Matrix infrared spectroscopic studies of the photo-dissociation at 266 nm of C1NO2 and of C1ONO

Article abstract of DOI:10.1016/S0301-0104(01)00607-3

Detailed infrared spectroscopic studies of the photo-dissociation at 266 nm of C1NO₂ trapped in argon matrices with subsequent experiments conducted with a xenon lamp at λ > 360 nm are reported. Formation of cis and trans C1ONO in equilibrium with C1NO₂ is observed after irradiation at 266 nm. λ > 360 nm the transformation of trans C1ONO into cis C1ONO occurs. On prolonged photolysis at 266 nm, C1ONO dissociates into C1ON and O(¹D) atom and into C10 + NO as evidenced in reactive matrices (solid oxygen and nitrogen).

Full text of DOI:10.1016/S0301-0104(01)00607-3

Chemical Physics 276 (2002) 309–320
www.elsevier.com/locate/chemphys
Matrix infrared spectroscopic studies of the photo-dissociation
at 266 nm of ClNO2 and of ClONO
J.M. Coangaa, L. Schriver-Mazzuolia,b,*, A. Schrivera, P.R. Dahooc
a
Laboratoire de Physique Moleꢀculaire et Applications, Laboratoire propre du CNRS, Universiteꢀ Pierre et Marie Curie, Tour 13, case 76,
4 place Jussieu, 75252 Paris Cedex 05, France
b
Laboratoire d’Etude des Nuisances Atmospheꢀriques et de leurs Effets, Universiteꢀ Paris-Nord Campus de Bobigny, IUP Ville et Santeꢀ,
Rue de la Convention, Bobigny 93017, France
c
Laboratoire de Magneꢀtisme et d’Optique de Versailles, Universiteꢀ de Versailles St Quentin en Yvelines, 45 Avenue des Etats Unis,
78035 Versailles Cedex, France
Received 4 September 2001; in final form 14 November 2001
Abstract
Detailed infrared spectroscopic studies of the photo-dissociation at 266 nm of ClNO2 trapped in argon matrices with
subsequent experiments conducted with a xenon lamp at k > 360 nm are reported. Formation of cis and trans ClONO
in equilibrium with ClNO2 is observed after irradiation at 266 nm. At k > 360 nm the transformation of trans ClONO
1
into cis ClONO occurs. On prolonged photolysis at 266 nm, ClONO dissociates into ClON and Oð DÞ atom and into
ClO þ NO as evidenced in reactive matrices (solid oxygen and nitrogen). Ó 2002 Published by Elsevier Science B.V.
1. Introduction
reactions between atomic chlorine and NO2 [4] or
between Cl2O and ClNO [5]. It isomerized slowly
into ClNO2.
Over the past two decades, it has become evi-
dent that nitryl chloride (ClNO2) and its isomers
(ClONO cis and trans) play an important role in
the chemistry of both the troposphere and strato-
sphere. In the troposphere, production of ClNO2
occurs from the reaction of gaseous N2O5 with
NaCl salt aerosol [1,2]. In the stratosphere, fast
reaction of N2O5 with HCl dissolved in ice leads to
HNO3 (solid) and ClNO2 [3]. The cis isomer
ClONO has been observed in the gas phase by
While in the atmosphere ClNO2 is assumed to
be rapidly photolysed after sunrise (in less than
1 h) yielding to Cl atoms and NO2, in condensed
phase, photo-dissociation of ClNO2 can be differ-
ent due to the cage effect. Tevault and
Smardzewski [6] reported an apparent conversion
from ClONO trans (produced in argon matrix
from Cl and NO2) to ClNO2. In their study of
photo-isomerization of nitryl bromide to trans
BrONO in argon matrix, Scheffer et al. [7] reported
that ClNO2 was converted by UV light into a
mixture of cis and trans ClONO. In the present
work, we report a detailed infrared spectroscopic
study of the photo-dissociation at 266 nm of
* Corresponding author. Fax: +1-4427-7033.
E-mail address: schriver@ccr.jussieu.fr (L. Schriver-Mazzu-
oli).
0301-0104/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V.
PII: S0301-0104(01)00607-3

310
J.M. Coanga et al. / Chemical Physics 276 (2002) 309–320
ClNO2 trapped in argon, oxygen and nitrogen
matrices. Subsequent experiments in argon matrix
have also been conducted with a xenon lamp at
k > 360 nm.
examined in the vapour state and solid state 30
years ago [9]. More recently the m1 ðtSNO symmetric
stretching mode) and 2m6 (out of plane wagging
harmonic mode) bands which are in Fermi reso-
nance have been studied by high resolution using
Fourier transform spectroscopy. The rotational
constants in the m1 ¼ 1 and m6 ¼ 2 vibrational
states of 35ClNO2 were determined by least square
calculations [10]. In matrices, relatively little is
known about the spectrum of ClNO2. Some bands
have been identified by Tevault and Smardzewski
[6] after the reaction of Cl with NO2. Preliminary
to photolysis experiments, we have recorded the
spectrum of ClNO2 highly diluted in argon, ni-
trogen and oxygen matrices (ClNO2=M ¼ 1=5000)
between 4000 and 500 cmÀ1. Fig. 1 shows the key
regions in argon matrices, obtained after deposi-
tion and after annealing. Frequencies and relative
intensities of the observed bands are summarized
in Table 1 and compared with those measured in
nitrogen and oxygen matrices and in the gas
phase.
2. Experimental
Normal ClNO2 was synthesized by the reaction
of an excess of ozone with nitrosyl chloride
(ClNO) at room temperature [8]. Ozone was pre-
pared by silent electrical discharge through oxygen
in a closed system cooled with liquid nitrogen us-
ing O2 N 45 pure from Air Liquide. ClNO from
Merck Schuhardt was condensed at 77 K and
subjected to several freeze–pump–thaw cycles be-
fore being mixed with ozone. After the reaction of
O3 with ClNO, residual oxygen was removed by
the pumping on the )180 °C condensate followed
by purification of ClNO2 by trap to trap distilla-
tion at )70 °C.
Matrix samples M=ClNO2 of molar ratio of 5000
were prepared using standard manometric proce-
dures. Ar, N2 and O2 were obtained commercially
with a purity greater than 99.9995% and used
without purification. The studies were performed
using a closed cycle helium refrigerator Air Product
Displex 202 A with a rotating base which allowed
one to place the sample either in front of the spec-
trometer beam or in a perpendicular direction in
front of the photolysis sources through KBr or
CaF2 windows, respectively. Matrix gases were
deposited onto a golden mirror held at 20 K (Ar,
O2) or 17 K (N2) with a deposition rate of about
10 m mol hÀ1. Reflected infrared spectra were re-
corded at 11 K by a Bruker IRTF 113 v over the
range 4000–500 cmÀ1 at a resolution of 0:5 cmÀ1
.
Two different photolysis sources were em-
ployed: a Nd-Yag laser from Quantel (YG 781-
20C) at 266 nm and operating at 20 Hzand a
xenon lamp, Cunow XC 150 S 150 W used with a
cut-off 360 nm filter (Schott WG 360).
3. Vibrational spectrum of ClNO2
Fig. 1. Infrared spectra of ClNO2=Ar matrix sample (1/5000)
at 10 K: (a) after deposition at 20 K; (b) after annealing at
38 K.
The IR spectrum of nitrogen 14 and nitrogen
15 isotopic species of nitryl chloride has been

J.M. Coanga et al. / Chemical Physics 276 (2002) 309–320
311
Table 1
Vibrational wavenumbers (cmÀ1) for IR band positions of ClNO2 in the gas phase and trapped in argon, nitrogen and oxygen matrices
and their assignment
Modes
Argon matrix
1673.8 (1)
Oxygen matrix
1674.4 (1)
Nitrogen matrix
1673.1 (1)
Gas phase [9]
1684.6
m4 ¼ mNASO ðB1Þ
m1 ¼ mNS O ðA1Þ
1261.7 (0.5)
1315.7
1265.0 (0.65)
1316.2
1270.5 (0.95)
1324.8
1267.1
1318.6
2m6
Fermi resonance
m2 ¼ ds ðA1Þ
786.9 ð35ClÞ ð0:9Þ
786.4 ð37ClÞ
651.5 (0.06)
787.5 (0.7)
787.0
797.3 (0.7)
796.5
792.6
–
m6 ðB2Þ
650.8 (0.04)
656.3(0.03)
652.3
408.1
369.6
m5 ¼ dAS ðB1Þ
m3 ¼ mCS lN ðA1Þ
–
–
–
–
–
–
4. Photo-dissociation of ClNO2
In all matrices the symmetric bending mode m2
of A1 symmetry appears as a doublet measured in
solid argon at 786.9 and 786:4 cmÀ1. The intensity
ratio of the two components is 1/3 suggesting that
m2 is coupled with m3 (stretching N–35Cl and
N–37Cl) of the same symmetry. As it can be seen in
Fig. 1 the harmonic 2m6 band is intense because of
Fermi resonance with the m1 mode. Attempts to
evaluate the unperturbed frequencies xi and the
Fermi coupling coefficient W were performed from
the observed perturbed frequencies ki and the in-
tegrated intensity ratio between mixed modes
using the simplified following expression estab-
lished from time independent perturbation theory:
4.1. Observations
ClNO2 highly diluted in argon matrix (1/5000)
was irradiated with the 266 nm laser line. FTIR
spectra were recorded after each of several suc-
cessive irradiation periods. Fig. 2 shows typical
spectra obtained after irradiation with a photo-
flux of 9 Â 1016 photons cmÀ2
s
À1. Irradiation
caused a decrease in intensity for all bands be-
longing to ClNO2 and new bands appeared at
1754.9, 1710.5, 852–850.7, 658.0 and 640:0 cmÀ1
.
However, as illustrated in Fig. 2, after 20 min of
irradiation time, all peaks belonging to ClNO2 and
those belonging to the photolysis products showed
a nearly stationary intensity suggesting a photo-
stationary state between ClNO2 and the photo-
products.
From ab initio calculations [11,12], and exper-
imental data [4,6,7] the band at 1754:9 cmÀ1, the
doublet at 852–850:7 cmÀ1 and the feature at
640:0 cmÀ1 are assigned to mNO; dONO and mClO
modes of the trans ClONO molecule, respectively.
Absorptions at 1710.5 and at 640:0 cmÀ1 are as-
signed to the mNO and mClO modes of the cis ClO-
NO molecule, respectively. All observed band
positions and the proposed assignment are col-
lected in Table 2 for comparison with the calcu-
lated literature data. Thus, under irradiation,
ClNO2 transforms into cis and trans ClONO iso-
mers but a steady-state concentration is reached
on approximately 1200 s of irradiation in this
k1 þ k2 ¼ x1 þ x2;
k1k2 ¼ x1x2 À W 2;
I2 ðk1 À k2Þ À ðx1 À x2Þ
¼
:
I1 ðk1 À k2Þ þ ðx1 À x2Þ
In argon matrix x1 ð2m6Þ and x2 ðm1Þ were found
to be 1292.6 and 1284:9 cmÀ1, respectively, with a
Fermi coefficient of 26:75 cmÀ1
.
Some weak satellite bands were observed near
m4 ð1673:05 cmÀ1Þ; 2m6 ð1314:3 cmÀ1Þ; m1 ð1261:7
cmÀ1Þ fundamentals. After annealing at 38 K
their relative intensities in regard to the monomer
band increased indicating that they belong to
dimer species. Other bands which appear after
annealing in the m4 region (1313.6–1313.3,
1263:9 cmÀ1) are probably due to higher aggre-
gates than dimers.

312
J.M. Coanga et al. / Chemical Physics 276 (2002) 309–320
NO in the same wavelength range to produce a
photochemical equilibrium. As it also can be seen
in Fig. 3, in the dONO region, the two components
of the 852–850:7 cmÀ1 doublet decrease in inten-
sity under irradiation, confirming that they belong
to the trans ClONO isomer and not, as suggested
in [7], to the trans and cis ClONO isomers, re-
spectively. Assuming that the total population is
conserved, i.e., only interconversion and not loss
occurs, it is possible to obtain a value of the ratio
of the appropriate IR absorption cross-sections of
mNO for the trans and cis ClONO from measure-
ments of the intensity of the mNO band of trans
ClONO which disappears and of the intensity of
the mNO band of cis ClONO which appears. The
ratio
Fig. 2. Product band growth during laser photolysis of
ClNO2=Ar (1/5000) mixture in the mNO of t-ClONO and c-
ClONO. For comparison decrease of ClNO2 is shown in the m1
region. Irradiation times in minutes: (a) 0; (b) 120; (c) 240; (d)
e1750 etClONO
¼
600; (e) 1200; (f) 1800 s. Photon flux 9 Â 1016 photons cmÀ2 sÀ1
.
e1715 ecClONO
was found equal to 1:2 Æ 0:2, a value in agreement
with that determined theoretically by Lee [11].
The knowledge of the ratio etClONO=ecClONO
allows one to calculate the relative concentration
of trans and cis ClONO produced at the photo-
stationary state after irradiation of ClNO2 at 266
nm as well as to obtain molar fractions of each
species present in the matrix as a function of the
irradiation time at 266 nm using measurements of
experiment. In order to obtain information about
the relative stability of the photo-products a sub-
sequent irradiation experiment was carried out
with a xenon lamp at k > 360 nm after photolysis
of ClNO2 at 266 nm and formation of cis and
trans ClONO. As shown in Fig. 3, after 3 h of
irradiation time, a decrease in intensity of the
1754:9 cmÀ1 absorption was observed with an in-
crease in intensity of the 1710:5 cmÀ1 absorption
in parallel. Intensity of the ClNO2 bands remained
unchanged. At k >360 nm ClONO trans trans-
forms partially into ClONO cis but total disap-
pearance of ClONO trans was not observed with a
longer irradiation time, suggesting a reverse iso-
merization process of cis ClONO into trans ClO-
integrated intensities of ClNO2 at time
0 ðA0ClNO Þ and of ClNO2, c-ClONO and t-ClONO
¼
2
(At) after the different irradiation times (t). At each
time one should have
½ClNO2Š ¼ ½ClNO2Št þ ½t-ClNO2Št þ ½c-ClNO2Š
and
0
t
Table 2
Comparison between frequencies (cmÀ1) of cis and trans ClONO observed experimentally in argon matrix and obtained by ab initio
calculations from [11]
Modes
CIS
TRANS
Argon matrix
1710.5
ab initio
Argon matrix
ab initio
m1 ¼ mNO ðA0Þ
m2 ¼ dONO ðA0Þ
m3 ¼ mClO ðA0Þ
m4 ¼ mON ðA0Þ
m5 ¼ dClON ðA0Þ
m6 ðA00Þ
1715
850
638
416
249
341
1754.9
852.0–850.7
658.0
1754
855
662
407
262
170
640.0

J.M. Coanga et al. / Chemical Physics 276 (2002) 309–320
313
Fig. 3. Effect of irradiation at k > 360 nm during 3 h upon spectra of t-ClONO and c-ClONO in the 1760–1750, 1715–1700,
860–845 cmÀ1 regions: (a) spectrum after photolysis at 266 nm of a ClNO2=Ar (1/5000) mixture; (b) spectrum (a) after photolysis at
k < 360 nm.
t
ClNO2
Att-ClONO
A
Atc-ClONO
A
A
A
þ
à a þ
à b ¼ 1
0
ClNO2
0
ClNO2
0
ClNO2
with
e
e
b
a
et-ClONO
ClNO2
ClNO2
a ¼
;
b ¼
;
¼
¼ 1:2
ec-ClONO
et-ClONO
ec-ClONO
From experimental measurements of the mNO in-
tensity for the three compounds, a, was found to
be 0.55. Time evolution of the molar fractions of
the various species following irradiation is pre-
sented in Fig. 4. At the photo-stationary state 60%
of ClNO2 is photolysed leading to the formation of
42% of trans ClONO and of 18% of cis ClONO.
Fig. 4. Curves of the molar fractions of ClNO2, c-ClONO,
t-ClONO species versus photolysis time.
4.2. Photolytic process
Photolysis of ClNO2 in the gas phase leads to
the formation of Cl and NO2. In matrices the
photo-dissociation of ClNO2 at 266 nm proceeds
probably along the same channel as in gas phase
but due to the cage effect, recombination of Cl and
NO2 in the matrix cage occurs and leads to ClNO2
or to cis and trans ClONO isomers which trans-
form back in ClNO2 through Cl and NO2 photo-
fragments probably without interconversion at
this wavelength as follows:
If we assume a stationary state for the ðCl þ NO2Þ
pair, the disappearance rate of ClNO2 at each time
can be written as

314
J.M. Coanga et al. / Chemical Physics 276 (2002) 309–320
At the equilibrium state (eq)
dbClNO2c
À
d½ClNO2Šeq d½c-ClONOŠeq
dt
À
¼
ꢀ
ꢁ
ꢀ
ꢁ
dt
dt
d½t-ClONOŠeq
dt
k1k
k
k
À1k2
k
À1
¼
k1 À
½ClNO2Š À
½c-ClONOŠ
ð1Þ
¼
¼ 0
ð6Þ
ꢀ
ꢁ
k
À1k3
k
and from (3) and (4)
À
½t-ClONOŠ
½t-ClONOŠeq k4k
À5
kÀ4k5
¼
:
½c-ClONOŠeq
with k ¼ kÀ1 þ k2 þ k
.
À3
This equation is similar to that which is ob-
tained with a model simplified as follows:
From kinetic curves of Fig. 3 it can be seen that
the ratio ½t-ClONOŠi=½c-ClONOŠi is nearly similar
to ½t-ClONOŠeq=½c-ClONOŠeq. Consequently
k4 ¼ k5 and Eq. (2) can be written as
d½ClNO2Š
À
¼ ðkÀ4 þ kÀ5Þ½ClNO2Š
dt
Àk4ð½c-ClONOŠ þ ½t-ClONOŠÞ ð7Þ
0
As ½c-ClONOŠþ½t-ClONOŠ¼½ClNO2Š À½ClNO 2Št,
0
(½ClNO2Š initial concentration of ½ClNO2Š at t¼0),
Eq. (7) becomes
d½ClNO2Š
À
¼ ðk4 þ kÀ4 þ kÀ5Þ½ClNO2Š
Indeed from this model
dt
0
d½ClNO2Š
À k4½ClNO2Š
À
¼ ðkÀ4 þ kÀ5Þ½ClNO2Š À k4½c-ClONOŠ
À k5½t-ClONOŠ ð2Þ
a differential equation equivalent to (1) supposing
dt
0
¼ k0½ClNO2Š À k4½ClNO2Š
Finally integration of Eq. (8) gives the concentra-
tion of ClNO2 at time t:
ð8Þ
0
0
0 k0 expÀk t Àk4 expÀk t þk4
k1k
k
k
À1k2
k
À1
½ClNO2Š ¼ ½ClNO2Š
ð9Þ
ðkÀ4 þ kÀ5Þ ¼ k1 À
;
k4 ¼
;
k0
As the ½ClNO2Š concentration is proportional to
the integrated absorption intensity of ClNO2 vib-
rational modes the experimental decrease in in-
tensity of the mNO band of ClNO2 versus irradiation
time was fitted from the kinetic equation (9) using
the program Excel Microsoftâ. The fitting was
satisfactory with appropriate coefficients k0 and k4
which were found from our experimental condi-
k
À1k3
k
k5 ¼
Furthermore
d½c-ClONOŠ
dt
¼ k ½ClNO2Š À k4½c-ClONOŠ
ð3Þ
ð4Þ
À4
d½t-ClONOŠ
dt
tions to be 2:36 Â 10À3 and 8:32 Â 10À4
s
À1, re-
¼ k ½ClNO2Š À k5½t-ClONOŠ
À5
spectively. Relative concentrations obtained for
trans and cis ClONO from Cl þ NO2 recombina-
tion (42% and 18%, respectively) could suggest that
the trans ClONO is more stable than the cis ClO-
NO an observation in disagreement with heats
of formation of cis ClONO and trans ClONO
which were predicted to be 15:4 Æ 1:5 and 18:0 Æ
At the beginning of the reaction (i) concentrations
of cis and trans ClONO are very weak and from
(3) and (4)
½t-ClONOŠ
k
k
À5
À4
i
¼
:
ð5Þ
½c-ClONOŠ
i

J.M. Coanga et al. / Chemical Physics 276 (2002) 309–320
315
1:5 kcal molÀ1, respectively, from ab initio calcu-
lations using isodesmic reactions involving H2O,
HOCl and cis HONO and trans HONO [11]. In
fact due to steric effects recombination of Cl and
NO2 in the matrix cage favours the formation of
trans ClONO in regard to that of cis ClONO.
5. Other photolysis channels
The equilibrium state between ClNO2, c-ClO-
NO and t-ClONO observed after irradiation at 266
nm of ClNO2 is the indication that just as ClNO2,
ClONO photo-dissociates into Cl and NO2.
However on prolonged photolysis (t > 2500 s in
the above experiment) bands characteristic of the
three products decreased slowly in intensity and
two new bands located at 1842 and 1804 cmÀ1
grew as illustrated in Fig. 5. From their previous
identification in this laboratory [13], the two bands
at 1842 and 1804 cmÀ1 can be assigned to the
stretching mNO mode of ClON and ClNO, respec-
tively. Appearance of ClON is the indication that
the photo-dissociation of ClONO occurs through
a second pathway namely ClON þ O.
The formation in parallel of ClNO could
also suggest a second dissociation channel for
ClNO2, but this assumption is very questionable
because it has been evidenced that ClON trans-
forms fastly into ClNO in argon matrix by the
tunnelling effect [13]. Thus the origin of ClNO is
probably due to isomerization of ClON into
ClNO. As a matter of fact, the dissociation process
ClNO2 ! ClNO þ O has a reaction enthalpy
equal to 288 kJ molÀ1 and at 248 nm, this decay
path was found to be a minor channel (U ¼ 0:02)
in the gas phase [14].
Fig. 5. Product band growth in the 1850–1800 cmÀ1 region
during prolonged photolysis at 266 nm of the sample of Fig. 2
ðClNO2=Ar ¼ 1=5000Þ. Irradiation times: (a) 1800; (b) 3000;
(c) 4800; (d) 16200 s.
5.1. Oxygen matrix
In order to verify the formation of an oxygen
atom as photofragment of ClONO photo-dissoci-
ation and to determine its energy state (O3P or
O1D), photolysis of ClNO2 trapped in two reactive
matrices, solid oxygen and nitrogen, was carried
out at 266 nm. In oxygen matrices ground state
atomic oxygen and excited atomic oxygen which is
quickly quenched can react with oxygen host
molecules and produce ozone. In nitrogen, excited
atomic oxygen O1D can react with N2 molecules
leading to N2O molecule production.
Oxygen matrix containing 1/5000 of ClNO2 was
subjected to 266 nm photolysis for up to 120 mn.
Product absorption regions of special interest
before photolysis and after different irradiation
times are shown in Fig. 6. A higher loss of ClNO2
than in argon was observed. It was accompanied
by the appearance of features associated with
cis ClONO ð1709:5–1708:3 cmÀ1Þ, trans ClONO
ð1754:6–1751:9 cmÀ1Þ, O3 (2108.2, 1037.9, 702:7
cmÀ1), O3 ꢀ ꢀ ꢀ O (2096.1, 1030.8, 697:8 cmÀ1) ClNO
(1804.6, 1803.7, 1802:7 cmÀ1
)
and ClONO2

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J.M. Coanga et al. / Chemical Physics 276 (2002) 309–320
Fig. 6. Product band growth during laser photolysis of a ClNO2=O2 (1/5000) mixture in typical spectral regions with photolysis times
of (a) 5; (b) 30; (c) 60; (d) 120 min.
Table 3
Wavenumbers of infrared absorptions due to products formed on photolysis of Ar, O2; N2 matrices containing ClNO2 and their
assignment
Argon matrix
Oxygen matrix
Nitrogen matrix
2235.5–2233.4
Assignment
N2O
2108.2
2096.1
O3 ðm1 þ m3Þ
O3 ꢀ ꢀ ꢀ O ðm1 þ m3Þ
NO, NO ꢀ ꢀ ꢀ X
ClON
1881.8–1874.8
1842
1832–1830–1829
1826.1
1825.5
asym-N2O4 ðD0Þ
asym-N2O3
ClNO
1804
1804.6–1803.7–1802.7
1754.6–1751.9
1823.3–1819
1757.0
1763.1
asym-N2O4 ðDÞ
sym-N2O4
t-ClONO
sym-N2O3
ClONO2
1754.9
1710.5
1759
1730.1–1728.9–1721.7
1709.5–1708.3
1723.5
c-ClONO
sym-N2O4
sym-N2O3
NO2
1716.7–1714.8
1691.3
1615.9
1295.7–1295.1
1291.3
1285.2–1282.1
asym-N2O4 ðDÞ
asym-N2O4 ðD0Þ
N2O
1037.9
1030.8
O3
O3 ꢀ ꢀ ꢀ O
853.3
850.1
sym N2O3
t-ClONO
O3
852.0–850.7
702.7
697.8
O3 ꢀ ꢀ ꢀ O
658
640
t-ClON
c-ClONO

J.M. Coanga et al. / Chemical Physics 276 (2002) 309–320
317
(1730.0, 1728.9, 1721:7 cmÀ1) as minor products.
The peak locations of product absorptions are
listed in column 2 of Table 3. The time dependence
of the products is illustrated in Fig. 7. For short
photolysis times, ClNO2 strongly disappears and
as in argon matrix cis and trans ClONO isomers
appear. At longer times, c-ClONO and t-ClONO
decrease at the same rate whatever ClNO grows
with a maximum in concentration after 60 mn of
irradiation. In parallel ozone appears as well as
traces of ClONO2 after an induction period. The
ClON isomer was not observed probably due to its
rapid conversion into ClNO. Appearance of ozone
and its complex with atomic oxygen both identi-
fied in the m3 region by absorptions at 1037.9 and
at 1030:8 cmÀ1, respectively [15] confirm the pho-
to-dissociation of ClONO into ClNO and atomic
oxygen. There is no net destruction of ozone be-
cause when photo-dissociation of O3 occurs the
oxygen atom produced combines with oxygen
molecule host and regenerates ozone [16].
occur leading to ClNO and an oxygen atom [18].
However, the induction period observed for
ClONO2 which is produced in small amounts is the
indication that this possible dissociation is a minor
channel for the production of ozone observed in
this experiment.
5.2. Nitrogen matrix
Exposure of ClNO2=N2 sample (1/5000) to 266
nm irradiation over 8 h produced a product
spectrum richer than that observed with argon and
oxygen matrices. ClNO2 decreased strongly at the
beginning of irradiation, and was totally destroyed
after 8 h of irradiation. Three new pattern ab-
sorptions appeared in the 2250–2000, 1880–1800,
1760–1700, 1300–1250 cmÀ1 regions with, in ad-
dition, absorptions at 1691, 1616, 853 and
852 cmÀ1. Fig. 8 shows excerpts of some spectra
recorded after different irradiation times and Fig. 9
displays changes in the intensities of main product
bands as a function of irradiation times. Table 2 of
column 3 lists the IR frequencies of absorptions
observed after irradiation. Identification of the
numerous close bands was not straightforward
principally in the 1800 and 1700 cmÀ1 regions
which is characteristic of products containing
ðNOÞx oscillators. Different trapping sites, com-
plexes and aggregates of produced species cannot
be excluded. Nevertheless on the basis of their
different time evolutions during photolysis corre-
lated to the literature data, the main products were
identified.
Only one secondary product, chlorine nitrate, is
observed as traces. It is probably produced from
the photolysis of ClNO diluted in solid oxygen as
reported by Tevault and Smardzewski [17]. Dis-
sociation of ClONO2, which is very slow, can also
Bands at 1763, 853 and at 1723:5 cmÀ1 which
showed the same time evolution than in oxygen
matrix were assigned to t-ClONO and to c-ClO-
NO, respectively. The bulky structure centred at
2235 cmÀ1 which was correlated to two weaker
bands at 1282 and 1280 cmÀ1 was assigned to N2O
in comparison with data reported in the literature
[19,20]. Absorption at 1825 cmÀ1 which grew in
concert with N2O was assigned to the NO stretch
ðm1Þ mode of ClNO molecule in spite of a red
5 cmÀ1 shift with respect to the frequency of di-
rectly deposited ClNO in nitrogen [13]. As previ-
ously evidenced, species produced from photolysis
of a site containing the reactant can exhibit a weak
shift from directly deposited species. Other
Fig. 7. Time evolution of photoproducts and ClNO2 after
photolysis at 11 K of a ClNO2=O2 (1/5000) mixture with the 266
nm laser line using integrated intensities of an absorption of
each product. ClNO: pattern bands between 1805.9 and
1802:0 cmÀ1; t-ClONO: 1754.6, 1751:9 cmÀ1; c-ClONO: 1709.5,
1708:3 cmÀ1; O3: 1037.9 cmÀ1; ClNO2: 1315.6 cmÀ1; ClONO2:
pattern absorptions between 1732 and 1718 cmÀ1. Integrated
intensities of absorption belonging at ClNO and (c) and (t)
ClONO have been reduced by 50%.

318
J.M. Coanga et al. / Chemical Physics 276 (2002) 309–320
Fig. 8. Product band growth during laser photolysis of a ClNO2=N2 (1/5000) mixture in typical spectral regions with photolysis times
of (a) 0; (b) 15; (c) 60; (d) 190; (e) 370; (f) 500 min.
remaining bands were assigned to NO (1881:6
)
N2O3, respectively. Indeed when the sample was
kept in the dark (12 h) after photolysis at 266 nm,
the absorption at 1826 cmÀ1 disappeared whatever
the absorptions at 1759, 1691 and 853:3 cmÀ1
grew. Observed frequencies for sym-N2O3 are in
good agreement with the literature data. Unfor-
tunately, the remaining observed absorptions
attributable to asym-N2O3 were overlapped by the
ClNO2 bands. As shown in Fig. 9(b), asym-N2O3
grows strongly at the beginning of the irradiation
then disappears. Consistent with published data,
the triplet at 1832.04–1830.7–1829.0 and the
doublet at 1823–1819 cmÀ1 with features at
1291–1295 cmÀ1 were assigned to asym-N2O4 (D
and D0) and features at 1757 and 1714:8 cmÀ1 to
sym-N2O4. As illustrated in Fig. 9(b), sym-N2O4 at
cmÀ1), NO ꢀ ꢀ ꢀ X ð1874:8 cmÀ1Þ, NO2 (1616 cmÀ1
from the literature data [21,22] and to N2O3 and
N2O4 species as discussed below. Two isomeric
forms of N2O3 (symmetric ONONO and asym-
metric ON–NO2) and of N2O4 (O2N–NO2 of D2h,
and D2d symmetries and asymmetric ONO–NO2)
have been spectroscopically identified several years
ago in gas phase [22], solid phase [22–25] and
matrices [26–31]. Varetti and Pimentel [29] showed
that in nitrogen matrix asym-N2O3 was converted
into sym-N2O3 upon prolonged exposure to the
radiation of the Nernst glower source of the
infrared Às1pectrometer. On this basis the band at
1826 cm and the absorptions at 1759, 1691 and
853:3 cmÀ1 were assigned to asym-N2O3 and sym-

J.M. Coanga et al. / Chemical Physics 276 (2002) 309–320
319
ClON is not observed, due to its fast conversion
into ClNO which grows in concert with N2O.
Formation of nitrogen oxides not observed in ar-
gon and oxygen matrix is the indication of a third
dissociation channel of ClONO. In regard to ar-
gon matrix, nitrogen matrix favours the diffusion
of small molecules [30,31] and hence cage recom-
bination of photo-fragments can be partially ruled
out. Formation of N2O3, NO and NO2 at the be-
ginning of irradiation with ClONO suggests that
ClONO dissociates also into NO þ ClO. In nitro-
gen matrices the NO molecule cannot result from
the decomposition of ClNO as previously observed
[13] and hence it is formed from ClONO and leads
to N2O3 for which the only known bimolecular
channel through which N2O3 can be produced is
via the reaction NO2 þ NO ! N2O3 [20–26]. Ap-
pearance of NO2 after total destruction of ClNO2
can be due to photo-dissociation of ClONO or
subsequent photolysis of N2O3, or of sym-N2O4,
or to reaction of NO produced by ClONO with
atomic oxygen. Further speculation on this point
must await other investigations with isotopically
substituted ClNO2 which are underway.
6. Conclusion
Photolysis of ClNO2 in solid argon at 266 nm as
shown by FTIR leads to the formation of cis and
trans ClONO in equilibrium with ClNO2. These
results indicate that photolysis of ClNO2 yields
Cl þ NO2 that may recombine in the matrix cage
to reform ClNO2 or to produce ClONO which
transforms back in Cl þ NO2 fragments. At
k > 360 nm transformation of trans ClONO into
cis ClONO leading to a photochemical equilibrium
occurs. The calculated ratio of absorption cross-
sections of mNO band for the trans and cis ClONO
shows that the trans ClONO is formed in a slightly
higher yield than the cis ClONO ðt-ClONO=c-
ClONO ¼ 2:3Þ.
Fig. 9. Time evolution of photoproducts and ClNO2 after
photolysis at 11 K of a ClNO2=N2 (1/5000) mixture with the
266 nm laser line using integrated intensities of an absorption of
each product. N2O: 2235.5 cmÀ1; ClNO: 1825.5 cmÀ1; t-ClO-
NO: 1763.1 cmÀ1; c-ClONO: 1723.5 cmÀ1; ClNO2: 1324.8 cmÀ1
;
NO2: 1615.9 cmÀ1; NO: 1881.8 cmÀ1; NO ꢀ ꢀ ꢀ X: 1874.8 cmÀ1
;
asym-N2O3: 1826.1 cmÀ1; sym-N2O3: 1759.0 cmÀ1; sym-N2O4:
1757.0 cmÀ1; sym-N2O4: 1757.0 cmÀ1; asym-N2O4: 1830–1827
cmÀ1. Integrated intensities of NO2; ClNO2; N2O have been
reduced by 50%, and integrated intensities of ClNO by 33%.
266 nm is sensitive to irradiation and transforms
partially into asym-N2O4. Note that in argon
matrix isomerization of asym-N2O4 into sym-
N2O4 was observed at 436 nm irradiation as re-
ported by Bandow et al. [32].
As in argon and oxygen matrix, photo-dissoci-
ation of ClNO2 leads to ClONO which in turns
dissociates into ClON and oxygen atom. Forma-
tion of N2O is the indication that produced oxygen
atom is in an excited state. As in oxygen matrix,
Prolonged photolysis of ClNO2 at 266 nm in
argon, nitrogen, oxygen matrices gives evidence
that photo-dissociation of ClONO proceeds along
1
two other different channels namely ClONþOð DÞ
and ClO þ NO. Thus photolysis of ClNO2 at 266
nm can be summarized as follows:

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J.M. Coanga et al. / Chemical Physics 276 (2002) 309–320
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This work was partly supported by PNCA
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