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?