Transcript Structural Chemistry from the Edge(s): An Introduction

Structural Chemistry from
the Edge(s): An
Introduction
Georgina Rosair
Department of Chemistry, Heriot-Watt University
.
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
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
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
Fe K-edge X-ray Absorption
Spectrum of Trevorite, Fe2NiO4
•Pre-edge: core to
valence level
•XANES and EXAFS:
scattering of
ejected
photoelectron
1
Absorbance
•Edge: Ionisation of
a core electron
1.2
•XANES
0.8
•EXAFS oscillations
0.6
0.4
0.2
Pre-edge
0
Eo
-0.2
-0.4
6800
7000
7200
7400
7600
Energy / eV
7800
8000
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
Fourier Transform
 The FT of the EXAFS
spectrum : approximate
radial distribution of
scatterers around the
absorbing atom, after
correction for phase
and amplitude
50
Fe…Fe,
Fe..Ni
40
Fe..O
30
20
 The theoretical fit is
generated by adding
shells of scatterers
and refining the model
to get the best fit
10
0
0
2
4
R/Å
6
8
10
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 data:parameter ratio, therefore accurate models are required
to act as constraints in refinement
J.E. Penner-Hahn, Coord. Chem. Revs., 1999, 1101
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.html
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
Diffraction Anomalous Fine
Structure
 The detector is set at the right scattering angle 2θ 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
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
Study of the excited state
 The compound [Fe(NCS)2(phen)2] has two spin states; low spin,
S=0; 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
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
XMCD at the V and Cr K edges for Cs(I)
V(II) V(III)1½[Cr(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, 190–192, 1023–
1047
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/
XANES and EXAFS: Metal
environment in metalloproteins
 Cytochrome-c on oxidation:
ΔFe-N negligible
ΔFe-S 2.29 to 2.33(2) Å
 Greater precision than previous
single crystal structure
determination
 Sulfur K pre-edge:Degree 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
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.
EDE spectra for the Pt LIII and Ge
K edges; 298–670 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
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