Photon Factory Activity Report 2005 #23 Part B (2006)
Materials Science

9A/2005G196

Local Structure of Br in Brominated Mesoporous Ethylenesilica
Tomohisa MIYAJIMA1, Koujiro NAKAI1, Hideaki YOSHITAKE*1
1
Yokohama National University, Yokohama 240-8501, Japan
Introduction
Recently-developed periodic mesoporous organosilica(PMO) has been considered as a soft material in a newcategory. In fact, PMO is signalized by its intermediate
properties between an organic polymer and amorphoussilica. In the bromination of C=C double bonds inmesoporous ethylenesilica at room temperature,
considerable number of double bonds in the bulk arereacted, suggesting bromine diffuses into the bulk where
nitrogen cannot penetrate at 77 K. Since the product canbe further converted by another organic reaction, the local
structure of Br is significantly important. For this reason,
we investigated Br K-edge EXAFS in this product.
Experiment
XAFS spectra were measured using Beamline 9A. The
spectra were acquired three times at 293 K, and the averageχ(k) was calculated from the extracted spectra. A standard
automated data reduction procedure was used to extractthe experimental EXAFS spectra using a REX 2000(Rigaku Co.) program assembly. The amplitudes and phaseshift functions were calculated using the FEFF code.
Results and Discussion
The figure shows the k3χ(k) EXAFS oscillation, theradial distribution function and the curve-fitting result ofbrominated ethylenesilica. The first peak around 1.5 Å isclearly assigned to the Br-C bond (fig (b)). However, thesecond largest peak around 2.8 Å is assigned to a sum of thescattering of the next nearest neighbors of C and Si. Thecurve-fitting results are summarized in the table. The
coordination number for these three atoms is nearly unity,
and is consistent with the structure of a brominated C=C
double bond where each carbon is bound to a silicon atom.
The appearance of the first shell as a strong peak reveals
the uniformity in Br-C bond, suggesting that there issufficient space around the C=C double bonds for the Br2
addition reaction without resulting in considerable
stereochemical distortion. The lack of a peak from Br-Br
bond in the Fourier transform provides an evidence for thatthe large uptake of bromine in the bromination (< 40%) is
not due to molecular Br2 adsorbed. A tiny peak around 2.1 A
was not able to be fitted with a Br-Br bond.
The Br-C bond length (1.83 Å) is significantly shorter
than that in bromoalkanes (ca. 1.9 Å). This length depends
on the bond order of the neighboring C-C bond; C-Br boundto an alkynyl carbon is shorter than to an alkenyl carbon, andthat to the alkenyl group is shorter than to an alkyl carbon.
The Br-C distance found in the table is close to that inbromobenzene (1.8674 Å), suggesting a similarcharacteristic of Br-C with that to an aryl C.

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Users' Report

Figure Br EXAFS of brominated ethylenesilica. (a) k χ(k),(b) Fourier transform and (c) curve-fitting of theinversely-Fourier transformed spectrum at r = 0.9 – 3.2 Å.
3Table : Curve-fitting results for the Br K edge EXAFS.
scattererNγ/Å
∆E0/ σ/Å
eV
C
1.829
0.82
15.4
0.109
C
3.077
1.00
5.19
0.089
Si
3.095
0.82
-3.73
0.180
γ: bond distance, N: coordination number, ∆E0: shift ofthe energy threshold, and σ: Debye-Waller factor.

*yos@ynu.ac.jp