Chemistry Reference
In-Depth Information
12.4 CHARACTERIZATION OF THE DEHYDROHALOGENATION
PRODUCTS
Numerous studies dealing with structural investigations of carbynoid
materials with the use of various spectroscopic methods have been reviewed
in details elsewhere [3]. Here I would like to briefly describe the most
important findings, while focusing on recent studies published since 1998.
12.4.1 V
IBRATIONAL
S
PECTROSCOPY
12.4.1.1 Infrared Spectroscopy
As the dehydrohalogenation of halogen-containing polymers proceeds, the
absorption bands, typical of C-Hal vibrations (mostly in the 400-750 cm
1
frequency region), disappear from the IR spectra of the reaction products.
New absorption bands appear therewith near 1600 (strong), 1720 (often
appears as a shoulder), and 2100-2200 (medium to weak) cm
1
, being attri-
butable to stretching vibrations of double C
¼
C-bonds, carbonyl groups,
and triple C
C-bonds, respectively [4,6-13,15-19]. Noteworthy is the obser-
vation by Danno et al. of the peak at 2040 cm
1
, which is a surprisingly low
frequency as compared with all other spectra of dehydrohalogenated poly-
mers published to date [4]. Additional absorption bands (weak to medium)
corresponding to C-H stretching vibrations in aliphatic groups appear
in the 2850-3000 cm
1
frequency range, along with a strong broad band
near 3400 cm
1
that can be assigned to both free (H
2
O) and bonded hydroxy
groups [7-13,15-19]. The peaks near 870 and 750 cm
1
are attributa-
ble to out-of-plane bending of aromatic protons [19], and the band at
1100-1110 cm
1
[19] is likely to result from C-O-C stretching vibrations in
ether groups. Aromatic structures can arise in the reaction products due to
dehydrohalogenation of six-membered rings formed via the Diels-Alder
type cyclization, whereas the ether linkages are produced as a consequence
of the alkoxylation side reactions through the nucleophilic substitution of
chlorine atoms by ethoxide species or polymeric alkoxide ions [19].
Finally, it is worthy of particular note that the IR spectra of profoundly
dehydrohalogenated polymers are amazingly similar (
Figure 12.1
)
, even if
the structure of these polymers allows one to expect the formation of dif-
ferent isomers of carbyne (polyyne and cumulene) by exhaustive elimination
of hydrogen halide (cf. Figure 12.1, spectra 6 and 7).
12.4.1.2 Raman Spectroscopy
Typically, Raman spectra of profoundly dehydrohalogenated polymers
reveal two well-resolved lines around 1550 and 2150 cm
1
generally assigned
to stretching modes of C
C-bonds, respectively [5,10,17]. The line
at 1540 cm
1
is common in all forms of polycrystalline graphite or amor-
phous carbons with predominantly sp
2
-type bonding. On the other hand,
¼
C and C
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