FTIR spectroscopic study of 1,1,1-trifluoro-2-chloroethyl and 1,1,1- trifluoro-2-chloroethylperoxyl radicals

Article abstract of DOI:10.1016/S0022-2860(98)00777-7

A combination of matrix isolation and FTIR spectroscopy was applied to investigate 1,1,1-trifluoro-2-chloroethyl (1) and 1,1,1-trifluoro-2- chloroethylperoxyl (2) radicals. Radical 2 was obtained by vacuum pyrolysis of 1,1,1-trifluoro-2-bromo-2-chloroethane (3). Corresponding peroxyl radicals was generated by co-condensation of pyrolysis products and molecular oxygen in an argon matrix. To assign the experimental bands DFT calculations (B3LYP/6-311G**) were carried out. The fundamental bands of O-O and C-O stretching vibrations of peroxyl radical CF₃CHClOO (1102.1, 972.7, cm^-1) were identified by their red shifts to 1044.7 and 954.1 cm^-1 in the spectra of ₁₈O substituted derivatives. UV photolysis of the radical 2 in the low- temperature matrix produced difluoroformaldehyde CF₂O, radicals ClCO, CF₃, as well as CO and HCl as the primary photoproducts.

Full text of DOI:10.1016/S0022-2860(98)00777-7

Journal of Molecular Structure 480–481 (1999) 519–522
FTIR spectroscopic study of 1,1,1-trifluoro-2-chloroethyl and
1
,1,1-trifluoro-2-chloroethylperoxyl radicals
*
E.G. Baskir , V.A. Korolev, O.M. Nefedov
N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47 Leninsky prosp., 117913 Moscow, Russia
Received 1 October 1998; accepted 19 October 1998
Abstract
A combination of matrix isolation and FTIR spectroscopy was applied to investigate 1,1,1-trifluoro-2-chloroethyl (1) and
,1,1-trifluoro-2-chloroethylperoxyl (2) radicals. Radical 2 was obtained by vacuum pyrolysis of 1,1,1-trifluoro-2-bromo-2-
1
chloroethane (3). Corresponding peroxyl radicals was generated by co-condensation of pyrolysis products and molecular
oxygen in an argon matrix. To assign the experimental bands DFT calculations (B3LYP/6–311G**) were carried out. The
Ϫ1
3
fundamental bands of O–O and C–O stretching vibrations of peroxyl radical CF CHClOO (1102.1, 972.7, cm ) were
Ϫ1
18
identified by their red shifts to 1044.7 and 954.1 cm in the spectra of O substituted derivatives. UV photolysis of the
radical 2 in the low-temperature matrix produced difluoroformaldehyde CF O, radicals ClCO, CF , as well as CO and HCl as
the primary photoproducts. ᭧ 1999 Elsevier Science B.V. All rights reserved.
2
3
Keywords: 1,1,1-trifluoro-2-chloroethyl radical; 1,1,1-trifluoro-2-chloroethylperoxyl radical; Pyrolysis; Photolysis; Matrix isolation; IR
spectroscopy
1
. Introduction
meters of these species have not been obtained until
quite recently. The purpose of this work is the IR
spectroscopic study of radicals 1 and 2 stabilized in
argon matrices.
A
wide class of atmospherically important
chemical reactions involves fluorine-containing free
radicals as intermediate species. Generally, in
chemical studies their participation is monitored
indirectly via the chemical kinetics or analysis of
stable reaction products [1–3]. A combination of
matrix isolation and IR spectroscopy was successfully
used for direct observation of primary intermediates
and to study of their structure. Radicals 1 and 2 were
predicted to exist by the theoretical calculations [4]
and were suggested on the basis of the UV spectro-
scopic study of the reactions of CF CH Cl with
2. Experimental
Radicals 1 were generated by the thermolysis of
1,1,1-trifluoro-2-bromo-2-chloroethane (3) at 1150–
Ϫ4
1300 K and 10 Torr in a 50–120 mm quartz reactor
(5 mm in diameter) attached to an optical helium
cryostat. The pyrolysis products were co-deposited
with excess of argon on the copper mirror cooled to
12 K by a closed cycle cryogenic system Displex
3
2
fluorine atoms [3]. However, reliable vibration para-
2
08R (Air Products & Chemicals). To form corre-
sponding peroxyl radicals 2, radicals 1 were allowed
*
Corresponding author. Fax: ϩ 7-095-135-5328.
E-mail address: bas@cacr.ioc.ac.ru (E.G. Baskir)
to react with argon matrices containing from 1 to 10%
0
022-2860/99/$ - see front matter ᭧ 1999 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(98)00777-7

5
20
E.G. Baskir et al. / Journal of Molecular Structure 480–481 (1999) 519–522
Fig. 1. IR spectra of 1,1,1-trifluoro-2-chloroethyl (1) and of 1,1,1-trifluoro-2-chloroethyl peroxyl (2) radicals in an argon matrix at 12 K: (a)
spectrum of pyrolysis products of 1,1,1-trifluoro-2-bromo-2-chloroethane; bands of radical 1 are denoted by asterisk; letters E designate bands
of trifloroethylene; (b) spectrum of the products of the reaction chloroethyl radical with oxygen, bands of radical 2 are denoted by arrows.
oxygen during the trapping process. IR spectra in the
[5] on a Power Challenge supercomputer at the
Computer Assistance of the Chemical Research
Center of the Russian Academy of Sciences.
Ϫ1
region 4000–450 cm were recorded with a FTIR-
Ϫ1
spectrometer Bruker IFS-113 v (1 cm resolution) by
the beam reflection scheme. The UV photolysis (l Ͼ
250 nm) was carried out by a high-pressure mercury
lamp (1000 W). Commercial 3 and isotope-substi-
3. Results and discussion
1
8
tuted O of 99% purity with 82% isotope saturation
2
were used without additional purification.
In the IR spectrum of matrix isolated products of
the pyrolysis precursor 3, two groups of new bands are
observed along with the bands of the starting
compound (Fig. 1a, Table 1). The first one corre-
sponds to the stable trifluoroethylene: 3158.2,
Quantum chemical calculations of molecular struc-
ture and vibrational spectra were performed by ab
initio (UHF) and density functional theory (B3LYP)
approximations using Caussian 94 program package

E.G. Baskir et al. / Journal of Molecular Structure 480–481 (1999) 519–522
521
Table 1
Experimental and calculated (B3LYP/6–311G**) vibrational
frequencies of 1,1,1-trifluoro-chloroethyl radical in the 4000–
Ϫ1
4
50 cm range
Preliminary, assignment Exp.
Calc.
Int. n, cm
Ϫ1
Ϫ1
n, cm
Int.(km/mol)
3
1354.5 102
1245.0 169
n CH
dHCC, n
3118.3
1353.3
1237.6
w
s
s
3245.6
s
CF
3
n
n
n
s
CF
asCF
asCF
3
3
3
1147.2 vs 1145.8 245
1129.6 vs 1121.3 323
Scheme 1.
nC–C, dHCC
nC–Cl, dHCC
943.3
824.7
654.9
572.0 vw
539.0 vw
m
w
w
934.2
818.8
648.8
565.0
532.1
94
19
22
6
trifluoroethylene. It was found that the CF CHCl
3
radical is generated at the first step of the reaction
(at lower temperature and in shorter pyrolysis tube).
It could be explained by the successive cleavage of the
C–Br and C–Cl bonds (Scheme 1).
d
s
CF
3
wHCCl, d
s
CF
3
d
asCF
3
4
It is known [8,9] that the reaction of free radicals
Ϫ1
1781.0, 1354.7, 1257.7, 1172.4, 924.9, 753.5 cm
with O results in the formation of peroxyl radicals
2
[
6]. The second set contains 10 bands assigned to a
(Scheme 1). Co-condensation of pyrolysis product of
3 with dioxygen (Scheme 1) lead to appearance of
new bands (Fig. 1b, Table 2). Intensities of these
signals were changed simultaneously when the matrix
was heated to the diffusion temperature (35 K) or
under UV irradiation. This fact and the presence of
the bands, that showed oxygen frequency shifts upon
labile species. As should be expected, these bands
decreased in intensity when matrix was heated up to
3
5–37 K, resulted in a diffusion of molecules and
simultaneous growth of intensities of bands of
CF CHCl) - the dimerization product of radical 1.
(
3
2
Similar products were detected previously [7] under
laser radiolysis of 3. This fact allowed us to assign the
last set of the bands to radical 1 and showed that the
thermal decomposition of the precursor 3 occurs via
two pathways: to form the trifluorochloroethyl radical
or trifluoromethylcarbene which isomerized to
1
6
18
substitution of O with O confirmed the assign-
2
2
ment of latter species to radical 2.
The full assignment of experimental bands was
performed by comparison of quantum chemical calcu-
lated and experimental FTIR spectra of radicals 1 and
Table 2
Experimental and calculated (B3LYP/6–311G**) vibrational frequencies of 1,1,1-trifluoro-2-chloroethyl peroxyl radical in the 4000–
Ϫ1
4
50 cm range
16
16
Ϫ1
18 18
Ϫ1
Preliminary assignment
CF
3
CHCl O O(n, cm
)
3
CF CHCl O O(n, cm )
Exp.
Calc.
Exp.
Calc.
nCH
dHCC
3023.9
1343.3
3150.3
1355.2
1302.5
1273.4
1205.5
1163.9
1143.6
975.3
3024.1
1345.0
3150.0
1353.8
1300.8
1273.2
1202.1
1153.7
1091.5
955.0
n
n
n
asCF
asCF
3
3
1275.6
1274.6
s
CF
3
1210.3, 1204.9
1155.9, 1154.3
1102.1
1209.1,1203.8
1155.3, 1154.0
1044.7
dHCCl, nO–O
nO–O, dHCCl
nC–O
972.7
954.1
nC–C, dHCO
nC–Cl
887.3
854.4
883.4
812.9
886.2
849.4
882.3
811.8
dHCO, nC–C
703.3, 686.8
699.1
701.2, 685.2
697.3
5
5
60.0
44.6
556.9
542.4

5
22
E.G. Baskir et al. / Journal of Molecular Structure 480–481 (1999) 519–522
The UV photolysis (l Ͼ 250 nm) of the radical
CF CHClOO in the low-temperature matrix produced
3
difluoroformaldehyde CF O (1939.6, 1907.3, 1236.7,
2
Ϫ1
Ϫ1
and 773.1 cm ) [11], radicals ClCO (1879.5 cm )
12], CF (1250.0, 1082.7 cm ) [13], as well as CO
and HCl (2150.8 and 2846.6 cm ) [14], as primary
photoproducts.
Ϫ1
[
3
Ϫ1
Fig. 2. DFT B3LYP/6–311G** calculated geometries of minimum
energy structure of 1,1,1-trifluoro-2-chloroethyl (1) and of 1,1,1-
trifluoro-2-chloroethyl peroxyl (2) radicals.
Acknowledgements
2
. Geometry were optimized by UHF and B3LYP
This work was supported by the RFBR (Projects
Nos. 97-03-33740, 98-07-90290) and the INTAS
methods with 3–21G, 6–31G** and 6–311G**
basis sets. The best agreement of calculated and
experimental data was achieved at the B3LYP/6–
(Grant No. 96-0866).
311G** level (Tables 1 and 2).
According to the results of calculations radical 1
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Ϫ1
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1
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Ϫ1
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[
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Ϫ1
9
72.7 cm ) were identified by their red shifts to
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Ϫ1
1
044.7 and 954.1 cm respectively in the spectra of
O substituted derivatives.
The appearance of doublets at 1210.3, 1204.9 cm
and at 1155.9, 1154.3 cm can be explained by the
1
8
Ϫ1
Ϫ1
[
[
Ϫ1
matrix effects. One of addition band at ca. 700 cm is
likely an overtone of lower vibrations.