Using X-ray Absorption Spectroscopy (XAS) to Investigate Ligand Field Theory in Coordination Compounds.

Using X-ray Absorption Spectroscopy (XAS) to Investigate Ligand Field Theory in Coordination Compounds.
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This resource explores how X-ray absorption spectroscopy (XAS) can be used to measure various factors related to ligand field theory in coordination compounds, such as electronic environment, MO energy levels, and magnitude of charge transfer (CT) transitions, among others. The resource is created by Karen McFarlane Holman at Willamette University and posted on the VIPEr website.

About Using X-ray Absorption Spectroscopy (XAS) to Investigate Ligand Field Theory in Coordination Compounds.

PowerPoint presentation about 'Using X-ray Absorption Spectroscopy (XAS) to Investigate Ligand Field Theory in Coordination Compounds.'. This presentation describes the topic on This resource explores how X-ray absorption spectroscopy (XAS) can be used to measure various factors related to ligand field theory in coordination compounds, such as electronic environment, MO energy levels, and magnitude of charge transfer (CT) transitions, among others. The resource is created by Karen McFarlane Holman at Willamette University and posted on the VIPEr website.. The key topics included in this slideshow are X-ray absorption spectroscopy (XAS), ligand field theory (LFT), coordination compounds, magnitude of d-d splitting, CT transitions, MO energy levels,. Download this presentation absolutely free.

Presentation Transcript


1. Created by Karen McFarlane Holman, Willamette University ( kholman@willamette.edu ) and posted on VIPEr ( www.ionicviper.org ) on June 27, 2013. Copyright Karen McFarlane Holman 2013. This work is licensed under the Creative Commons Attribution-NonCommerical-ShareAlike 3.0 Unported License. To view a copy of this license visit http://creativecommons.org/about/license/ . XAS & LFT: X-ray absorption spectroscopy (XAS) as a tool to investigate ligand field theory

2. For coordination compounds, how does one measure Magnitude of d-d splitting? Magnitude of charge transfer (CT) transitions? MO energy levels? Extent of M-L orbital overlap? Electronic environment (oxidation state, Z eff )?

3. For coordination compounds, how does one measure Magnitude of d-d splitting? Magnitude of charge transfer (CT) transitions? MO energy levels? Extent of M-L orbital overlap? Electronic environment (oxidation state, Z eff )? X-ray absorption spectroscopy (XAS)can be used to determine all of these things simultaneously!

4. What is XAS ? X -ray High energy photons eject core e - s (1s, 2s, etc.) A bsorption S pectroscopy Scan over a spectrum, measure absorbance of photons that eject e - s or fluorescence of photons emitted when valence e - s relax into holes 1s h h 3d 4p Elemental Fe:

5. Just another spectrometer Stanford Synchrotron Radiation Lightsource Beamline 11-2 Ion Chamber 0 Ion Chamber 1 Ion Chamber 2 Sample Detector standard Slits Although you do need a special photon source

6. X-rays emerge from the source: A particle accelerator called a synchrotron Dave, an undergraduate student from Willamette University, collects data at the Advanced Light Source, Lawrence Berkeley National Laboratory.

7. X-rays reach the sample chamber Look through The leaded glass window:

8. Ligand 1s Metal d manifold h h Sample Backside of Detector Inside the sample chamber 5d 5 metal complex, D 4h symmetry

9. Synchrotron The light source for intense, coherent X-rays used for XAS Advanced Photon Source The Advanced Light Source

10. Origin of each part of an X-ray absorption spectrum Pre-edge, K-edge (XANES), and EXAFS regions Transmission Absorption/fluorescence Thank you to Chris Kim, Chapman U., for providing most of this slide!

11. Some definitions XANES: X-ray absorption near-edge structure is the region of the XAS spectrum leading up to and at the K -edge of an element. K -edge: The energy required to eject a 1s electron (akin to ionization energy, but for core electrons). Pre-edge features: Peaks in the XANES spectrum corresponding to electronic transitions from core electrons to bound states that occur slightly below the K -edge energy.

12. Features of XAS Example: Ru(bpy) 3 2+

13. Element Specific: Edges energies well resolved for different elements

14. Element Specific Ru

15. Element Specific N

16. Element Specific C

17. Oxidation State: K-edge energies shift when an element is oxidized or reduced or Z eff changes

18. Bond lengths: Can be determined via EXAFS (another type of XAS experiment not discussed here)

19. Identify Ligand: Useful for bioinorganic systems or when identity of ligand is ambiguous

20. Orbital mixing Extent of M-L overlap can be determined from spectral features in the pre-edge region

21. Coordination number/symmetry: Can be determined from XANES and EXAFS

22. In situ experiments are possible: Extremely useful for dilute samples such as probing a metal catalytic site in an enzyme; Often non-destructive

23. Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998 , 441 , 11-14. Sulfur K-edge XANES Spectra SO 4 2- SO 3 2- RSH RSSR MS x 2460 2470 2480 2490 Energy (eV) Normalized Absorbance EXAMPLE 1:

24. Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998 , 441 , 11-14. Sulfur K-edge XANES Spectra SO 4 2- SO 3 2- RSH RSSR MS x 2460 2470 2480 2490 Energy (eV) Normalized Absorbance K-edge = 2480 eV S 6+

25. Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998 , 441 , 11-14. Sulfur K-edge XANES Spectra SO 4 2- SO 3 2- RSH RSSR MS x 2460 2470 2480 2490 Energy (eV) Normalized Absorbance K-edge = 2475 eV S 4+

26. Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998 , 441 , 11-14. Sulfur K-edge XANES Spectra SO 4 2- SO 3 2- RSH RSSR MS x 2460 2470 2480 2490 Energy (eV) Normalized Absorbance

27. Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998 , 441 , 11-14. Sulfur K-edge XANES Spectra SO 4 2- SO 3 2- RSH RSSR MS x 2460 2470 2480 2490 Energy (eV) Normalized Absorbance 2GSH GSSG -2H + , -2e - +2H + , +2e -

28. Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998 , 441 , 11-14. Sulfur K-edge XANES Spectra SO 4 2- SO 3 2- RSH RSSR MS x 2460 2470 2480 2490 Energy (eV) Normalized Absorbance

29. Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998 , 441 , 11-14. Sulfur K-edge XANES Spectra SO 4 2- SO 3 2- RSH RSSR MS x 2460 2470 2480 2490 Energy (eV) Normalized Absorbance K-edge

30. Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998 , 441 , 11-14. Sulfur K-edge XANES Spectra SO 4 2- SO 3 2- RSH RSSR MS x 2460 2470 2480 2490 Energy (eV) Normalized Absorbance K-edge

31. Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998 , 441 , 11-14. Sulfur K-edge XANES Spectra SO 4 2- SO 3 2- RSH RSSR MS x 2460 2470 2480 2490 Energy (eV) Normalized Absorbance Pre-edge

32. Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998 , 441 , 11-14. Sulfur K-edge XANES Spectra SO 4 2- SO 3 2- RSH RSSR MS x 2460 2470 2480 2490 Energy (eV) Normalized Absorbance Pre-edge Due to M-L bonding

33. Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990 , 112 , 1643-1645. Chlorine K-edge XANES Spectra in MCl 4 2- 2820 2830 2840 Energy (eV) Normalized Absorbance CuCl 4 2- D 4h CuCl 4 2- D 2d ZnCl 4 2- D 2d EXAMPLE 2:

34. Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990 , 112 , 1643-1645. Chlorine K-edge XANES Spectra in MCl 4 2- 2820 2830 2840 Energy (eV) Normalized Absorbance CuCl 4 2- D 4h CuCl 4 2- D 2d ZnCl 4 2- D 2d

35. Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990 , 112 , 1643-1645. Chlorine K-edge XANES Spectra in MCl 4 2- 2820 2830 2840 Energy (eV) Normalized Absorbance CuCl 4 2- D 4h CuCl 4 2- D 2d ZnCl 4 2- D 2d Cl 1 s Cl 3 p Cu 3 d x2-y2 continuum

36. Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990 , 112 , 1643-1645. Chlorine K-edge XANES Spectra in MCl 4 2- 2820 2830 2840 Energy (eV) Normalized Absorbance CuCl 4 2- D 4h CuCl 4 2- D 2d ZnCl 4 2- D 2d Cl 1 s Cl 3 p Cu 3 d x2-y2 continuum

37. Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990 , 112 , 1643-1645. Chlorine K-edge XANES Spectra in MCl 4 2- 2820 2830 2840 Energy (eV) Normalized Absorbance CuCl 4 2- D 4h CuCl 4 2- D 2d ZnCl 4 2- D 2d Cl 1 s Cl 3 p Cu 3 d x2-y2 continuum K-edge energy: oxidation state, local environment

38. Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990 , 112 , 1643-1645. Chlorine K-edge XANES Spectra in MCl 4 2- 2820 2830 2840 Energy (eV) Normalized Absorbance CuCl 4 2- D 4h CuCl 4 2- D 2d ZnCl 4 2- D 2d Cl 1 s Cl 3 p Cu 3 d x2-y2 continuum

39. Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990 , 112 , 1643-1645. Chlorine K-edge XANES Spectra in MCl 4 2- 2820 2830 2840 Energy (eV) Normalized Absorbance CuCl 4 2- D 4h CuCl 4 2- D 2d ZnCl 4 2- D 2d Cl 1 s Cl 3 p Cu 3 d x2-y2 continuum

40. Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990 , 112 , 1643-1645. Chlorine K-edge XANES Spectra in MCl 4 2- 2820 2830 2840 Energy (eV) Normalized Absorbance CuCl 4 2- D 4h CuCl 4 2- D 2d ZnCl 4 2- D 2d Cl 1 s Cl 3 p Cu 3 d x2-y2 continuum Pre-edge Energy: MO energy levels

41. Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990 , 112 , 1643-1645. Chlorine K-edge XANES Spectra in MCl 4 2- 2820 2830 2840 Energy (eV) Normalized Absorbance CuCl 4 2- D 4h CuCl 4 2- D 2d ZnCl 4 2- D 2d Cl 1 s Cl 3 p Cu 3 d x2-y2 continuum Pre-edge peak area: Orbital overlap (covalency of M-L bond)

42. Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990 , 112 , 1643-1645. Chlorine K-edge XANES Spectra in MCl 4 2- 2820 2830 2840 Energy (eV) Normalized Absorbance CuCl 4 2- D 4h CuCl 4 2- D 2d ZnCl 4 2- D 2d Cl 1 s Cl 3 p Zn 3 d x2-y2 continuum

43. Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990 , 112 , 1643-1645. Chlorine K-edge XANES Spectra in MCl 4 2- 2820 2830 2840 Energy (eV) Normalized Absorbance CuCl 4 2- D 4h CuCl 4 2- D 2d ZnCl 4 2- D 2d Cl 1 s Cl 3 p Zn 3 d x2-y2 continuum No Pre-edge Feature

44. e g a 1 g t 1 u e g e g * a 1 g a 1 g * t 1 u t 1 u * t 1 u a 1 g e g + t 2 g 4 d 5 s 5 p 6 L (LGOs) M LMCT UV- Vis t 2 g nb ML 6 The Full Picture: LMCT transitions are not always measureable with UV-Vis (instrument limitations)

45. The Full Picture: Energy levels of various valence shell MOs are measurable via XANES. Couple with Density Functional Theory (DFT) calculations.

46. The Full Picture: Energy levels of various valence shell MOs are measurable via XANES. Couple with Density Functional Theory (DFT) calculations. Plus, you can determine Z eff , oxidation state, and extent of M-L overlap. XANES rules!