XAS LFT Xray absorption spectroscopy XAS as a
XAS & LFT: X-ray absorption spectroscopy (XAS) as a tool to investigate ligand field theory Created by Karen Mc. Farlane Holman, Willamette University (kholman@willamette. edu) and posted on VIPEr (www. ionicviper. org) on June 27, 2013. Copyright Karen Mc. Farlane Holman 2013. This work is licensed under the Creative Commons Attribution-Non. Commerical-Share. Alike 3. 0 Unported License. To view a copy of this license visit http: //creativecommons. org/about/license/.
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, Zeff)?
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, Zeff)? X-ray absorption spectroscopy (XAS)can be used to determine all of these things simultaneously!
What is XAS? X-ray Absorption Spectroscopy High energy photons eject core e-s (1 s, 2 s, etc. ) Scan over a spectrum, measure absorbance of photons that eject e-s or fluorescence of photons emitted when valence e -s relax into holes 4 p Elemental Fe: 3 d hn 1 s hn
Just another spectrometer Ion Chamber 2 Ion Chamber 1 Sample standard Stanford Synchrotron Radiation Lightsource Beamline 11 -2 Although you do need a special photon source… Ion Chamber 0 Slits Detector
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.
Look through The leaded glass window: X-rays reach the sample chamber
5 d 5 metal complex, D 4 h symmetry Metal d manifold hn Ligand 1 s hn Sample Backside of Detector Inside the sample chamber
Synchrotron The light source for intense, coherent X-rays used for XAS Advanced Photon Source The Advanced Light Source
Origin of each part of an X-ray absorption spectrum Pre-edge, K-edge (XANES), and EXAFS regions Absorption/fluorescence Transmission Thank you to Chris Kim, Chapman U. , for providing most of this slide!
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 1 s 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.
Features of XAS Example: Ru(bpy)32+
Element Specific: Edges energies well resolved for different elements
Element Specific Ru
Element Specific N
Element Specific C
Oxidation State: K-edge energies shift when an element is oxidized or reduced or Zeff changes
Bond lengths: Can be determined via EXAFS (another type of XAS experiment not discussed here)
Identify Ligand: Useful for bioinorganic systems or when identity of ligand is ambiguous
Orbital mixing Extent of M-L overlap can be determined from spectral features in the pre-edge region
Coordination number/symmetry: Can be determined from XANES and EXAFS
In situ experiments are possible: Extremely useful for dilute samples such as probing a metal catalytic site in an enzyme; Often non-destructive
EXAMPLE 1: Sulfur K-edge XANES Spectra Adapted from I. J. Pickering, R. C. Prince, T. Divers, G. N. George, FEBS Letters 1998, 441, 11 -14. Normalized Absorbance SO 42 SO 32 RSH RSSR MSx 2460 2470 2480 2490 Energy (e. V)
Sulfur K-edge XANES Spectra Adapted from I. J. Pickering, R. C. Prince, T. Divers, G. N. George, FEBS Letters 1998, 441, 11 -14. K-edge = 2480 e. V Normalized Absorbance SO 42 SO 32 RSH RSSR MSx 2460 2470 2480 2490 Energy (e. V) S 6+
Sulfur K-edge XANES Spectra Adapted from I. J. Pickering, R. C. Prince, T. Divers, G. N. George, FEBS Letters 1998, 441, 11 -14. Normalized Absorbance SO 42 SO 32 RSH RSSR MSx 2460 2470 2480 2490 Energy (e. V) K-edge = 2475 e. V S 4+
Sulfur K-edge XANES Spectra Adapted from I. J. Pickering, R. C. Prince, T. Divers, G. N. George, FEBS Letters 1998, 441, 11 -14. Normalized Absorbance SO 42 SO 32 RSH RSSR MSx 2460 2470 2480 2490 Energy (e. V)
Sulfur K-edge XANES Spectra Adapted from I. J. Pickering, R. C. Prince, T. Divers, G. N. George, FEBS Letters 1998, 441, 11 -14. Normalized Absorbance SO 42 SO 32 RSH 2 GSH RSSR MSx 2460 2470 2480 2490 Energy (e. V) -2 H+, -2 e+2 H+, +2 e- GSSG
Sulfur K-edge XANES Spectra Adapted from I. J. Pickering, R. C. Prince, T. Divers, G. N. George, FEBS Letters 1998, 441, 11 -14. Normalized Absorbance SO 42 SO 32 RSH RSSR MSx 2460 2470 2480 2490 Energy (e. V)
Sulfur K-edge XANES Spectra Adapted from I. J. Pickering, R. C. Prince, T. Divers, G. N. George, FEBS Letters 1998, 441, 11 -14. Normalized Absorbance SO 42 SO 32 RSH RSSR MSx 2460 2470 2480 2490 Energy (e. V) K-edge
Sulfur K-edge XANES Spectra Adapted from I. J. Pickering, R. C. Prince, T. Divers, G. N. George, FEBS Letters 1998, 441, 11 -14. Normalized Absorbance SO 42 SO 32 RSH RSSR MSx 2460 2470 2480 2490 Energy (e. V) K-edge
Sulfur K-edge XANES Spectra Adapted from I. J. Pickering, R. C. Prince, T. Divers, G. N. George, FEBS Letters 1998, 441, 11 -14. Normalized Absorbance SO 42 SO 32 RSH RSSR MSx 2460 2470 2480 2490 Energy (e. V) Pre-edge
Sulfur K-edge XANES Spectra Adapted from I. J. Pickering, R. C. Prince, T. Divers, G. N. George, FEBS Letters 1998, 441, 11 -14. Normalized Absorbance SO 42 SO 32 RSH RSSR MSx 2460 2470 2480 2490 Energy (e. V) Due to M-L bonding Pre-edge
EXAMPLE 2: Chlorine K-edge XANES Spectra in MCl 42 - Adapted from B. Hedman, K. O. Hodgson, E. I. Solomon J. Am. Chem. Soc. 1990, 112, 1643 -1645. Cu. Cl 42 - Normalized Absorbance D 4 h Cu. Cl 42 D 2 d Zn. Cl 42 D 2 d 2820 2830 2840 Energy (e. V)
Chlorine K-edge XANES Spectra in MCl 42 Adapted from B. Hedman, K. O. Hodgson, E. I. Solomon J. Am. Chem. Soc. 1990, 112, 1643 -1645. Cu. Cl 42 - Normalized Absorbance D 4 h Cu. Cl 42 D 2 d Zn. Cl 42 D 2 d 2820 2830 2840 Energy (e. V)
Chlorine K-edge XANES Spectra in MCl 42 Adapted from B. Hedman, K. O. Hodgson, E. I. Solomon J. Am. Chem. Soc. 1990, 112, 1643 -1645. Cu. Cl 42 - continuum Normalized Absorbance D 4 h Cu. Cl 42 D 2 d Cu 3 dx 2 -y 2 Cl 3 p Zn. Cl 42 D 2 d 2820 2830 2840 Energy (e. V) Cl 1 s
Chlorine K-edge XANES Spectra in MCl 42 Adapted from B. Hedman, K. O. Hodgson, E. I. Solomon J. Am. Chem. Soc. 1990, 112, 1643 -1645. Cu. Cl 42 - continuum Normalized Absorbance D 4 h Cu. Cl 42 D 2 d Cu 3 dx 2 -y 2 Cl 3 p Zn. Cl 42 D 2 d 2820 2830 2840 Energy (e. V) Cl 1 s
Chlorine K-edge XANES Spectra in MCl 42 Adapted from B. Hedman, K. O. Hodgson, E. I. Solomon J. Am. Chem. Soc. 1990, 112, 1643 -1645. Cu. Cl 42 - continuum Normalized Absorbance D 4 h Cu. Cl 42 D 2 d Cu 3 dx 2 -y 2 Cl 3 p Zn. Cl 42 D 2 d K-edge energy: oxidation state, local environment 2820 2830 2840 Energy (e. V) Cl 1 s
Chlorine K-edge XANES Spectra in MCl 42 Adapted from B. Hedman, K. O. Hodgson, E. I. Solomon J. Am. Chem. Soc. 1990, 112, 1643 -1645. Cu. Cl 42 - continuum Normalized Absorbance D 4 h Cu. Cl 42 D 2 d Cu 3 dx 2 -y 2 Cl 3 p Zn. Cl 42 D 2 d 2820 2830 2840 Energy (e. V) Cl 1 s
Chlorine K-edge XANES Spectra in MCl 42 Adapted from B. Hedman, K. O. Hodgson, E. I. Solomon J. Am. Chem. Soc. 1990, 112, 1643 -1645. Cu. Cl 42 - continuum Normalized Absorbance D 4 h Cu. Cl 42 D 2 d Cu 3 dx 2 -y 2 Cl 3 p Zn. Cl 42 D 2 d 2820 2830 2840 Energy (e. V) Cl 1 s
Chlorine K-edge XANES Spectra in MCl 42 Adapted from B. Hedman, K. O. Hodgson, E. I. Solomon J. Am. Chem. Soc. 1990, 112, 1643 -1645. Cu. Cl 42 - continuum Normalized Absorbance D 4 h Cu. Cl 42 D 2 d Cu 3 dx 2 -y 2 Cl 3 p Zn. Cl 42 - Pre-edge D 2 d Energy: MO energy levels 2820 2830 2840 Energy (e. V) Cl 1 s
Chlorine K-edge XANES Spectra in MCl 42 Adapted from B. Hedman, K. O. Hodgson, E. I. Solomon J. Am. Chem. Soc. 1990, 112, 1643 -1645. Normalized Absorbance Cu. Cl 42 - D 4 h Pre-edge peak area: Orbital overlap (covalency of M-L bond) Cu. Cl 42 D 2 d continuum Cu 3 dx 2 -y 2 Cl 3 p Zn. Cl 42 D 2 d 2820 2830 2840 Energy (e. V) Cl 1 s
Chlorine K-edge XANES Spectra in MCl 42 Adapted from B. Hedman, K. O. Hodgson, E. I. Solomon J. Am. Chem. Soc. 1990, 112, 1643 -1645. Cu. Cl 42 - continuum Normalized Absorbance D 4 h Cu. Cl 42 D 2 d Zn 3 dx 2 -y 2 Cl 3 p Zn. Cl 42 D 2 d 2820 2830 2840 Energy (e. V) Cl 1 s
Chlorine K-edge XANES Spectra in MCl 42 Adapted from B. Hedman, K. O. Hodgson, E. I. Solomon J. Am. Chem. Soc. 1990, 112, 1643 -1645. Cu. Cl 42 - continuum Normalized Absorbance D 4 h Cu. Cl 42 D 2 d Zn 3 dx 2 -y 2 Cl 3 p Zn. Cl 42 D 2 d No Pre-edge Feature 2820 2830 2840 Energy (e. V) Cl 1 s
The Full Picture: LMCT transitions are not always measureable with UV-Vis (instrument limitations) M t 1 u 5 p a 1 g 5 s ML 6 6 L t 1 u* a 1 g* (LGOs) eg* eg+t 2 g 4 d t 2 gnb eg t 1 u a 1 g UV-Vis LMCT eg t 1 u a 1 g
The Full Picture: Energy levels of various valence shell MOs are measurable via XANES. Couple with Density Functional Theory (DFT) calculations. M t 1 u 5 p a 1 g 5 s ML 6 6 L t 1 u* a 1 g* (LGOs) eg* eg+t 2 g 4 d t 2 gnb eg t 1 u a 1 g UV-Vis LMCT eg t 1 u XANES Pre-Edge a 1 g L 1 s
The Full Picture: Energy levels of various valence shell MOs are measurable via XANES. M t 1 u 5 p a 1 g 5 s ML 6 6 L t 1 u* a 1 g* (LGOs) eg* eg+t 2 g Couple with Density Functional Theory (DFT) calculations. 4 d t 2 gnb eg t 1 u a 1 g UV-Vis LMCT eg t 1 u XANES Pre-Edge a 1 g Plus, you can determine Zeff, oxidation state, and extent of M-L overlap. XANES rules! L 1 s
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