XAS & LFT:
X-ray absorption spectroscopy (XAS) as
a tool to investigate ligand field theory
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/.
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
(1s, 2s, 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
4p
Elemental
Fe:
3d
hn
1s
hn
Just another spectrometer
Ion
Chamber2
Ion
Chamber1
Sample
Ion
Chamber0
standard
Slits
Stanford Synchrotron Radiation
Lightsource Beamline 11-2
Although you do need a special photon source…
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
5d5 metal complex,
D4h symmetry
Metal
d manifold
hn
Ligand 1s
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 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.
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
SO42-
SO32RSH
RSSR
MSx
2460 2470 2480 2490
Energy (eV)
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 eV
Normalized Absorbance
SO42-
SO32RSH
RSSR
MSx
2460 2470 2480 2490
Energy (eV)
S6+
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
SO42-
SO32RSH
RSSR
MSx
2460 2470 2480 2490
Energy (eV)
K-edge = 2475 eV
S4+
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
SO42-
SO32RSH
RSSR
MSx
2460 2470 2480 2490
Energy (eV)
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
SO42-
SO32RSH
-2H+, -2e-
2GSH
RSSR
MSx
2460 2470 2480 2490
Energy (eV)
GSSG
+2H+, +2e-
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
SO42-
SO32RSH
RSSR
MSx
2460 2470 2480 2490
Energy (eV)
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
SO42-
SO32RSH
RSSR
MSx
2460 2470 2480 2490
Energy (eV)
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
SO42-
SO32RSH
RSSR
MSx
2460 2470 2480 2490
Energy (eV)
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
SO42-
SO32RSH
RSSR
MSx
2460 2470 2480 2490
Energy (eV)
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
SO42-
SO32RSH
RSSR
MSx
2460 2470 2480 2490
Energy (eV)
Due to M-L
bonding
Pre-edge
EXAMPLE 2:
Chlorine K-edge XANES Spectra in MCl42Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, 1643-1645.
CuCl42-
Normalized Absorbance
D4h
CuCl42D2d
ZnCl42D2d
2820 2830 2840
Energy (eV)
Chlorine K-edge XANES Spectra in MCl42Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, 1643-1645.
CuCl42-
Normalized Absorbance
D4h
CuCl42D2d
ZnCl42D2d
2820 2830 2840
Energy (eV)
Chlorine K-edge XANES Spectra in MCl42Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, 1643-1645.
CuCl42-
continuum
Normalized Absorbance
D4h
CuCl42D2d
Cu 3dx2-y2
Cl 3p
ZnCl42D2d
2820 2830 2840
Energy (eV)
Cl 1s
Chlorine K-edge XANES Spectra in MCl42Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, 1643-1645.
CuCl42-
continuum
Normalized Absorbance
D4h
CuCl42D2d
Cu 3dx2-y2
Cl 3p
ZnCl42D2d
2820 2830 2840
Energy (eV)
Cl 1s
Chlorine K-edge XANES Spectra in MCl42Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, 1643-1645.
CuCl42-
continuum
Normalized Absorbance
D4h
CuCl42D2d
Cu 3dx2-y2
Cl 3p
ZnCl42D2d
K-edge energy: oxidation state,
local environment
2820 2830 2840
Energy (eV)
Cl 1s
Chlorine K-edge XANES Spectra in MCl42Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, 1643-1645.
CuCl42-
continuum
Normalized Absorbance
D4h
CuCl42D2d
Cu 3dx2-y2
Cl 3p
ZnCl42D2d
2820 2830 2840
Energy (eV)
Cl 1s
Chlorine K-edge XANES Spectra in MCl42Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, 1643-1645.
CuCl42-
continuum
Normalized Absorbance
D4h
CuCl42D2d
Cu 3dx2-y2
Cl 3p
ZnCl42D2d
2820 2830 2840
Energy (eV)
Cl 1s
Chlorine K-edge XANES Spectra in MCl42Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, 1643-1645.
CuCl42-
continuum
Normalized Absorbance
D4h
CuCl42D2d
Cu 3dx2-y2
Cl 3p
ZnCl42Pre-edge
D2d
Energy: MO energy levels
2820 2830 2840
Energy (eV)
Cl 1s
Chlorine K-edge XANES Spectra in MCl42Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, 1643-1645.
Normalized Absorbance
CuCl42D4h
Pre-edge peak area:
Orbital overlap
(covalency of M-L bond)
CuCl42D2d
continuum
Cu 3dx2-y2
Cl 3p
ZnCl42D2d
2820 2830 2840
Energy (eV)
Cl 1s
Chlorine K-edge XANES Spectra in MCl42Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, 1643-1645.
CuCl42-
continuum
Normalized Absorbance
D4h
CuCl42D2d
Zn 3dx2-y2
Cl 3p
ZnCl42D2d
2820 2830 2840
Energy (eV)
Cl 1s
Chlorine K-edge XANES Spectra in MCl42Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, 1643-1645.
CuCl42-
continuum
Normalized Absorbance
D4h
CuCl42D2d
Zn 3dx2-y2
Cl 3p
ZnCl42D2d
No
Pre-edge
Feature
2820 2830 2840
Energy (eV)
Cl 1s
The Full Picture:
LMCT transitions
are not always
measureable with
UV-Vis (instrument
limitations)
M
t1u 5p
a1g 5s
ML6
6L
t1u*
a1g*
(LGOs)
eg*
eg+t2g
4d
t2gnb
eg
t1u
a1g
UV-Vis
LMCT
eg
t1u
a1g
The Full Picture:
Energy levels of
various valence
shell MOs are
measurable via
XANES.
M
t1u 5p
a1g 5s
ML6
6L
t1u*
a1g*
(LGOs)
eg*
eg+t2g
4d
t2gnb
eg
t1u
a1g
UV-Vis
LMCT
Couple with Density
Functional Theory
(DFT) calculations.
eg
t1u
XANES
Pre-Edge
a1g
L 1s
The Full Picture:
Energy levels of
various valence
shell MOs are
measurable via
XANES.
M
t1u 5p
a1g 5s
ML6
6L
t1u*
a1g*
(LGOs)
eg*
eg+t2g
4d
t2gnb
eg
t1u
a1g
UV-Vis
LMCT
Couple with Density
Functional Theory
(DFT) calculations.
eg
t1u
XANES
Pre-Edge
a1g
Plus, you can determine
Zeff, oxidation state, and
extent of M-L overlap.
XANES rules!
L 1s