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

Bio-Chem Question Br- I- Cl-

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

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

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

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

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

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

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

Bromide XANES 4 6 8 10

Iodide XANES 4 6 8 10

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!

Iodide Solvation Shell Bromide Solvation Shell

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

Funding Institutions

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

Thank You! Questions? Comments?

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?