XAFS Analysis of Triiodide Ion in Solutions
Hideto Sakanea*, Takayuki Mitsuib, Hajime Tanidac, and Iwao WatanabedCenter for Instrumental Analysis, Yamanashi University, Kofu, Yamanashi, Japanb
Faculty of Engineering, Yamanashi University, Kofu, Yamanashi, JapancJapan Synchrotron Radiation Research Institute, Mikazuki, Hyogo, Japan
dGraduate School of Science, Osaka University, Toyonaka, Osaka, Japan

Abstract

Search for the second peakI]-I

[ISolvent
DMF
DMA
TMS
ACANNM
DMSO
EtOH
MeOH
Aq(×2)

0.02

0.01

0.04
0.03
-30.03|ρ|/Å-3

Iodine K-edge XAFS of triiodide ions in various solventswere measured at SPring-8 BL01B1 and analyzed. Though
the anion takes a linear and symmetric form, the second
peak expected from enhanced multiple scatterings can be
hardly observed because of its large vibrations. The bonddistances and the Debye-Waller factors for the I−I couple
vary when protic solvents were used and they were similar
when aprotic solvents were used. It was indicated thatthe larger the Mayers’ acceptor number AN of the solvent,the larger the Debye-Waller factor. Also found was thatamong the aprotic solvents, the larger the Gutmann's
donor number DN, the smaller the Debye-Waller factor.

|ρ| / Åamissing

0.02
0.01
missing

Experimental

03456

FT for NaI3 solutions

AC(TPA)

2.88
10DMA(Na),DMA(TPA)30

0.10

MeOH(Na)MeOH(TPA)

AN(TPA)DMSO(Na)
AN(Na)DMSO(TPA)

20

7

0.1

0.03
-30.02|ρ| / Å-3

0.08
0.06
appears

0.04

appears

0.01

0.02

01

2

34r/Å

5670

FT for the compound data

40

1234r/Å

567

FT for TPAI3 powder at ~10 K

00.076

0.2

β ( )
0.4

0.6

0.8

AN

0.074

0.10

Solvents
proticaprotic
MeOH(Na)

0.08
0.08

aq

50

0.12

σ/Å

EtOH

60.12aq

DMSO(TPA) MeOH(TPA)
MeOH(Na)NMDMA(TPA)
EtOH

AC(Na) DMF
2.89 TMS NM

50.12

TMS

2.90

34r/Å

cool until ~10 K

summation of 12 independent spectra

AN(Na) AN(TPA)

σ/Å ( )

AC(TPA)

2

FT for TPAI3 powder at room temperature60

I−I distance r and the Debye-Waller factor σ vs. theaccetor number AN

σ/Å

2.92

17aq

DMA(Na)AC(Na) DMF DMSO(Na)r/Å ( )

2

Relations to solvent parameters

2.93

2.91

1r /Å

|ρ| / ÅSamples
Solute: NaI and I2 (NaI3 solution), *tetra-n-propylammonium
triiodide (TPAI3 solution)
Solvent: acetone (AC)*, N, N-dimethylacetamide (DMA)*,
nitromethane (NM), N, N-dimethylformamide (DMF),
tetrahydrothiophen-1, 1-dioxide (TMS), methanol (MeOH)*,
acetonitrile (AN)*, dimethylsulfoxide (DMSO)*, ethanol (EtOH),water (aq)Concentration: 0.01 mol/L I3(aqueous solution: 0.04 mol/L NaI and 0.01 mol/L I2)
XAS measurements
Conditions: transmission mode at room temperature
Monochromator: silicon double crystal monochromator in Si(311)diffraction mode. Rh-coted quartz mirror
Path length: 5 − 20 cm
Analysis
Software: Xanadu (Sakane et al., 1993)
Parameters: Calculated with FEFF 6.01a for TPAI3 crystal
structure model.
Fourier transforms: k-range of 3.6 − 15.4 Å-1. Hamming windowfunction. Phase shifts and backscattering amplitude werecorrected.
Curve fitting: k-range of 3.75 − 13.95 Å-10NM0EtOH

MeOH(TPA)DMA(Na),AC(Na)DMA(TPA)AN(TPA)DMSO(Na)DMF
AN(Na)DMSO(TPA)TMS AC(TPA)

10DN

2030

σ vs. the donor number DN

0.072
0.070
0.068
01020
DN ( )

30

σ vs. DN and the Kamlet-Taft’s basicity parameter βof the aprotic solvents

Conclusion
The second peak, including enhanced multiple
scatterings, of polyiodine species is found for the firsttime. It results from the highest quality of thespectra obtained from SPring-8 BL01B1 and the proper
analysis treatments.
Protic solvent, water and alcohols, solutions
behave correlation of decrease of the I−I distance r
and increase of the Debye-Waller factor σ withincreasing the acceptor number AN. The group of the
aprotic solvents also lies on the correlation. σ for the
aprotic solvent solutions is declining lines with thedonor number D N and the Kamlet-Taft basicity
parameter β. These relations can be unified as themore the electron of I 3 - is withdrawn by solvent
molecules and the lower the electron density on theanion, the larger its σ becomes and then the weaker
the bond in the anion is. According to simple
molecular orbital interpretations, HOMO of I3 - hasantibonding character. But, from this study HOMO
should be bonding. A problem of this unified
interpretation is the limited application scope of theeach solvent parameters. The classification that asolvent is either protic or aprotic has been found forBr- and I- solvation studies by XAFS and may reflect afeature of anion-solvent interactions. r becomes
smaller when its σ becomes larger. This phenomenon
cannot be understood by common knowledge. For theaqueous solution, σ is so large that the thermalvibration of I3- would not be approximated with theDebye-Waller factor alone. Higher order cumulants
should be considered in the aqueous solution. Othersolutions conclude that the solvent effects affect theI−I bond strength in I3- in a complex manner.