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S=1/2 systems. PRINCIPLE. h . Energy. B 0. B r. z. y. x. EPR of isotrope systems: the g-value. For a bound electron: additional magnetic field B. In EPR:. EPR of isotrope systems. E. Energy. B 0 = 0. B 0 > 0. 1 mT. EXAMPLE.
E N D
PRINCIPLE h Energy B0 Br
z y x EPR of isotrope systems: the g-value For a bound electron: additional magnetic field B In EPR:
EPR of isotrope systems E Energy B0 = 0 B0 > 0
1 mT EXAMPLE X-band EPR of 2,2-diphenyl-1-picrylhydrazyle radical (DPPH) = 9,4424 GHz 298 K 336,7 mT
EPR of anisotrope systems: anisotropy of the electronic density
z B0 y B// B x B0//z B0//x B0//y any other direction powder or frozen solution: powder integration of single-crystal type spectra EPR of anisotrope systems: axial system B0
EPR of anisotrope systems: axial systems g > g// g < g//
X-ray irradiation, low temperature: e- is removed from one of the six oxygen ions g = 2,0386 g// = 2,0033 EPR of anisotrope systems: example Mg2+ vacancy model for the V- center in MgO
EPR of anisotrope systems: exemple square-planar copper(II) dx2-y2 dxy dz2 dxz dyz g// = 2.17 g = 2.05 ge < g < g//
g = 2,193 g// = 2,034 EPR of anisotrope systems: exemple dx2-y2 dz2 dxy dxz dyz g > g//
EPR of anisotrope systems: exemple substituted nitronyle nitroxyde radical x y gx= 2.0070 gy= 2.0069 gz= 2.0030 oriented crystal studies
EPR of anisotrope systems: exemple Fe(III) low-spin dx2-y2 dz2 dxy dxz, dyz
S=1/2 systems Hyperfine coupling
HYPERFINE COUPLING Interaction between magnetic moments of the electron and the nuclei (nuclear spin I)
HYPERFINE COUPLING: ORIGIN nuclear magnetic moment modification of the field Nuclear spin I -I ≤ mI ≤ I 2I+1 magnetic states A = coupling constant
CASE OF H: one nucleus I = 1/2 S = 1/2, mS = ±1/2 I = 1/2, mI = ±1/2 isotrope ms mI Transition rules: +1/2 +1/2 -1/2 Energy -1/2 -1/2 +1/2 B0 = 0 B0 > 0
Absorption h Energy derivative B0 CASE OF H same intensity difference of population too small
CASE OF H 49.7 mT g=2.0023 = 9,495 GHz 360.2 mT 310.5 mT Can be expressed in cm-1 Or in magnetic field units
S=1/2 systems Anisotrope Hyperfine coupling
Axial Copper(II) Cu(II) d9 S = 1/2 I = 3/2 2I+1 = 4
Influence of temperature: isotrope / anisotrope S = 1/2 I = 7/2 Vanadium(IV) complex : T = 295 K, 8 transitions B (mT)
g, A g//, A// Influence of temperature: isotrope / anisotrope S = 1/2 I = 7/2 Vanadium(IV) complex : T = 77K
Spin hamiltonian • Electronic zeeman effect: • Hyperfine coupling • Total spin-hamiltonian
systems with S > 1/2 • half-integer • integer
Energy 5 transitions same field B0 Half-integer spin systems S = 5/2 -5/2 ≤ ms ≤ +5/2 One resonance not the observed situation !!
not pure ms levels intra-doublet transitions Half-integer spin systems Kramers doublet Zero-field splitting, ZFS Energy E,D E,D B0 D > 0 Zero-field parameters: D (axial), E (rhombic)
S=5/2 Rhombogram Practically: EPR spectra described by a single parameter. Spectra = independent of the numerical values of D and E
EXAMPLE: iron(III)-porphyrine g = 6 and g = 2
E/D = 0 axial system
O2, ACC Oxydase, Fe(II) 2e- (ascorbate) + 2 H+ CO2 EXAMPLE: ACCO
E/D = 1/3 Rhombic system E/D = 0.24
Cyt. P450CAM Pseudomonas Putida I. Schlichting et al. Science 2000, 287, 1615-1622
RH H2O LS, S = 1/2 HS, S = 5/2
Energy E,D B0 D > 0 Integer spin systems spin hamiltonian applies BUT : No intra-doublet transitions high-field EPR or parallel-mode EPR
IN BIOLOGY Maximum spin for mononuclear centre: S = 5/2 (d5 configuration) S = 4 (d4 or d7 configuration) no natural lanthanide proteins Higher spin can only occur for clusters of metals
Iron-sulfur proteins Ferredoxin electron transfer proteins iron-sulfur cluster [2Fe-2S] spirulina patensis
Iron-sulfur proteins Fe(III)-------Fe(III) Fe(III)-------Fe(II) 5/2 5/2 5/2 2 resulting spin S = 0 (ground state) resulting spin S =1/2 (ground state)