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

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

High energyphotons

eject core e-s(1s, 2s, etc.)

Absorption SpectroscopyScan over a spectrum, measure

absorbance of photons that eject e-s or fluorescence of

photons emitted when valence e-s relax into holes

1s

hh

3d

4p

ElementalFe:

Just another spectrometer

Stanford Synchrotron Radiation Lightsource Beamline 11-2

IonChamber0

IonChamber1

IonChamber2

Sample

Detector

standardSlits

Although you do need a special photon source…

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.

X-raysreach thesamplechamber

LookthroughTheleadedglasswindow:

Ligand 1s

Metald manifold

hh

Sample Backside of Detector

Inside the sample chamber

5d5 metal complex,D4h symmetry

SynchrotronThe 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 spectrumPre-edge, K-edge (XANES), and EXAFS regions

TransmissionAbsorption/fluorescence

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 viaEXAFS (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 determinedfrom 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

Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998, 441, 11-14.

Sulfur K-edge XANES Spectra

SO42-

SO32-

RSH

RSSR

MSx

2460 2470 2480 2490Energy (eV)

EXAMPLE 1:

Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998, 441, 11-14.

Sulfur K-edge XANES Spectra

SO42-

SO32-

RSH

RSSR

MSx

2460 2470 2480 2490Energy (eV)

K-edge = 2480 eV

S6+

Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998, 441, 11-14.

Sulfur K-edge XANES Spectra

SO42-

SO32-

RSH

RSSR

MSx

2460 2470 2480 2490Energy (eV)

K-edge = 2475 eV

S4+

Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998, 441, 11-14.

Sulfur K-edge XANES Spectra

SO42-

SO32-

RSH

RSSR

MSx

2460 2470 2480 2490Energy (eV)

Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998, 441, 11-14.

Sulfur K-edge XANES Spectra

SO42-

SO32-

RSH

RSSR

MSx

2460 2470 2480 2490Energy (eV)

2GSH GSSG-2H+, -2e-

+2H+, +2e-

Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998, 441, 11-14.

Sulfur K-edge XANES Spectra

SO42-

SO32-

RSH

RSSR

MSx

2460 2470 2480 2490Energy (eV)

Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998, 441, 11-14.

Sulfur K-edge XANES Spectra

SO42-

SO32-

RSH

RSSR

MSx

2460 2470 2480 2490Energy (eV)

K-edge

Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998, 441, 11-14.

Sulfur K-edge XANES Spectra

SO42-

SO32-

RSH

RSSR

MSx

2460 2470 2480 2490Energy (eV)

K-edge

Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998, 441, 11-14.

Sulfur K-edge XANES Spectra

SO42-

SO32-

RSH

RSSR

MSx

2460 2470 2480 2490Energy (eV)

Pre-edge

Adapted from I.J. Pickering, R.C. Prince, T. Divers, G.N. George, FEBS Letters 1998, 441, 11-14.

Sulfur K-edge XANES Spectra

SO42-

SO32-

RSH

RSSR

MSx

2460 2470 2480 2490Energy (eV)

Pre-edge

Due to M-Lbonding

Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, 1643-1645.

Chlorine K-edge XANES Spectra in MCl42-

2820 2830 2840Energy (eV)

CuCl42-

D4h

CuCl42-

D2d

ZnCl42-

D2d

EXAMPLE 2:

Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, 1643-1645.

Chlorine K-edge XANES Spectra in MCl42-

2820 2830 2840Energy (eV)

CuCl42-

D4h

CuCl42-

D2d

ZnCl42-

D2d

Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, 1643-1645.

Chlorine K-edge XANES Spectra in MCl42-

2820 2830 2840Energy (eV)

CuCl42-

D4h

CuCl42-

D2d

ZnCl42-

D2d

Cl 1s

Cl 3p

Cu 3dx2-y2

continuum

Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, 1643-1645.

Chlorine K-edge XANES Spectra in MCl42-

2820 2830 2840Energy (eV)

CuCl42-

D4h

CuCl42-

D2d

ZnCl42-

D2d

Cl 1s

Cl 3p

Cu 3dx2-y2

continuum

Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, 1643-1645.

Chlorine K-edge XANES Spectra in MCl42-

2820 2830 2840Energy (eV)

CuCl42-

D4h

CuCl42-

D2d

ZnCl42-

D2d

Cl 1s

Cl 3p

Cu 3dx2-y2

continuum

K-edge energy: oxidation state, local environment

Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, 1643-1645.

Chlorine K-edge XANES Spectra in MCl42-

2820 2830 2840Energy (eV)

CuCl42-

D4h

CuCl42-

D2d

ZnCl42-

D2d

Cl 1s

Cl 3p

Cu 3dx2-y2

continuum

Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, 1643-1645.

Chlorine K-edge XANES Spectra in MCl42-

2820 2830 2840Energy (eV)

CuCl42-

D4h

CuCl42-

D2d

ZnCl42-

D2d

Cl 1s

Cl 3p

Cu 3dx2-y2

continuum

Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, 1643-1645.

Chlorine K-edge XANES Spectra in MCl42-

2820 2830 2840Energy (eV)

CuCl42-

D4h

CuCl42-

D2d

ZnCl42-

D2d

Cl 1s

Cl 3p

Cu 3dx2-y2

continuum

Pre-edgeEnergy: MO energy levels

Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, 1643-1645.

Chlorine K-edge XANES Spectra in MCl42-

2820 2830 2840Energy (eV)

CuCl42-

D4h

CuCl42-

D2d

ZnCl42-

D2d

Cl 1s

Cl 3p

Cu 3dx2-y2

continuumPre-edge peak area: Orbital overlap(covalency of M-L bond)

Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, 1643-1645.

Chlorine K-edge XANES Spectra in MCl42-

2820 2830 2840Energy (eV)

CuCl42-

D4h

CuCl42-

D2d

ZnCl42-

D2d

Cl 1s

Cl 3p

Zn 3dx2-y2

continuum

Adapted from B. Hedman, K.O. Hodgson, E.I. Solomon J. Am. Chem. Soc. 1990, 112, 1643-1645.

Chlorine K-edge XANES Spectra in MCl42-

2820 2830 2840Energy (eV)

CuCl42-

D4h

CuCl42-

D2d

ZnCl42-

D2d

Cl 1s

Cl 3p

Zn 3dx2-y2

continuum

NoPre-edgeFeature

eg

a1g

t1u

eg

eg*

a1g

a1g*

t1u

t1u*

t1ua1g

eg+ t2g 4d

5s5p

6 L(LGOs)

M

LMCT UV-Vis

t2gnb

ML6The Full Picture:

LMCT transitionsare not alwaysmeasureable withUV-Vis (instrumentlimitations)

eg

a1g

t1u

eg

eg*

a1g

a1g*

t1u

t1u*

t1ua1g

eg+ t2g 4d

5s5p

6 L(LGOs)

M

LMCT UV-Vis

L 1s

t2gnb

Pre-Edge XANES

ML6The Full Picture:

Energy levels ofvarious valenceshell MOs aremeasurable viaXANES.

Couple with DensityFunctional Theory(DFT) calculations.

eg

a1g

t1u

eg

eg*

a1g

a1g*

t1u

t1u*

t1ua1g

eg+ t2g 4d

5s5p

6 L(LGOs)

M

LMCT UV-Vis

L 1s

t2gnb

Pre-Edge XANES

ML6The Full Picture:

Energy levels ofvarious valenceshell MOs aremeasurable viaXANES.

Couple with DensityFunctional Theory(DFT) calculations.

Plus, you can determineZeff, oxidation state, andextent of M-L overlap.

XANES rules!