S=1/2 systems

1. S=1/2 systems

2. PRINCIPLE h Energy B0 Br

3. z y x EPR of isotrope systems: the g-value For a bound electron: additional magnetic field B In EPR:

4. EPR of isotrope systems E Energy B0 = 0 B0 > 0

5. 1 mT EXAMPLE X-band EPR of 2,2-diphenyl-1-picrylhydrazyle radical (DPPH) = 9,4424 GHz 298 K 336,7 mT

6. EPR of anisotrope systems: anisotropy of the electronic density

7. 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

8. EPR of anisotrope systems: axial systems g > g// g < g//

9. 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

10. EPR of anisotrope systems: exemple square-planar copper(II) dx2-y2 dxy dz2 dxz dyz g// = 2.17 g = 2.05 ge < g < g//

11. g = 2,193 g// = 2,034 EPR of anisotrope systems: exemple dx2-y2 dz2 dxy dxz dyz g > g//

12. EPR of anisotrope systems: rhombic systems g1 g2 g3

13. EPR of anisotrope systems: exemple substituted nitronyle nitroxyde radical x y gx= 2.0070 gy= 2.0069 gz= 2.0030 oriented crystal studies

14. EPR of anisotrope systems: exemple Fe(III) low-spin dx2-y2 dz2 dxy dxz, dyz

15. S=1/2 systems Hyperfine coupling

16. HYPERFINE COUPLING Interaction between magnetic moments of the electron and the nuclei (nuclear spin I)

17. HYPERFINE COUPLING: ORIGIN nuclear magnetic moment modification of the field Nuclear spin I -I ≤ mI ≤ I 2I+1 magnetic states A = coupling constant

18. 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

19. Absorption h Energy derivative B0 CASE OF H same intensity difference of population too small

20. 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

21. S=1/2 systems Anisotrope Hyperfine coupling

22. Axial Copper(II) Cu(II) d9 S = 1/2 I = 3/2 2I+1 = 4

23. Influence of temperature: isotrope / anisotrope S = 1/2 I = 7/2 Vanadium(IV) complex : T = 295 K, 8 transitions B (mT)

24. g, A g//, A// Influence of temperature: isotrope / anisotrope S = 1/2 I = 7/2 Vanadium(IV) complex : T = 77K

25. Spin hamiltonian • Electronic zeeman effect: • Hyperfine coupling • Total spin-hamiltonian

26. systems with S > 1/2 • half-integer • integer

27. Energy 5 transitions same field B0 Half-integer spin systems S = 5/2 -5/2 ≤ ms ≤ +5/2 One resonance not the observed situation !!

28. 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)

29. S=5/2 Rhombogram Practically: EPR spectra described by a single parameter. Spectra = independent of the numerical values of D and E

30. EXAMPLE: iron(III)-porphyrine g = 6 and g = 2

31. E/D = 0 axial system

32. O2, ACC Oxydase, Fe(II) 2e- (ascorbate) + 2 H+ CO2 EXAMPLE: ACCO

33. EXAMPLE: ACCO

34. E/D = 1/3 Rhombic system E/D = 0.24

35. EXAMPLE: cyt P450

36. E/D ~ 0.1

37. Cyt. P450CAM Pseudomonas Putida I. Schlichting et al. Science 2000, 287, 1615-1622

38. RH H2O LS, S = 1/2 HS, S = 5/2

39. EXAMPLE: cyt P450 LS

40. Energy E,D B0 D > 0 Integer spin systems spin hamiltonian applies BUT : No intra-doublet transitions high-field EPR or parallel-mode EPR

41. 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

42. Polynuclear systems

43. Iron-sulfur proteins Ferredoxin electron transfer proteins iron-sulfur cluster [2Fe-2S] spirulina patensis

44. 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)

45. Iron-sulfur proteins