water of NaI was removed before experiments by heating in
vacuum.
pound and d the thickness of the in situ cell]. If an organic
solvent of low absorption is used (such as acetone, ethanol or
diethyl ether), the necessary concentration increases with
increasing X-ray photon energy because the cross-section for
X-rays decreases for heavier atoms.
XAFS yields quantitative information on short-range order.
This requires that the part of the molecule that undergoes a
transformation must contain the absorbing element. To facili-
tate the XAFS analysis, it is advantageous if the monitored
element is present only in a few di†erent local environments in
the course of the reaction. The fact that the sample is contin-
uously irradiated with X-rays during the experiment does not
seem to have a strong inÑuence on the kinetics of the Finkel-
stein reaction. Were such an e†ect present, one would expect a
much faster reaction due to an easier breaking of the BrwC
bond.
We are grateful to HASYLAB for beamtime. We thank A.
Reller and M. Martin for generous support. Financial support
was granted by the Deutsche Forschungsgemeinschaft and by
the Fonds der Chemischen Industrie.
References
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Experimental
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0
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*
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Ðxed pathlength in the calorimeter. The measured spectra
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2
7
8
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constant in subsequent Ðts of the reaction mixture:
3
1
G. Li, F. Bridges and G. S. Brown, Phys. Rev. L ett., 1992, 68,
p2(BrwC ) \ 0.0011
Ó2,
Ó2,
p2(BrwC ) \ 0.0137
Ó2,
Ó,
a
b
1609.
p2(BrwNa) \ 0.02593
R(BrwC ) \ 2.858
b
32 P. DÏAngelo, A. Di Cicco, A. Filipponi and N. V. Pavel, Phys.
S 2(BrwC ) \ S 2(BrwC ) \ 1.082,
S 2(BrwNa) \ 1.10,
Rev. A, 1993, 47, 2055.
0
a
0
b
0
E (BrwC ) \ E (BrwC ) \ [0.89 eV, E (BrwNa) \ [1.65
33 E. Burattini, P. DÏAngelo, A. Di Cicco, A. Filipponi and N. V.
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0
a
0
b
0
eV. These values are in good agreement with data for similar
34
35
36
J. J. Rehr, C. H. Booth, F. Bridges and S. I. Zabinsky, Phys. Rev.
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compounds.13,33,35 N(BrwC ) was set to 2N(BrwC ). Conse-
b
a
quently, the only Ðt variables were the mean coordination
M. Epple, H. Kirschnick, G. N. Greaves, G. Sankar and J. M.
Thomas, J. Chem. Soc., Faraday T rans., 1996, 5035.
numbers N(BrwC ) and N(BrwNa), and the mean distances
a
R(BrwNa) and R(BrwC ). The evaluated XAFS data range
M. Epple and L. Tr o ger, J. Chem. Soc., Dalton T rans., 1996, 11.
a
was k \ 2.0È7.1 Ó~1 and R \ 0.8È3.1 Ó throughout all experi-
ments. 2-Bromopropane (99]%), acetone (99.9%; HPLC
grade) and NaI (p.a.) were purchased from Aldrich. Hydrate
Paper 7/01433C; Received 28th February, 1997
J. Chem. Soc., Faraday T rans., 1997, V ol. 93 3037