 /solid Kr matrix at 77 K
• hu
• -? Mn(CO)5
• A?(55Mn) 6.5 mT
• A-(55Mn) 3.5 mT

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• transition metal complexes
• the number of d electrons
• high or low spin complex
• consequence of Jahn-Teller distortion
• zero-field splitting and Kramers degeneracy
• ESR spectra of second and third row
• transition metal complexes are often hard to
• observed, however, rare-earth metal
• complexes give clear, useful spectra
• short spin-lattice relaxation times
• gt broad spectral lines
• low temperature experiments will be needed
• to observe spectra
• Ex. 10 d3 system

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• trans-Cr(pyridine)4Cl2
• (a) frozen solution in DMF/H2O/MeOH
• (b) in transRh(pyridine)4Cl2Cl6H2O
• powder
• Ex. 11 d6 system
• low-spin diamagnetic
• Oh tetragonal
• high-spin 5D -? 5T2 ---? 5B2

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• Ex. 12 d9 system
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• CuII(TPP) complex (frozen solution in CCl3H)
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• multiple resonance
• ENDOR (electron-nuclear double resonance)
• Ex. 13 Ti(C8H8)(C5H5) in toluene (frozen
• solution)
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• (a) ESR spectrum (b) 1H ENDOR spectrum
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