Environmental Engineering Reference
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structural determination of inorganic and metal-organic species because
absorption due to stretching and bending vibrations of bonds between
metal atoms and both inorganic and organic ligands generally occur at
frequencies lower than
650
cm
1
(
>
15
m
m).
The interpretation of IR spectrum is not a straightforward task. To simplify,
Figure 8.12 can be used as a general guide for the common stretching and bending
vibrations. The reader can relate the IR spectrum of benzene (Fig. 8.11) for an
example use of Figure 8.12. In benzene, every carbon has a single bond to hydrogen
and each carbon is bonded to two other carbons and the carbon-carbon bonds are alike
for all six carbons. The aromatic CH stretch appears at 3100-3000 cm
1
. There are
aromatic CC stretch bands (for the carbon-carbon bonds in the aromatic ring) at
about 1500 cm
1
. Two bands are caused by bending motions involving CH bonds.
The bands for CH bends appear at approximately 1000 cm
1
for the in-plane bends
and at about 675 cm
1
for the out-of-plane bend. As shown, the IR spectrum for
benzene is relatively simple because benzene has only a few prominent bands owing
to its symmetric and planar molecular structure.
Wavenumbers (cm
-1
)
700
40003000
2000
1500
1200
1000 900
800
625
OH & NH
Stretching
C=O
Stretching
C-O
Stretching
C-H
Stretching
C-N
Stretching
C-N
Stretching
-
-
-
-
CC
C
X
C
X
Y
C
-
C
Stretching
Stretching
Stretching
NH
CH
-
-
Bending
Rocking
CH
-
NH
-
Bending
Rocking
OH
-
Bending
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
m)
Figure 8.12
Chart of characteristic vibrations (http://www.wag.caltech.edu/home/jang/genchem/
infrared.htm)
Microns (
μ
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