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 (L I ) and 2p (L II and L III ) L II : state 2P 1 / 2 L III : state 2P 3 / 2

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

2

NiO

4

• Pre-edge: core to valence level • Edge: Ionisation of a core electron • XANES and EXAFS: scattering of ejected photoelectron 1.2

1 0.8

0.6

0.4

0.2

Pre-edge 0 •XANES •EXAFS oscillations -0.2

-0.4

6800 7000 Eo 7200 7400 7600 Energy / eV 7800 8000

Features of the Absorption Edge

 The higher the frequency of the oscillations the between absorber and scatterer lower the distance  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 U eq ) 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 50 40 30 20 10 0 0 Fe..O

2 Fe…Fe, Fe..Ni

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 maximise real part ( e.g.

the change in the f' ) of the anomalous scattering and minimise the change in the imaginary ( part f" )  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: Co 3 O 4 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 sites respectively.

hkl reflections 2 2 2 and 4 2 2 were used for the octahedral site and tetrahedral  I.J. Pickering, M. Sansome, J. Marsch, G. N. George, 1993, 115, 6302 J. Am Chem. Soc .

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

2

(phen)

2

]

   XAS of the Fe K, L edges are measured after the sample is irradiated with He/Ne laser II and L III Fe-N distances from the K edge Metal spin state - ratio between the intensities of the L II and L III 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 ] S=0; high spin S = 2 has two spin states; low spin, 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 magnetic field with 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 ] · nH 2 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 1047 et al . Coord. Chem. R e v., 1999 , 190–192, 1023–

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 XANES spectra multiple scattering calculations of EXAFS and 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: Δ Fe-N negligible Δ Fe-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. Res., 2000, 33,959 Acc. Chem.  M-C Cheng, A. M. Rich, R. S. Armstrong, P.J. Ellis and P. A. Lay, Chem., 1999, 38, 5703 Inorg.

Reduction by H Ge(Bu)

4

silica support

2

of Pt(acac)

2

and to form Pt particles on a

 The catalytic activity of Pt is enhanced by the presence of Ge  Multi edge energy dispersive EXAFS (EDE) follows the changes in the Pt L III 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