Structural Chemistry from the Edges: An Introduction

1
Structural Chemistry from the Edge(s) An
Introduction
Georgina Rosair Department of Chemistry,
Heriot-Watt University
.
2
Overview

• Why use absorption edges?
• X-ray absorption XANES, EXAFS
• Anomalous scattering Diffraction at different
  wavelengths
• Case Histories
• Molecular magnets
• Excited states
• Metalloproteins
• Catalysis
• Conclusions

3
Why use absorption edges?

• Study of local environment in liquids and
  amorphous solids, including surfaces
• Electronic and magnetic structure
• Element specific
• Can use elements above Ca in atomic weight
• Below Ca vacuum environmental cell needed for P
  and S K edges
• Changes in anomalous components of the Scattering
  factor

4
Edge origins

• Edge Ionisation of a core electron
• K edge electron originates from 1s orbital
• L edge electron from 2s (LI) and 2p (LII and
  LIII)
• LII state 2P1/2
• LIII state 2P3/2

5
Fe K-edge X-ray Absorption Spectrum of Trevorite,
Fe2NiO4

• Pre-edge core to valence level
• Edge Ionisation of a core electron
• XANES and EXAFS scattering of ejected
  photoelectron

• XANES

Absorbance

• EXAFS oscillations

Pre-edge
Eo
Energy / eV
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Features of the Absorption Edge

• The higher the frequency of the oscillations the
  lower the distance between absorber and scatterer
• Phase of the EXAFS and shape of the amplitude are
  dependent on the identity of the scatterer, but
  weakly so - O and S can be distinguished but not
  O and N
• Intensity of oscillations proportional to the
  number of neighbours i.e. coordination no.
• The EXAFS function is dampened by thermal motion.
  
• Debye Waller factor (similar to Ueq) Structural
  disorder also influences this parameter.
• The pre-edge height is proportional to the number
  of vacancies in the valence levels

7
Fourier Transform

• The FT of the EXAFS spectrum approximate radial
  distribution of scatterers around the absorbing
  atom, after correction for phase and amplitude
• The theoretical fit is generated by adding shells
  of scatterers and refining the model to get the
  best fit

FeFe, Fe..Ni
Fe..O
Transform Amplitude
R / Å
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Some limitations

• Reference compounds needed
• If there's a high uncertainty in a distance then
  the peak may not be visible in the EXAFS
• Low dataparameter ratio, therefore accurate
  models are required to act as constraints in
  refinement
• J.E. Penner-Hahn, Coord. Chem. Revs., 1999, 1101

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Anomalous scattering

• Collect diffraction data at two or more
  wavelengths near the absorption edge
• Chosen wavelengths e.g. maximise the change in
  the real part (f') of the anomalous scattering
  and minimise the change in the imaginary (f")
  part
• Position of anomalous scatterer found by f
  difference Patterson or Fourier maps

• http//www.bmsc.washington.edu/scatter/AS_index.ht
  ml

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Some Applications of Anomalous Scattering

• Distinguish between neighbouring elements in the
  periodic table
• particularly when a site is disordered and
  occupied by two different elements
• A change in valence states shift the position of
  the absorption edge
• Many macromolecular crystal structures are solved
  by using MAD (Multiwavelength Anomalous
  Dispersion) or SAD if they contain an anomalous
  scatterer

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Diffraction Anomalous Fine Structure

• The detector is set at the right scattering angle
  2? for a particular hkl value and a DAFS spectrum
  is measured.
• The contribution of each component to the total
  absorption spectrum can be separated
• Example Co3O4
• Tetrahedral Co sites are high spin Co(II)
• Octahedral Co sites are low spin Co(III)
• Because the Co atoms are on special positions,
  the hkl reflections 2 2 2 and 4 2 2 were used
  for the octahedral site and tetrahedral sites
  respectively.
• I.J. Pickering, M. Sansome, J. Marsch, G. N.
  George, J. Am Chem. Soc. 1993, 115, 6302

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Light-induced low spin to high spin transition
in Fe(NCS)2(phen)2

• XAS of the Fe K, LII and LIII edges are measured
  after the sample is irradiated with He/Ne laser
• Fe-N distances from the K edge
• Metal spin state - ratio between the intensities
  of the LII and LIII edges

• J-J Lee, H-S. Sheu, C-R Lee, J-M Chen, J-F Lee,
  C-C. Wang, C-H Huang and Y.Wang, J. Am. Chem.
  Soc, 2000, 122, 5742 and refs therein

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Study of the excited state

• The compound Fe(NCS)2(phen)2 has two spin
  states low spin, S0 high spin S 2
• Two high spin states, thermal and light-induced
• Light-induced HS state trapped at 17K
• K edge
• Fe-N(Phen) 1.985(5) at 17K to
• 2.12(1) Å on light excitation at 17 K
• 2.190(5) Å at 300K.
• L edge relaxation of high spin to low spin
• Crystal field multiplet calculations
  theoretical fit

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Metal cyanide complexes as molecular magnets

• X-ray Magnetic Circular Dichroism
• Direction and magnitude of the local magnetic
  moment
• Collect data with magnetic field
• Need circularly polarised X-rays - synchrotron
  radiation

15
XMCD at the V and Cr K edges for Cs(I) V(II)
V(III)1½Cr(III)(CN)6 nH2 O
Vanadium K edge Chromium K edge

• Antiferromagnetic coupling between V and Cr ions
  is shown by the inversion of the dichroic signal
  at the V and Cr K edges
• M. Verdaguer et al. Coord. Chem. Rev., 1999,
  190192, 10231047

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XANES

• XANES region distance travelled by photoelectron
  longer than in EXAFS region
• Multiple scattering provides angular as well as
  radial information - 3D structure around a
  photoabsorber, even determine chirality
• Multiple scattering analysis to simulate the
  spectrum.
• e.g. FEFF, ab initio multiple scattering
  calculations of EXAFS and XANES spectra
• Accurate models needed to provide a constraint in
  refinement.
• FEFF http//leonardo.phys.washington.edu/feff/

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XANES and EXAFS Metal environment in
metalloproteins

• Cytochrome-c on oxidation
• ?Fe-N negligible
• ?Fe-S 2.29 to 2.33(2) Å
• Greater precision than previous single crystal
  structure determination
• Sulfur K pre-edgeDegree of covalency in M-L
  bonds
• E.I. Solomon et al. Acc. Chem. Res., 2000, 33,959

• M-C Cheng, A. M. Rich, R. S. Armstrong, P.J.
  Ellis and P. A. Lay, Inorg. Chem., 1999, 38, 5703

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Reduction by H2 of Pt(acac)2 and Ge(Bu)4 to form
Pt particles on a silica support

• The catalytic activity of Pt is enhanced by the
  presence of Ge
• Multi edge energy dispersive EXAFS (EDE) follows
  the changes in the Pt LIII edge and Ge K edge
  simultaneously as the temperature is increased
  from 300 to 630 K
• Ideally, an elliptically bent monochromator
  delivers a focused X-ray beam containing a range
  of X-ray energies
• The detector is a photodiode array
• S. G. Fiddy, M. A. Newton, A. J. Dent, I.
  Harvey, G. Salvini, J. M. Corker, S. Turin, T.
  Campbell and J. Evans, Chem. Commun., 2001, 445.

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EDE spectra for the Pt LIII and Ge K edges
298670 K.

• Above 460 K Pt..Pt coordination declines
• Above 540 K Evidence of Pt-Ge interactions and
  alloy formation
• C/O coordination to Ge retained up to 650 K

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Conclusions

• Absorption edges can be used for
• Determining the spin state of metals
• Resolution of disorder
• Local structure around the metal in
  metalloproteins
• Follow the change in local environment around a
  metal during a chemical reaction in the bulk
  and/or on a surface
• Thanks to Dr Andrew Dent at Daresbury and
• research groups who carried out the work