Matrix isolation study of the photochemically induced reaction of ozone with dibromochloromethane and bromodichloromethane in solid argon at 14 K. FT-IR spectra of the complexes C(O)HCl ... BrX, C(O)HBr ... XCl, C(O)BrCl ... HX, (OC)(HCl)(Br2), (OC)(HBr)(Cl2), and (OC)(BrCl)(HX) (where X = Br or Cl)

Article abstract of DOI:10.1039/a805799k

Quartz-filtered (λ > 240 nm) photolysis of either ozone/dibromochloromethane or ozone/bromodichloromethane in an argon matrix at 14 K has been shown by FT-IR spectroscopy to lead to the formation of the carbonyl ... Lewis acid complexes C(O)HCl ... BrX, C(O)HBr ... XCl, C(O)BrCl ... HX, and C(O)Cl₂ ... HBr, where X = Br or Cl, depending on the trihalogenomethane used. Several of these complexes are new and are formed by either hydrogen halide or dihalogen abstraction reactions. Upon further irradiation the carbonyl complexes dissociate to form the new carbon monoxide complexes (OC)(HCl)(Br₂), (OC)(HBr)(Cl₂), and (OC)(BrCl)(HX). Thus the photo-induced reactions of ozone with trihalogenomethanes lead to the formation of new carbonyl and carbon monoxide complexes whose vibrational properties are well characterised by FT-IR spectroscopy. Suggested pathways for the photolyses are presented.

Full text of DOI:10.1039/a805799k

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Matrix isolation study of the photochemically induced reaction
of ozone with dibromochloromethane and bromodichloromethane
in solid argon at 14 K. FT-IR spectra of the complexes
C(O)HCl ؒ ؒ ؒ BrX, C(O)HBr ؒ ؒ ؒ XCl, C(O)BrCl ؒ ؒ ؒ HX,
(
OC)(HCl)(Br ), (OC)(HBr)(Cl ), and (OC)(BrCl)(HX) (where
2
2
X ؍ Br or Cl)
Robin J. H. Clark,* Jonathan R. Dann and Loraine J. Foley
Christopher Ingold Laboratories, University College London, 20 Gordon Street, London,
UK WC1H 0AJ
Received 24th July 1998, Accepted 30th October 1998
Quartz-filtered (λ > 240 nm) photolysis of either ozone/dibromochloromethane or ozone/bromodichloromethane in
an argon matrix at 14 K has been shown by FT-IR spectroscopy to lead to the formation of the carbonyl ؒ ؒ ؒ Lewis
acid complexes C(O)HCl ؒ ؒ ؒ BrX, C(O)HBr ؒ ؒ ؒ XCl, C(O)BrCl ؒ ؒ ؒ HX, and C(O)Cl ؒ ؒ ؒ HBr, where X = Br or Cl,
2
depending on the trihalogenomethane used. Several of these complexes are new and are formed by either hydrogen
halide or dihalogen abstraction reactions. Upon further irradiation the carbonyl complexes dissociate to form the
new carbon monoxide complexes (OC)(HCl)(Br ), (OC)(HBr)(Cl ), and (OC)(BrCl)(HX). Thus the photo-induced
2
2
reactions of ozone with trihalogenomethanes lead to the formation of new carbonyl and carbon monoxide
complexes whose vibrational properties are well characterised by FT-IR spectroscopy. Suggested pathways for the
photolyses are presented.
Introduction
trum by a fast Fourier-transform algorithm using a zero filling
factor of times four. The spectrum was converted to a double-
beam absorbance spectrum. The band wavenumbers observed
Matrix isolation provides an excellent means whereby photo-
chemical reactions occurring between ozone and halogeno-
Ϫ1
are accurate to either ±0.1 or ±0.2 cm , depending on the
1
–3
carbons may be analysed, the information obtained being
relevant to the mechanisms of ozone depletion from the atmos-
phere. Previous studies have shown that the photolysis of ozone
resolution. Ozone was generated by Tesla coil discharge
through normal oxygen at low pressures (≈38 Torr) in a 15 cm
Pyrex finger immersed in liquid nitrogen, and purified by
multiple freeze–thaw cycles. British Oxygen Co. supplied the
4
3
1
5
with CH I, CH X2 (X = Cl or Br), CHCl3, CH ClI or
3
5
2
2
CH BrCl produces novel carbonyl ؒ ؒ ؒ Lewis acid complexes,
2
research grade oxygen (>99.99%) and argon, while CHBr Cl
4
2
for example H C(O) ؒ ؒ ؒ HI, from ozone with iodomethane.
The infrared spectra of such complexes differ slightly from
those of isolated species on account of perturbation by the
Lewis acids. The dihalide halogenocarbons CH XY (X and
Y = Cl or Br, X and Y ≠ I) react with ozone, first via hydrogen
halide abstraction to form the carbonyl complexes C(O)-
HX ؒ ؒ ؒ HY etc. and then, on further photolysis, to form vari-
ous carbon monoxide complexes (OC)(HX)(HY). By varying X
and Y of the halogenoalkane, comparisons can be made
between the spectra of complexes having identical carbonyls
but different Lewis acids and vice versa, and between the spec-
tral shifts of the carbon monoxide complexes having different
Lewis acid partners.
2
and CHBrCl2 were used as supplied by Aldrich and were
degassed by multiple freeze–thaw cycles with liquid nitrogen
prior to use. The halogenocarbons were then diluted separately
at species-to-argon (S/Ar) ratios in the range 1:400 – 1:600.
The precursors were then separately deposited for 8 h at rates of
2
Ϫ1
approximately 3 mmol h onto the CsI cold window (14 K) of
a Displex closed-cycle helium cryostat (Air Products DE 202 S).
The vacuum shroud surrounding the cold window could be
aligned for infrared transmission studies, for gas deposition and
for sample photolysis. Spectra were recorded after each matrix
irradiation or warming cycle to monitor any changes caused by
these processes.
The matrices were photolysed for various periods of time
with an Oriel xenon mercury lamp, a 5 cm thick water filter
being placed between the lamp and the sample to reduce the
infrared output of the lamp. The matrices were photolysed with
filtered radiation in the visible and ultraviolet regions by use of
the following transmission filters: Corning 7 mm blue/green
This study is concerned with the ultraviolet induced reactions
of ozone with dibromochloromethane (CHBr Cl) and bromo-
dichloromethane (CHBrCl ), with the identification of the
photoproducts, and with a consideration of possible photo-
chemical pathways.
2
2
(
550 > λ > 350 nm), Pyrex (λ > 290 nm) and quartz (λ > 240
Experimental
nm).
The infrared spectra were recorded on a Bruker IFS 113v
Fourier-transform infrared spectrometer over the range 500–
Results and discussion
Precursor deposition in argon
Ϫ1
Ϫ1
4
000 cm at a resolution of 0.5 or 1.0 cm using a germanium-
coated KBr beam splitter and a MCT detector cooled with
liquid nitrogen. The matrices were scanned 500 times, the inter-
ferograms being added and converted to a single beam spec-
The infrared spectra of CHBr Cl isolated in an argon matrix
2
(CHBr Cl/Ar = 1:450) and in an oxygen matrix (CHBr Cl/
2
2
J. Chem. Soc., Dalton Trans., 1999, 73–78
73

View Article Online
Ϫ1
Table 1 Infrared bands/cm detected for CHBr Cl after deposition in
2
a
an argon matrix and in a solid oxygen matrix at 14 K
Ar
O2
Assignment
3
3
3
1
1
1
1
1
1
1
1
1
070.5w
058.9m
040.1w
361.2mw
353.9w
3057.2mw
νC–H
1360.9w


 δa C–H
321.2w

308.6mw
217.0wm
198.7ms
160.1m
155.3ms
147.1w (sh)

1216.3w
1197.9m
1154.1m


 δ
s C–H


7
7
7
6
6
5
5
51.7s
37.1s
30.0w
70.7vs
47.6w
73.3s
29.5w
751.0s
737.0s

 νC–Cl


669.9s

 νC–Br
572.1ms


a
s = strong, m = medium, w = weak, v = very, sh = shoulder.
Fig. 1 Infrared spectra of CHBr Cl/O /Ar matrix after (a) deposition
2
3
and (b) quartz-filtered photolysis (λ > 240 nm) for ≈50 h. Spectrum (b)
Ϫ1
Ϫ1
was recorded at a resolution of 1.0 cm and shows new bands attrib-
Table 2 Infrared bands/cm detected after deposition of CHBrCl in
2
a
uted to νC ᎐᎐ O of different carbonyl ؒ ؒ ؒ Lewis acid complexes. Infrared
spectra of CHBrCl /O /Ar matrix after (c) deposition and (d) quartz-
an argon matrix and in a solid oxygen matrix at 14 K
2
3
filtered photolysis for ≈50 h. Spectrum (d) was recorded at a resolution
Ϫ1
Ar
O2
Assignment
of 0.5 cm and shows new bands attributed to ν of different carbon-
C᎐O
᎐
yl ؒ ؒ ؒ Lewis acid complexes.
3
3
3
1
1
1
1
1
1
1
1
1
1
1
1
1
058.4s
3016.0m




νC–H
055.5m
040.6w
362.9vw (sh)
361.5w
354.0vw
320.9w
315.3w
307.8w
288.8w
266.5w
220.3mw
179.7w
173.9w
118.9w
117.5w
resemble those detected in the spectra of CHBr Cl, CHBrCl ,
2
2
6
or ozone isolated separately in argon. No bands could be
assigned to a complex formed between ozone and either of the
two halogenomethanes; indeed even after photolysis with UV-
VIS irradiation (λ > 350 nm) no new bands appeared. This situ-








δ
a C–H
ation is different from that for a monoiodoalkane co-deposited
4,7,8
with ozone,
for which the spectroscopic evidence indicates
that a molecular complex is formed immediately after
deposition; this complex in turn breaks up after just ≈10–20
min of photolysis (λ > 800 nm or λ > 650 nm). Thus the pres-
ence of an iodine atom, as opposed to a bromine or chlorine
atom, in a halogenocarbon dramatically changes the photo-
chemistry of ozone. Irradiation of either matrix, CHBr Cl/O /
1211.6m
1169.4m


 δ

s C–H

2
3
8
8
7
7
7
7
7
6
6
6
6
6
6
6
5
5
21.4mw
20.6mw
67.9vs (br)
63.0vs
44.1mw
26.8vs (br)
23.0vs

Ar or CHBrCl /O /Ar, with Pyrex- (λ > 290 nm) or quartz-
2
3

filtered radiation (λ > 240 nm) for tens of hours was required
before a new set of bands was produced; for similar photolysis
times the Pyrex-filtered irradiation cycles produced weaker
bands than those detected after quartz-filtered photolysis. The
most informative experiments proved to be ones in which the
matrices were photolysed at fixed wavelengths (quartz-filtered)
for different periods of time. The bands detected are grouped as
reported below.
772.2 (sh)
757.5vs (br)


νC–Cl












720.3vs (br)
698.6w
96.3vw
91.4vw
87.7mw
62.8mw
61.4mw
58.5mw
07.9vs (br)
88.7w
νC–Br
Behaviour of deposited precursors after irradiation ë > 240 nm
603.9s
586.2vw
Carbonyl complexes. Bands attributable to several carbon-
yl ؒ ؒ ؒ Lewis acid complexes were detected after Pyrex- (λ > 290
nm) and quartz-filtered (λ > 240 nm) irradiation of argon
matrices containing either CHBr Cl/O or CHBrCl /O (Tables
29.8mw
a
br = broad.
2
3
2
3
3
and 4). In both cases the ν
bands of the various carbon-
C᎐O
᎐
O = 1:150) have been recorded (Table 1), as have the infrared
yl ؒ ؒ ؒ Lewis acid complexes produced are the most diagnostic.
C(O)HCl ؒ ؒ ؒ BrX (where X = Br or Cl). Bands attributable
to C(O)HCl are detected after UV irradiation of argon matrices
containing either CHBr Cl/O (Table 3 and Fig. 1) or CHBrCl /
2
spectra of CHBrCl isolated in an argon matrix (CHBrCl /
2
2
Ar = 1:500) and in an oxygen matrix (CHBrCl /O = 1:150)
2
2
(
Table 2). In both cases the bands are assigned using as guides
2
3
2
the assignments of bands for CH BrCl and other halogeno-
O (Table 4 and Fig. 1). The intensities of the bands increased
3
2
methanes in the gas phase. Ultraviolet (λ > 240 nm) photolysis
as the photolysis times increased up to ≈50 h, beyond which
of CHBr Cl/Ar or CHBrCl /Ar matrices produced no new
they began to decrease upon further irradiation. Employing
2
2
bands.
The infrared spectra of either CHBr Cl or CHBrCl co-
CHBr Cl as the initial precursor, a medium-weak band at
2
Ϫ1
1779.7 cm is assigned to ν of formyl chloride isolated in
C᎐O
᎐
2
2
9
deposited with ozone in argon matrices (CHBr Cl/O /Ar =
argon; in the analogous CHBrCl /O experiment this band
2 3
Ϫ1
2
3
1
:2:400; CHBrCl /O /Ar = 1:2:600) exhibited bands that
occurs at 1785.0 cm . In each case the ν
bands are accom-
C᎐O
2
3
᎐
7
4
J. Chem. Soc., Dalton Trans., 1999, 73–78

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Ϫ1
Table 3 Infrared bands/cm assigned to the carbonyl products detected after photolysis (λ > 240 nm) of a mixture of ozone/argon and CHBr Cl/
2
argon at 14 K
Complex
ν
δC–H
νC–Cl
νC–Br
Other
C᎐O
᎐
C(O)BrCl ؒ ؒ ؒ Ar
1824.6w
1
1
820.3w
817.4w (sh)
a
8
24.1mw
626.8vw
C(O)BrCl ؒ ؒ ؒ HBr
1813.5m
820.7m
814.9m
812.0m
805.7m
1
1
1
801.4mw
796.1m
794.7m (sh)
7
7
97.0mw
91.2mw
c
C(O)HBr ؒ ؒ ؒ BrCl
1783.1m
1285.9w
b642.0w
1073.8vw
1
1
279.7w
273.1mw
d
d
C(O)HCl ؒ ؒ ؒ Ar
C(O)HCl ؒ ؒ ؒ Br2
C(O)HCl ؒ ؒ ؒ (HX)2
1779.7mw
1776.3w
1753.7m
1307.9w
1302.8w
e
f
716.9w
712.7w
1147.1m
1
298.5w
a
b
Ϫ1
c
d
Combination ν
ϩ δC–Br–Cl
.
νC–Br bands of C(O)HBr are obscured by those of CHBr Cl at 670.7 cm
.
Combination νC–Br ϩ ν5. νC–Cl bands
C᎐O
2
᎐
Ϫ1
e
f
obscured by those of CHBr Cl at 751.7 cm
.
HX of the nearest neighbour. Combination ν
ϩ ν5.
2
C–Cl
Ϫ1
Ϫ1
Table 4 Infrared bands/cm assigned to carbonyl products detected
after photolysis (λ > 240 nm) of a mixture of ozone/argon and
7.2 cm , respectively. Further assignments are made in Tables 3
and 4 to support the presence of C(O)HBr complexes.
CHBrCl /argon at 14 K
2
C(O)BrCl ؒ ؒ ؒ HX (where X = Br or Cl). The C(O)BrCl
bands (Tables 3 and 4, Fig. 1) behave similarly, after photolysis,
to those of formyl chloride and formyl bromide and were found
to be at similar wavenumbers to those detected for C(O)BrCl
in the gas phase. Several νC᎐O bands were detected after
photolysis of CHBr Cl and O in argon matrices, those between
Complex
ν
δC–H
νC–Cl
νC–Br
C᎐O
᎐
C(O)Cl ؒ ؒ ؒ Ar
1813.5m (sh)
1804.1ms
802.4ms (sh)
1794.3m
792.6m
1785.0mw
1778.6w
776.5w
849.9m
2
10
C(O)Cl ؒ ؒ ؒ HBr
2
1
2
3
Ϫ1
C(O)BrCl ؒ ؒ ؒ HCl
823.3m (sh)
821.1ma
1824.6 and 1817.4 cm being attributed to C(O)BrCl isolated
in argon, the least perturbing environment. The other ν
1
C᎐O
᎐
C(O)HCl ؒ ؒ ؒ Ar
C(O)HBr ؒ ؒ ؒ Cl2
1315.4w 726.8s
1286.2w
bands were shifted to lower wavenumbers as a result of per-
turbation by the Lewis acid, HBr, and are considered to arise
from the complex C(O)BrCl ؒ ؒ ؒ HBr, the shifts being of a simi-
lar magnitude to those reported for other carbonyls perturbed
662.3m
661.6m
1
1279.7w
6
58.8m
C(O)HCl ؒ ؒ ؒ BrCl
1774.8w
772.9w
1762.0mw
1749.2s (sh)
747.5s
3
,5
1
by HBr. However, the three weak bands at 1824.6, 1820.3, and
Ϫ1
b
a
C(O)HCl ؒ ؒ ؒ HX
767.9s
1
817.4 cm , detected after 25 h of photolysis (λ > 240 nm)
b
C(O)HCl ؒ ؒ ؒ (HX)2
1303.1w
could, alternatively, be assigned to ν of C(O)Br ؒ ؒ ؒ HCl
since the ν
828 cm . Nevertheless formation of this complex seems
C᎐O
2
᎐
1
11
band of C(O)Br in the gas phase occurs at
2
C᎐O
᎐
a
b
Obscured by a broad precursor band. HX of the nearest neighbour.
Ϫ1
1
unlikely, as breakage of the two strongest bonds (C–H and
C–Cl) would be required while the weaker C–Br bond would
need to remain intact. The large number of bands assigned to
νC–Cl suggests that either further species or more than one
panied by some slightly red-shifted bands which are assigned to
ν
of formyl chloride in perturbing environments, i.e. com-
C᎐O
᎐
plexed with Lewis acids, C(O)HCl ؒ ؒ ؒ Br2 and C(O)HCl ؒ ؒ ؒ
distinct environment may be present. Photolysis of CHBrCl2/
Ϫ1
BrCl. Moreover, in both experiments carbonyl bands detected
O matrices gave rise to ν
3
bands at 1794.3 and 1792.6 cm
C᎐O
᎐
Ϫ1
in the 1762–1747 cm region (wavenumber shifts of ν , ≈20–
4
and are attributed to the complex C(O)BrCl ؒ ؒ ؒ HCl. The
shift in ν from its value for the uncomplexed carbonyl
C᎐O
᎐
Ϫ1
0 cm ) are attributed to C(O)HCl strongly perturbed by two
C᎐O
᎐
Ϫ1
5
hydrogen halides. These and other assignments for formyl
chloride complexes can be found in Tables 3 and 4. Warming
the matrices led to either growth or destruction of some bands;
this further supports the idea that more than one formyl
chloride species must be present.
(1817.3 cm ) increases with the Lewis acid strength, the
complex C(O)BrCl ؒ ؒ ؒ HCl exhibiting highly perturbed carb-
onyl bands.
C(O)Cl ؒ ؒ ؒ HBr. Bands attributable to this species were
2
detected after UV photolysis of a matrix containing CHBrCl2
C(O)HBr ؒ ؒ ؒ XCl (where X = Br or Cl). These bands are
formed after Pyrex- and quartz-filtered photolysis of matrices
containing either CHBr Cl/O or CHBrCl /O and are attrib-
and O (Table 4 and Fig. 1) and were found to be at similar
3
wavenumbers to those detected previously for carbonyl chloride
12
13
in the gas phase and in solid argon. The bands behaved,
after photolysis, almost identically to those of the carbonyl
species mentioned above. A medium band occurring at 1813.5
2
3
2
3
uted to formyl bromide (Tables 3 and 4, Fig. 1). Like the formyl
chloride bands, their intensities increased progressively until
after ≈50 h of irradiation, whereupon some of the bands began
to weaken. In matrices containing CHBr Cl and O , a medium
Ϫ1
cm is assigned to the isolated species C(O)Cl . Other ν
2
C᎐O
᎐
bands attributed to C(O)Cl were red-shifted to 1804.1 and
2
3
2
Ϫ1
Ϫ1
band occurring at 1783.1 cm is assigned to ν of the complex
C(O)HBr ؒ ؒ ؒ BrCl, the wavenumber shift being 18.4 cm from
that (at 1801.5 cm ) of isolated formyl bromide. In the
CHBrCl /O matrix, ν bands detected after photolysis are
1802.4 cm suggesting that a complex of the form C(O)-
C᎐O
᎐
Ϫ1
Cl ؒ ؒ ؒ HBr is present.
2
Ϫ1
5
Carbon monoxide complexes. The bands in this group are
attributed to several carbon monoxide ؒ ؒ ؒ Lewis acid com-
plexes (Tables 5 and 6) and appear to increase in intensity with
increase in photolysis time at the expense of those attributed to
the carbonyl ؒ ؒ ؒ Lewis acid complexes discussed above. There-
2
3
C᎐O
᎐
considered to arise from the complex C(O)HBr ؒ ؒ ؒ Cl , due to
2
Ϫ1
the wavenumber shifts of 22.9 and 25.0 cm of ν
from that
C᎐O
᎐
for isolated C(O)HBr. As a comparison, the corresponding
1
shifts for C(O)HCl ؒ ؒ ؒ Cl2 and C(O)HCl ؒ ؒ ؒ Br are 23.5 and
2
J. Chem. Soc., Dalton Trans., 1999, 73–78
75

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Ϫ1
Table 5 Infrared bands/cm assigned to νH–Cl, νH–Br and ν
᎐
of carbon
C᎐O
᎐
monoxide complexes detected after UV photolysis (λ > 240 nm) of a
mixture of ozone/argon and CHBr Cl/argon at 14 K
2
Complex
νH–Cl
νH–Br
ν
᎐
C᎐O
᎐
OC ؒ ؒ ؒ (HCl)2
2157.6mw
2150.7mw
OC ؒ ؒ ؒ Br ؒ ؒ ؒ HCl
2813.5m
2
2
807.6mw (sh)
OC ؒ ؒ ؒ HCl ؒ ؒ ؒ Br2
2794.6mw
2153.1m
2
791.4m
HBr ؒ ؒ ؒ Ar
2541.6vw
2525.2vw
2510.4w
a
(
OC) (HBr)n
OC ؒ ؒ ؒ BrCl ؒ ؒ ؒ HBr
2142.3w
m
2
507.8w
Fig. 3 Infrared spectra recorded after (a) deposition and (b) ≈120 h of
BrCl ؒ ؒ ؒ OC ؒ ؒ ؒ HBr
OC ؒ ؒ ؒ Ar
2496.7w
2146.6mw
2138.0mw
quartz-filtered photolysis (λ > 240 nm) of a CHBr Cl/O /Ar matrix.
2
3
Ϫ1
Spectrum (b) was recorded at a resolution of 1.0 cm and shows new
bands attributed to νH–Br of different carbon monoxide complexes,
OC ؒ ؒ ؒ HBr, OC ؒ ؒ ؒ HBr ؒ ؒ ؒ Cl , and OC ؒ ؒ ؒ Cl ؒ ؒ ؒ HBr.
a
m and n are not defined.
2
2
Ϫ1
Table 6 Infrared bands/cm assigned to carbon monoxide complexes
detected after UV photolysis (λ > 240 nm) of a mixture of ozone/argon
than one complex (Tables 5 and 6). Similarly the large number
of bands detected in both the νH–Cl and νH–Br regions indicates
that either HCl or HBr forms part of more than one complex
and CHBrCl /argon at 14 K
2
present in the matrix (HCl in solid argon absorbs at 2888.0,
Complex
ν
᎐
νH–Cl
νH–Br
C᎐O
᎐
Ϫ1 5,14,16
2
869, and 2853.3 cm ;
HBr isolated in argon absorbs at
a
Ϫ1 14
OC ؒ ؒ ؒ (HCl)2
2159w (sh)
2797.7w
2568.4 and 2549.6 cm
In previous studies
complexes such as OC ؒ ؒ ؒ HX ؒ ؒ ؒ HX could be distinguished
by the wavenumber of the ν or νH–Br band. In the complex
OC ؒ ؒ ؒ HCl ؒ ؒ ؒ HCl for example, the HCl in the a position
absorbs between 2791.3 and 2780.5 cm , and in the b position
between 2811.2 and 2803.0 cm . Likewise for the OC ؒ ؒ ؒ
HBr ؒ ؒ ؒ HBr complex, the HBr in the a position absorbs at
≈
).
2
2
786.3wb
5,14
the position of HX at either a or b in
774.9w
a
b
OC ؒ ؒ ؒ HCl ؒ ؒ ؒ BrCl
OC ؒ ؒ ؒ HCl
2156.7w
2154.4w
2
2823.8w
2850.0w
H–Cl
a
b
153.0w
c
Ϫ1
OC ؒ ؒ ؒ HBr
2152.1w (sh)
2540.8w
Ϫ1
2
515.8w
OC ؒ ؒ ؒ BrCl ؒ ؒ ؒ HCl
OC ؒ ؒ ؒ HBr ؒ ؒ ؒ Cl2
OC ؒ ؒ ؒ Cl ؒ ؒ ؒ HBr
2151.3w
2149.5w
2143.1w
2836.9w
a
b
2494.7vw
2509.3w
2507.7w
Ϫ1
Ϫ1
2484.8 cm and in the b position at ≈2509.7 cm . Thus in the
2
CHBr Cl/O experiment the complex OC ؒ ؒ ؒ Br ؒ ؒ ؒ HCl could
2
3
2
2
142.1w
Ϫ1
be identified by the νH–Cl values of 2813.5 and 2807.6 cm . A
2
506.3w
OC ؒ ؒ ؒ Ar ؒ ؒ ؒ Cl ?
OC ؒ ؒ ؒ Ar
2140.6w
2137.9w
second group of similar bands was detected and attributed to
2
Ϫ1
another complex whose νH–Cl values (2794.6 and 2791.4 cm )
a
b
c
are typical of that of HCl in the a position; OC ؒ ؒ ؒ HCl ؒ ؒ ؒ Br2.
HX of the nearest neighbour. (OC) (HCl) . HBr ؒ ؒ ؒ Ar.
m
n
Also the wavenumbers of the ν
᎐
bands detected for both com-
C᎐O
᎐
plexes compare reasonably well with those of related carbon
Ϫ1 14,17
monoxide complexes, i.e. OC ؒ ؒ ؒ (HCl)2 (2157.2 cm ,
2
Ϫ1 3
Ϫ1
5
156.7 and 2155.6 cm ), OC ؒ ؒ ؒ HCl ؒ ؒ ؒ HBr (2154.9 cm ),
Ϫ1 5
OC ؒ ؒ ؒ HCl ؒ ؒ ؒ HI (2151.5 and 2141.4 cm ), and OC ؒ ؒ ؒ HCl
Ϫ1 16,17
(
2151 cm ).
The bands of complexes OC ؒ ؒ ؒ Br ؒ ؒ ؒ HCl
2
and OC ؒ ؒ ؒ HCl ؒ ؒ ؒ Br have similar intensities and, since no
2
evidence for the complex C(O)Br ؒ ؒ ؒ HCl was detected, it is
2
believed that they form from photodissociated C(O)HCl ؒ ؒ ؒ
Br . The two structures exist because, unlike the complex
2
5
OC ؒ ؒ ؒ HCl ؒ ؒ ؒ HBr, bromine does not form a sufficiently
strong hydrogen bond for one type of arrangement to
dominate.
Fig. 2 Infrared spectra of an argon matrix containing CHBr Cl and
2
Likewise in the CHBrCl /O experiment the bands observed
2
3
O after (a) deposition and (b) quartz-filtered photolysis (λ > 240 nm)
3
are attributed to OC ؒ ؒ ؒ HBr ؒ ؒ ؒ Cl and OC ؒ ؒ ؒ Cl ؒ ؒ ؒ HBr.
Ϫ1
2
2
for ≈120 h. Spectrum (b) was recorded at a resolution of 1.0 cm and
shows new bands in the νH–Cl region attributed to the carbon mon-
Ϫ1
The band at 2494.7 cm is close to the value obtained when
HBr is located in the a position, while those detected between
oxide complexes, OC ؒ ؒ ؒ (HCl) , OC ؒ ؒ ؒ HCl, OC ؒ ؒ ؒ HCl ؒ ؒ ؒ BrCl, and
2
Ϫ1
OC ؒ ؒ ؒ BrCl ؒ ؒ ؒ HCl.
2509.3 and 2506.3 cm are typical of HBr in the b position.
Bands attributed to the complex OC ؒ ؒ ؒ HBr were also
fore a mechanism in which the carbonyl ؒ ؒ ؒ Lewis acid complex
photodissociates to form the carbon monoxide complex is
proposed.
detected. The blue-shift in ν
plexed CO in solid argon (2138.4 cm )
᎐
from its value for the uncom-
C᎐O
Ϫ1 1,15
᎐
increases with the
Lewis acid strength. This is due to the degree of removal of
electron density from the antibonding orbital of CO (located
(
OC)(HX)(Y ) (where X = Br or Cl and Y = Br or Cl ).
2
2
2
2
18
After prolonged UV photolysis of argon matrices containing
either CHBr Cl/O or CHBrCl /O , bands were detected which
primarily on the C atom), leading to an increase in bond
strength and therefore of ν , cf. the values of ν in the related
᎐
᎐
C᎐O
᎐
2
3
2
3
C᎐O
᎐
Ϫ1
3
could be assigned to the stretches of CO, HCl, or HBr (Tables 5
and 6, Figs. 2 and 4), their intensities continuing to increase
throughout the entire photolysis period (120 h). Bands are
attributed to carbon monoxide species on the basis that their
wavenumbers are near to that of carbon monoxide in the gas
complexes OC ؒ ؒ ؒ (HBr) (2153.1 cm ), OC ؒ ؒ ؒ HBr ؒ ؒ ؒ HCl
2
Ϫ1 5 Ϫ1 14 Ϫ1 19
(2149.0 cm ), and OC ؒ ؒ ؒ HBr (2152.4 cm , 2150 cm
(OC)(BrCl)(HX) (where X = Br or Cl). Once more bands
assigned to perturbed ν , νH–Cl, or νH–Br could be detected after
).
᎐
C᎐O
᎐
prolonged UV photolysis of CHBr Cl/O3 and CHBrCl /O3
matrices and are attributed to various carbon monoxide com-
plexes (Tables 5 and 6, Figs. 3 and 4). Bands assigned to ν and
2
2
Ϫ1
Ϫ1 14
phase (≈2138 cm ) and in solid argon (2138.3 cm , 2138.4
Ϫ1 1,15
cm
); shifts of ν
᎐
to higher wavenumbers suggest that
᎐
C᎐O
᎐
C᎐O
᎐
carbon monoxide is perturbed by different species to form more
νH–Br were detected after photolysis of matrices containing
7
6
J. Chem. Soc., Dalton Trans., 1999, 73–78

View Article Online
CHBr2Cl + O3
(a)
λ > 240 nm 14 K in Ar
H
H
Br
Cl
C
O
C
O
C
O
Cl
Br2
Br
Br
Cl
H
Br
formyl chloride complex formyl bromide complex carbonyl bromide chloride
complex
hν
hν
O
C
O
C
H
Cl
Br
Cl
and
Br2
Cl
H
H
Br
Br
and
Br
Cl
O
C
Br2
O
C
H
Carbon monoxide ...Lewis acid complexes
CHBrCl2 + O3
(b)
λ > 240 nm
14 K in Ar
H
H
Br
Cl
Cl
Cl Cl
C
O
C
O
C
O
C
O
Br
Cl
Br
Cl
H
Cl2
H
Br
formyl chloride
complex
formyl bromide
complex
carbonyl bromide chloride
complex
carbonyl chloride
complex
hυ
hυ
hυ
hυ
O
C
O
C
O
C
O
C
Cl2
H
Cl
H
Br
Br
Cl
Br
Cl
Cl2
H
Cl
H
Br
Carbon monoxide ... Lewis acid complexes
Scheme 1
Ϫ1
CHBr Cl and ozone. Those detected at 2510.4 and 2507.8 cm
effect on the shift of νH–Cl from its value for the isolated
2
are assigned to νH–Br in the complex OC ؒ ؒ ؒ BrCl ؒ ؒ ؒ HBr on the
basis that νH–Br compares well with those of the other carbon
molecule.
1,3,14,16,17,20
Photochemical interconversion
monoxide ؒ ؒ ؒ HBr complexes.
at 2496.7 and 2146.6 cm are assigned to νH–Br and ν
Other bands detected
Ϫ1
᎐
,
The photochemical reactions of ozone with iodine-containing
C᎐O
᎐
respectively, for the complex BrCl ؒ ؒ ؒ OC ؒ ؒ ؒ HBr. These
complexes are formed only weakly, possibly from the photo-
dissociation of the complexes C(O)HBr ؒ ؒ ؒ BrCl or C(O)Br-
Cl ؒ ؒ ؒ HBr.
compounds reveal evidence for species containing C–IO
x
4,7,8
and C–O–I bonds;
containing compounds (CH Cl2 or CH Br ) the initial
whereas with chlorine- or bromine-
3
2
2
2
products are carbonyl complexes in which insertion of an O atom
into the C–H bond has taken place. Although other mechan-
isms have been considered for the formation of carbonyl prod-
In the CHBrCl /O experiment, bands attributed to CO and
2
3
HCl were detected after prolonged UV photolysis. On the basis
3,5,19
19
that νH–Cl in the a position occurs at a lower value than that in
ucts,
the favoured one in this study and elsewhere, involves
Ϫ1
the b position, the νH–Cl bands at 2823.8 and 2836.9 cm are
the insertion of an O atom (from ozone or oxygen) into either a
C–H or a C–X bond of the precursors CHBr Cl and CHBrCl ,
attributed to the complexes OC ؒ ؒ ؒ HCl ؒ ؒ ؒ BrCl and
OC ؒ ؒ ؒ BrCl ؒ ؒ ؒ HCl, respectively. Both of these values for
νH–Cl occur slightly higher than those referred to earlier because
BrCl is a much weaker Lewis acid than HCl and thus has less
2
2
resulting in hydrogen halide (HX) or halogen (X2 or XY)
abstraction; all the required reaction products have, in fact,
been detected. After prolonged periods of photolysis the band
J. Chem. Soc., Dalton Trans., 1999, 73–78
77

View Article Online
onyl ؒ ؒ ؒ Lewis acid complexes that, upon further photolysis,
dissociate to form several carbon monoxide complexes (Scheme
1
); no evidence was obtained for the presence of species having
C–Br–O or C–O–Br bonds.
Acknowledgements
J. R. D. and L. J. F. thank the EPSRC for financial support and
Miss K. O’Neale for her preliminary work.
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2
3
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Fig. 4 Infrared spectra recorded after (a) deposition and (b) ≈120 h
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matrix. Spectrum (b) was recorded at a resolution of 1.0 cm and
shows new bands attributed to of different carbon mon-
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1
a
2
3
Ϫ1
1
1
1
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᎐
C᎐O
᎐
C᎐O
᎐
region of a
᎐
CHBrCl /O /Ar matrix after (c) deposition and (d) quartz-filtered
2
3
photolysis for ≈120 h. Spectrum (d) was recorded at a resolution of
Ϫ1
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0
.5 cm and shows new bands attributed to different carbon mon-
1
oxide ؒ ؒ ؒ Lewis acid complexes.
1
1
2
intensities of the carbonyl products decreased while new
bands attributed to several carbon monoxide complexes were
detected. This supports the idea that the carbonyl com-
plexes dissociate via elimination of molecular CO and Lewis
acids to form the carbon monoxide ؒ ؒ ؒ Lewis acid complexes
6 A. J. Barnes, H. E. Hallam and G. F. Schrimshaw, Trans. Faraday
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17 A. J. Barnes, H. E. Hallam and G. F. Schrimshaw, Trans. Faraday
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1
1
2
8 W. M. Huo, J. Chem. Phys., 1965, 43, 624.
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(
Scheme 1).
Thus the photochemical reaction of ozone with either
1
983, 100, 317.
CHBr Cl or CHBrCl is dependent on time and photolysis
2
2
wavelength and proceeds via the formation of a range of carb-
Paper 8/05799K
7
8
J. Chem. Soc., Dalton Trans., 1999, 73–78