A matrix isolation study of the photochemically induced reactions of nitrogen dioxide with 1,2-dibromoethene and 1,2-dichloroethene using Fourier transform infrared spectroscopy

Article abstract of DOI:10.1016/j.saa.2004.10.045

Photolyses of matrices of either BrCHCHBr/NO₂/Ar or ClCHCHCl/NO₂/Ar using quartz-filtered radiation (λ > 240 nm) led to the appearance of infrared bands attributable to carbonyl, carbon monoxide, and ketene species; no bands belonging to a precursor complex NO ₂...XCHCHX (where X = Br or Cl) were observed upon matrix deposition. The possible reaction pathway is discussed.

Full text of DOI:10.1016/j.saa.2004.10.045

Spectrochimica Acta Part A 61 (2005) 1389–1393
A matrix isolation study of the photochemically induced reactions of
nitrogen dioxide with 1,2-dibromoethene and 1,2-dichloroethene using
Fourier transform infrared spectroscopy
Robin J.H. Clark∗, Loraine J. Foley
Christopher Ingold Laboratories, University College London, 20 Gordon Street, London WC1H 0AJ, UK
Received 9 July 2004; received in revised form 14 October 2004; accepted 15 October 2004
Abstract
Photolyses of matrices of either BrCH CHBr/NO2/Ar or ClCH CHCl/NO2/Ar using quartz-filtered radiation (λ > 240 nm) led to the
appearance of infrared bands attributable to carbonyl, carbon monoxide, and ketene species; no bands belonging to a precursor complex
NO2· · ·XCH CHX (where X = Br or Cl) were observed upon matrix deposition. The possible reaction pathway is discussed.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Matrix isolation; Fourier transform infrared spectroscopy; Photochemical induced reaction; Nitrogen dioxide; Dibromoethene and dichloroethene
1. Introduction
for understanding the role of NO2 in the presence of C C
bonds.
There have been several studies involving the reaction
of nitrogen dioxide with alkenes in solution [1,2], in the
gas-phase [3,4] and in matrices [5–7], a variety of prod-
ucts having been observed. Chatterjee et al. [1] reported that
the reaction of an alkene with NO2 in solution led only to
addition products, whereas Bosch and Kochi [2] observed
the epoxidation of sterically hindered alkenes with NO2
in dichloromethane solution. Epoxidation of alkenes using
NO2 has also been achieved by photochemical activation
of the reactants [2,8,9]. The products identified by Frei and
co-workers [5,6] included ethene oxide, ethanal, NO, and
ethyl nitrite radical (•CH2CH2ONO) and were formed after
photolysis (575–488 nm) of the ethene/NO2 matrix. More-
over, in the photo-induced reaction of 1,4-cyclohexadiene
by NO2 in an argon matrix, Tanaka et al. observed dehy-
drogenation of the diene to form benzene along with NO
and HNO species [10]. The variety of products formed in
these alkene/NO2 reactions has prompted our investigation
into further alkene/NO2 systems in order to provide a basis
The matrix reactions between halogenated ethenes (1,2-
dibromoethene and 1,2-dichloroethene) and NO2 were inves-
tigated in a similar way to those with oxygen and ozone [11],
so as to compare the observed products with those reported
previously [11] as well as with the products reported above
[1–10]. The reactants, BrCH CHBr and ClCH CHCl, were
chosen since the halogen substituents may affect the reaction
mechanism and give rise to different products from those
produced in the ethene/NO2 reaction [5,6].
2. Experimental
Matrix suspensions of the halogenated ethenes and NO2
in solid Ar were prepared by slow continuous deposition
of the gas mixtures (either BrCH CHBr/NO2/Ar = 1:3:2000
or ClCH CHCl/NO2/Ar = 1:2.5:2500) onto a CsI window
cooled at 14 K and housed in a rotating vacuum shroud of
a cryostat (Air Products Displex Model DE 202 S). The vac-
uum shroud could be aligned for infrared transmission stud-
ies, gas deposition, or sample photolysis. The photochemistry
was followed by infrared spectroscopy using a Bruker IFS
∗
Corresponding author. Tel.: +44 20 7679 7457; fax: +44 20 7679 7463.
E-mail address: r.j.h.clark@ucl.ac.uk (R.J.H. Clark).
1386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2004.10.045

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R.J.H. Clark, L.J. Foley / Spectrochimica Acta Part A 61 (2005) 1389–1393
Table 1
113v FT-IR spectrometer over the range of 500–4000 cm−1
at a resolution of 0.5 cm−1 using a MCT detector cooled with
liquid nitrogen. The matrices were irradiated for various pe-
riods with an Oriel xenon mercury lamp with a 5 cm thick
water filter placed between the lamp and the sample to re-
duce the infrared output of the lamp. Filtered radiation in the
ultraviolet region was selected with Pyrex (λ >290 nm) and
quartz (λ >240 nm) transmission filters. Annealing the ma-
trix was achieved by using a variable voltage heater wound
around the cold head of the cryostat and the temperature was
monitored by both a chromel versus gold (0.07% Fe) ther-
mocouple and a hydrogen vapour pressure bulb. The matrices
were warmed up to 25–30 K for 20 min in these studies.
cis- and trans-1,2-dibromoethene and cis- and trans-1,2-
dichloroethene were purchased commercially by Aldrich and
used as supplied (>98% pure and 50:50 cis/trans mixtures).
Argon was used as supplied by British Oxygen Co. and ni-
trogen dioxide was used as supplied by Argo.
Infrared bands (cm−1) detected after deposition of either cis/trans-
BrCH CHBr/Ar or cis/trans-ClCH CHCl/Ar matrices at 14 Ka
Assignment
BrCH CHBr/Ar
3107.9 w
νa C−H
ꢀ
3076.1 vw, sh
3073.7 wb
νs C−H
ꢀ
1592.7 wb
1590.3 mw
νC=C
1536.9 w
?
1494.5 vw
2νa C−Br

1261.1 wb
1259.4 mw
1258.9 w, shb


δCHBr (i-p)



1162.2 mw
1161.4 w, shb
1151.0 vwb


δCHBr (i-p)



914.5 vwb
912.4 mwb
908.5 wb
900.2 vw
898.9 vw, shb
897.3 vwb







3. Results and discussion
δCHBr (o-o-p)







3.1. Deposition of BrCH CHBr/NO2 and
ClCH CHCl/NO2 matrices
ꢀ
753.2 m
752.2 mw, shb
νa C−Br
The infrared spectra of cis- and trans-1,2-dibromoethene,
BrCH CHBr, and cis- and trans-1,2-dichloroethene,
ClCH CHCl, isolated in argon matrices were recorded
and the band wavenumbers were assigned using as guides
the assignments for the bands of ClCH CHCl in the
gas-phase [12,13], ethene [14–16] and other halogenated
ethenes [17–19] in matrices (Table 1). Ultraviolet photolysis
(λ > 240 nm) of BrCH CHBr/Ar or ClCH CHCl/Ar matri-
ces produced no new bands. The infrared spectra of either
BrCH CHBr or ClCH CHCl co-deposited with NO2 in
solid argon exhibited bands that resembled those detected
in the infrared spectra of BrCH CHBr and ClCH CHCl
isolated separately in argon and of NO2 in the gas-phase
[20,21]. Traces of N2O4 and other nitrogen oxides were
present in the matrices. In neither case were infrared absorp-
tions belonging to a precursor complex NO2· · ·XCH CHX
observed, unlike the situation with ozone [11].

694.5 mw
690.2 vwb
675.1 mwb
673.4 wb





δCHBr (o-o-p)





γCHBr
672.0 mwb
587.8 w
ClCH CHCl/Ar
3109.2 m
3105.8 mw, shb
3103.0 w, shb



νa C−H


3080.1 w
νs C−H

1601.9 mwb
1599.4 mw
1597.1 wb
1593.4 wb
1589.4 wb





νC=C





1211.3 vwb
1203.3 mw, shb
1201.8 msb



δCHCl (i-p)


3.2. Photolysis of BrCH CHBr/NO2 and
ClCH CHCl/NO2 matrices
1200.5 m, sh
909.7 sb
905.9 ms
902.8 m, shb
901.9 m, shb



Infrared spectra recorded after UV photolysis
(λ > 240 nm) of BrCH CHBr/NO2 and ClCH CHCl/NO2
matrices led to the appearance of new bands (Tables 2 and 3).
In the BrCH CHBr/NO2 experiment, the new very weak
bands detected at 2149.4, 2145.4, 2140.8, and 2138.5 cm−1
are indicative of the C O stretch of carbon monoxide isolated
in different environments [22–25] while those at 2144.2 and
2142.2 cm−1 are attributed to a C O stretch of the ketene
species BrHC C O, cf. νC O = 2142.2 cm−1 for H2C C O
δCHCl (o-o-p)



851.2 w


827.3 vs
824.4 vs
821.5 s


c
νa C−Cl




818.9 ms, sh

R.J.H. Clark, L.J. Foley / Spectrochimica Acta Part A 61 (2005) 1389–1393
1391
Table 1 ( Continued )
Table 3
Infrared bands recorded after quartz-filtered (λ >240 nm) photolysis of cis-
and trans-ClCH CHCl and NO2 in argon at 14 K
Assignment

705.2 mw, shb
699.6 m
Band maxima (cm−1
)
Assignment




692.3 mwb
687.2 mwb
2809.3 vw
νH−Cl (· · ·HCl)
δCHCl (o-o-p)


2147 w, br
νC≡O (CO), νC=O (ClHC
C
O)
a

νa = antisymmetric stretching, νs = symmetric stretching, δ = bending, o-
o-p = out-of-plane, i-p = in-plane, γ = torsion.
1876.5 mw, sh
1871.9 mw
1863.2 mw

νNO (NO)a
b

Bands due to aggregates or matrix site effects.
Many bands may be due to isotopic splitting.
c

1771.0 vw, sh
1768.1 vw
1764.4 vw
1761.8 vw



[24,25] (Table 2; Fig. 1). This assignment is based on the
observation of similar bands detected after photolysis of
a BrCH CHBr/16O3−x18Ox/Ar matrix [11] which were
assigned to νC=O of a bromoketene species rather than to
νC≡O bands of carbon monoxide which occur in the same
region, because the 18O-shifted bands are characteristic
of ketene C 18O stretches (2115.4 cm−1) [26,27] and not
νC=O (HC(O)Cl)a or (CH2ClC(O)Cl)a




1756.7 vw
1751.1 w
1748.8 vw, sh

νC=O (HC(O)Cl· · ·HCl)a,b

1300.1 mw
1243.3 w
δC−H (HC(O)Cl)
νC−O (epoxy group) or δ(HCH) (CH2Cl2)
C
18O stretches (∼2087 cm−1) [22–25]. The absorptions of
the other vibrational modes of BrHC C O were either too
weak to be detected or were hidden by precursor bands. The
bands could alternatively belong to ketene itself, H2C C O,
although the breaking and addition of more bonds would
be required to form this species than BrHC C O. Some
new bands also appeared in the carbonyl stretching region
(Table 2), those at 1757.0 and 1751.6 cm−1 being assigned
to νC=O of HC(O)Br complexed by the Lewis acid HBr
cf. νC=O bands detected elsewhere for HC(O)Br· · ·HBr
(νC=O = 1756.3 cm−1) [28]. Note that the νC=O bands of
1120.7 vw
νC=C (ClHC C O)
a
Bands due to aggregates or matrix site effects.
HCl from nearest neighbour in the matrix.
b
HC(O)Br isolated in solid argon occur at 1801.5 [29] and
1799 cm−1 [22]. The other bands detected in this region
occurred between 1777.0 and 1768.3 cm−1 and are possibly
attributed to the acyl bromide species CH2BrC(O)Br on
the basis that its carbonyl vibrations absorb in the region
1815–1770 cm−1 [14,30,31]. These bands, however, could
also be attributed to nitrogen oxide species. The weak
band at 1242.9 cm−1 could possibly belong to the epoxy
group of a brominated epoxide species because a band at
1265 cm−1 has previously been attributed to the epoxy group
of ethene oxide [14,20,32]. However, the lack of other bands
associated with this ethene oxide suggests that its presence
is uncertain, although ethene oxide was a product in the
analogous C2H4/NO2 reaction studied by Fitzmaurice and
Table 2
Infrared bands recorded after quartz-filtered (λ > 240 nm) photolysis of cis-
and trans-BrCH CHBr and NO2 in argon at 14 K
Band maxima (cm−1
)
Assignment

2509.2 vw
2507.7 vw, sh
2506.3 vw, sh

νH−Br (· · ·HBr)


2149.4 vw
2145.4 vw, sh
2140.8 vw
2138.5 vw


a
νC≡O


ꢀ
2144.2 vw
2142.2 vw, sh
νC=O (BrHC
C
O)a
1872.1 vw
νNO (NO)
1863.5 w
cis-(s-NO)2

1777.0 w, sh
1776.1 mw
1771.0 w
1770.1 w, sh
1769.6 w






a
cis-(a-NO)2 or νC=O (CH2BrC(O)Br)a






1768.3 w
1758.4 mw, sh
trans-(a-NO)2
ꢀ
1757.0 mw, sh
1751.6 m
νC=O (HC(O)Br· · ·HBr)a,b
1242.9 w
νC−O (epoxy group) or δ(HCH) (CH2Br2)
Fig. 1. Infrared spectra of a BrCH CHBr/NO2/Ar matrix after (a) deposi-
tion and (b) quartz-filtered photolysis (λ > 240 nm). The spectra show the
a
Bands due to aggregates or matrix site effects.
HBr from nearest neighbour in the matrix.
b
growth of new bands assigned to νC≡O (C O) and νC=O (BrHC
C O).

1392
R.J.H. Clark, L.J. Foley / Spectrochimica Acta Part A 61 (2005) 1389–1393
Frei [6]. Alternatively, the band at 1242.9 cm−1 could be
attributed to the HCH bend of CH2Br2 based on IR gas-phase
data [12,20]. No band attributable to the associated C Br
stretch could, however, be detected, although it is possible
that such a band may have been hidden by the many δCHBr
precursor bands occurring in the 670 cm−1 region. If CH2Br2
were present in the matrix then a likely reaction pathway
is the photodissociation of CH2BrC(O)Br to give CH2Br2
and CO (which had been identified earlier), thus providing
further support to the idea that CH2BrC(O)Br could indeed
be responsible for some of the bands detected in the carbonyl
region. Warming of the matrix had no significant effect on
the intensities of the bands.
UV photolysis of the ClCH CHCl/NO2 matrix led to the
detection of a broad band occurring around 2147 cm−1; this
is attributable to the unresolved νC≡O and νC=O modes of
carbon monoxide species and a ketene species ClHC C O,
respectively. This attribution is made on the basis that simi-
lar bands were observed in the comparable mixed-ozone ex-
periments with BrCH CHBr and ClCH CHCl carried out
in this laboratory [11]. Other new bands were situated be-
tween 1771.0 and 1748.8 cm−1 and are assigned to νC=O
of HC(O)Cl isolated in the matrix, the acyl chloride species
CH2ClC(O)Cl [14], and HC(O)Cl perturbed by HCl [28,29]
(Table 3; Fig. 2). Similar to the BrCH CHBr/NO2 experi-
ment, a weak band appeared at 1243.3 cm−1 and is tenta-
tively attributed to either the epoxy group of a chlorinated
epoxide species (on the basis of the previous arguments
[6,14,20,32]) or to δ(HCH) of CH2Cl2 [12,20] formed from
the photodissociation of CH2ClC(O)Cl. Photolysis of iso-
lated NO2/Ar matrices showed that all other new bands ap-
pearing after UV irradiation of either BrCH CHBr/NO2/Ar
or ClCH CHCl/NO2/Ar matrices are due to the photodis-
sociation of NO2/N2O4 and the subsequent chemistry of the
Fig. 2. InfraredspectraofaClCH CHCl/NO2/Armatrixafter(a)deposition
and (b) quartz-filtered photolysis (λ > 240 nm). The spectra show the growth
of new bands assigned to νC=O of different carbonyl species.
fragments NO and (NO)2 [6,33–35] (Tables 2 and 3). No new
bands or changes in the intensities of bands were observed
after annealing of the matrix.
3.3. Photochemical interconversion
The first step in the photochemical pathway (Scheme 1)
is the photodissociation of NO2, forming the NO and O
fragments, followed by the addition of O(3P) to the C C
bond. The excited species CHXCHXO* represents sev-
eral possible structures, including that of the epoxide (i)
which has been tentatively identified in these experiments
and elsewhere [2]. The detection of bands attributable to
a ketene species suggests that XHC C O (ii) is produced,
probably by the 1,1-elimination of HX from CHXCHXO*,
Scheme 1.

R.J.H. Clark, L.J. Foley / Spectrochimica Acta Part A 61 (2005) 1389–1393
1393
while bands attributable to acyl halide species suggest that
CH2XC(O)X (iii) is probably formed from H atom migra-
tion on CHXCHXO*. Other carbonyl bands indicated the
presence of HC(O)X complexes (iv) which could form from
the dissociation of CHXCHXO*. Further, photodissociation
of the matrix stabilised species (iii) and (iv) can lead to the
formation of the observed carbon monoxide species (v) and
(vi), as seen in previous studies [28,29,36].
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There are many feasible routes whereby these reactions
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Acknowledgement
The authors thank the EPSRC for financial support.
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