Structural Chemistry from the Edge(s): An Introduction

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Georgina RosairDepartment of Chemistry, Heriot-Watt University

Overview

Why use absorption edges? X-ray absorption: XANES, EXAFS Anomalous scattering : Diffraction at different

wavelengths Case Histories:

Molecular magnetsExcited statesMetalloproteinsCatalysis

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

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

6800 7000 7200 7400 7600 7800 8000

•XANES

•EXAFS oscillations

Ab

sorb

ance

Energy / eV

Pre-edge

•Pre-edge: core to valence level

•Edge: Ionisation of a core electron

•XANES and EXAFS: scattering of ejected photoelectron

Eo

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

The theoretical fit is generated by adding shells of scatterers and refining the model to get the best fit

0

10

20

30

40

50

0 2 4 6 8 10

R / Å

Tra

nsf

orm

Am

plit

ud

e

Fe..O

Fe…Fe, Fe..Ni

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 K2.190(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

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

Vanadium K edge Chromium K edge

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 negligibleΔFe-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 declinesAbove 540 K: Evidence of Pt-Ge interactions and alloy formationC/O coordination to Ge retained up to 650 K

Conclusions

Absorption edges can be used for:Determining the spin state of metalsResolution of disorderLocal 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