Chemistry Reference
In-Depth Information
(narrow line in Figure 1.4a,b; central units of 1 (Figure 1.5) is decreased. In the same
manner a decreasing proportion of the sum over all fourfold Al species is found and,
contrary to that, an increasing amount of fivefold coordinated aluminium species AlO
5
(signals at about 35 ppm) [23].
27
Al
19
F
-156 ppm
-147 ppm
a
a
-165 ppm
-163 ppm
-161 ppm
-171 ppm
b
b
c
c
d
d
-200
-300
-400
-100
-120
-140
-160
-180
-200
500
400
300
200
100
0
-100
(ppm)
(ppm)
Figure 1.7
27
Al NMR and
19
F NMR spectra of different sols and wet gels(B
0
¼9.4 T). For all:
solid line: experimental spectrum, dashed: simulation, dotted: decomposition. From a to d
increasing content of fluorine. Molar ratios Al: F: (a) 4:1,(b) 2:1,(c) 1:1,(d) 1 : 2. (Reprinted
with permission from [23] Copyright (2007) American Chemical Society.)
All
19
F spectra are characterized by a group of three sharp signals (-161 ppm, -163 ppm,
-165 ppm) with different intensities. All these signals are in a typical region for fluorine
bounded on aluminium centres in a mixed oxygen-fluorine coordination with different
fluorine ratios [13, 24-28]. With higher fluorine content, the intensity of the
19
FNMR
spectrum is more and more dominated by a broad peak at about -160 ppm (Figure 1.7,
19
F,d). These line-broadening effects result mainly from
19
F-
19
F homonuclear dipolar
couplings ending up in one broad peak in the static
19
F NMR spectrum for the gel with
molar ratio Al:F as 1:3.
1
H and
13
C NMR spectra of sols and gels show two main effects
