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Solvation Sphere Structure of Halides in Aqueous Solution. Matt Antalek Aug 12 th 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. - PowerPoint PPT Presentation
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Solvation Sphere Structure of Halides in Aqueous Solution
Matt AntalekAug 12th 2010
SLACSMB-SSRL
Bio-Chem Question
I-
Br-
Cl-
Why use XAS?Allows us to find the “needle in a haystack”Element specific techniqueGives electronic and structural information
XAS-EXAFS
EXAFSQuantitative Local Structure.
Extended X-ray Absorbption Fine Structure
Br-
EXAFS
Fourier Transform peak is fn of coordination number
BUT also disorder! This poses problems in EXAFS experiments
Model is green line
ExperimentCollected I- L1 edge data on BL 4-3Br- 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?
XANES Oxidation state, Molecular composition, structure.
X-Ray Absorption Near Edge 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
Parting Notes
New theory is needed to explain this result!
Funding Institutions
AcknowledgementsDr. Ritimukta Sarangi (mentor)Dr. Maurizio BenfattoDr. Apurva MehtaDr. Stephen Rock (SULI Director)SSRL StaffSULI Students and Organization
Thank You!
Questions?
Comments?
Speculations and Future StudiesPerhaps size not importantCharge delocalization plays a bigger role
than previously thought8 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?