1. Solvation Sphere Structure of Halides in Aqueous Solution
Matt Antalek
Aug 12th 2010
SLAC
SMB-SSRL

2. Bio-Chem Question
Br- I-
Cl-

3. Why use XAS? Allows us to find the “needle in a haystack”
Element specific technique
Gives electronic and structural information

4. XAS-EXAFS Br- Extended X-ray Absorbption Fine Structure EXAFS
Quantitative Local Structure.

5. EXAFS Model is green line
Fourier Transform peak is fn of coordination number
BUT also disorder! This poses problems in EXAFS experiments

6. Experiment Collected I- L1 edge data on BL 4-3
Br- data was previously collected on BL 10-2

7. Bromide EXAFS
Χ²error=7.1%
Χ²error=4.0%
Χ²error=1.8%
Χ²error=1.6%

8. Iodide EXAFS
Χ²error=0.1%
Χ²error=0.2%
Χ²error=0.06%
Χ²error=0.4%

9. What else can we do? X-Ray Absorption Near Edge Structure
XANES
Oxidation state,
Molecular composition,
structure.
NOT sensitive to thermal noise! Fits rigorously to Multiple Scattering theory, complex

10. Bromide XANES 4 6 8 10

11. Iodide XANES 4 6 8 10

12. What does this mean?
We have found that both solvation shells best fit data with 8 water molecules coordinated to central anion
Different from accepted theory!

13. Iodide Solvation Shell
Bromide Solvation Shell

14. New theory is needed to explain this result!
Parting Notes
New theory is needed to explain this result!

15. Funding Institutions

16. Acknowledgements Dr. Ritimukta Sarangi (mentor) Dr. Maurizio Benfatto
Dr. Apurva Mehta
Dr. Stephen Rock (SULI Director)
SSRL Staff
SULI Students and Organization

17. Thank You!
Questions? Comments?

18. Speculations and Future Studies
Perhaps size not important
Charge delocalization plays a bigger role than previously thought
8 water molecules is all that can “fit” into solvation cage or this is enough to spread charge sufficiently?
How is this structure affected by molecular interactions?