VUV irradiation of carbon dioxide (CO2) and ammonia (NH3) complexes in argon matrix

Article abstract of DOI:10.1016/j.chemphys.2008.10.026

In this study, we characterized by FTIR spectroscopy the 1:1 and 2:1 molecular complexes between NH₃ and CO₂ in argon matrix at low temperature. In addition we tentatively identified the 1:2 molecular complex. The structures of the c

Full text of DOI:10.1016/j.chemphys.2008.10.026

Chemical Physics 354 (2008) 211–217
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VUV irradiation of carbon dioxide (CO2) and ammonia (NH3) complexes
in argon matrix
*
J.B. Bossa, F. Duvernay , P. Theulé, F. Borget, T. Chiavassa
Physique des Interactions Ioniques et Moléculaires, UMR 6633, Université de Provence et CNRS, Centre de St Jérôme, case 252, 13397 Marseille Cedex 20, France
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, we characterized by FTIR spectroscopy the 1:1 and 2:1 molecular complexes between NH3
and CO2 in argon matrix at low temperature. In addition we tentatively identified the 1:2 molecular com-
plex. The structures of the complexes were established from B3LYP calculations. Moreover, we showed
that the VUV irradiation of the 1:1 complex forms the NH2OH:CO complex.
Received 23 June 2008
Accepted 23 October 2008
Available online 30 October 2008
Ó 2008 Elsevier B.V. All rights reserved.
Keywords:
Matrix isolation
FTIR spectroscopy
Photochemistry
Carbamic acid
Molecular complexes
1. Introduction
cryogenic matrix by FTIR spectroscopy. We identify the complex
structures and give their vibrational assignments with the help of
The interstellar medium (ISM) is a rich reservoir of molecules.
Two of them, CO2 and NH3 coexist in a variety of astrophysical
environments and are two of the most abundant molecules to have
been identified into the icy mantle of the interstellar grains [1]. In
several places of the ISM, those grains are exposed to VUV radia-
tion coming from the stars that can induce the formation of new
products [2,3]. Low temperature irradiation of a NH3:CO2 binary
ice in laboratory has been investigated in the past and these stud-
ies showed that carbamic acid can be produced in such conditions
[4,5]. Moreover, it has been shown that the photolysis of a mixture
of methylamine and CO2 leads to the formation of glycine, the sim-
plest amino acid [6]. Therefore, the understanding of the photo-
chemical processes of amine–CO2 binary mixtures are of
particular importance for astrochemistry.
quantum calculation. Secondly, we focus on the VUV photolysis of
the 1:1 NH3:CO2 molecular complex. We show that the main pho-
tochemical product is a 1:1 molecular complex between NH2OH
and CO. We proposed a mechanism to explain the formation of this
complex.
2. Experimental
Argon (Air liquide, 99.99% purity), carbon dioxide (Linde, 99.99%
purity) and ammonia (Air liquide, 99.9% purity) gases are mixed in
different ratios in a pyrex bulb using standard manometric tech-
niques. The gaseous sample is deposited at a 8 mbar/min rate on
a gold platted surface kept at 10 K at a constant pressure of
10À7 mbar.
The rare gas cryogenic matrices isolation technique is a conve-
nient tool to study molecular complexes by infrared spectroscopy
and provides a simplified model of photoreactivity in an interstel-
lar grain. Experimental and theoretical vibrational modes fre-
quency provide valuable information on the complex formation
and its possible geometrical structure. Moreover, the isolated
molecular complex formed in a cryogenic matrix can induce a
selective mechanism for a particular reaction induced by UV pho-
tolysis and thus give an insight into the mechanism [7].
The annealing of the sample is achieved by a 4 K/min heating
rate warming up of the surface at 10 K, 20 K, 30 K and 36 K and
then re-cooled at 10 K. The VUV irradiation (k > 120 nm) is realized
using a microwave discharge hydrogen flow lamp (Opthos instru-
ments). The infrared spectra of the samples are recorded at 10 K,
in a reflection mode between 4000 and 600 cmÀ1 using a Nicolet
Magna 750 FTIR spectrometer with a MCT detector. Each spectrum
is averaged over one hundred scans and has a 0.12 cmÀ1 resolution.
The goal of these studies is twofold. First, we characterize the
1:1, 2:1 and 1:2 molecular complexes between CO2 and NH3 in a
3. Computational
All calculations are performed using the GAUSSIAN 98 package
[8]. The structures and energies of the monomers and complexes
* Corresponding author. Tel.: +33 49 128 8580; fax: +33 49 163 6510.
E-mail address: fabrice.duvernay@univ-provence.fr (F. Duvernay).
0301-0104/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.chemphys.2008.10.026

212
J.B. Bossa et al. / Chemical Physics 354 (2008) 211–217
are calculated using the B3LYP method with the 6-311++ G(2d,2p)
basis set [9]. The interaction energy of the complexes are estimated
as the difference between the total energy the complexes and the
monomers. Each term is evaluated using the entire orbital set as
usual in Boy’s counterpoise method (BSSE correction) [10].
Harmonic vibrational frequencies and intensities were computed
for the monomers and for the complexes optimized geometries.
The vibrational shifts for the molecular complexes are calculated
from the harmonic frequency values, and then compared to the
experimental shifts.
4. Results and discussion
4.1. CO2 isolated in an Ar matrix
The most intense bands in the FTIR spectrum of CO2 isolated in
an Ar matrix appear at 2345 and 2339 cmÀ1 (Table 1, Fig. 1a). These
two bands are assigned to the asymmetric stretching mode m3
mode of monomeric CO2 in two different matrix sites [11]. The cor-
responding bands for the 13CO2 are located at 2280 and 2274 cmÀ1
Fig. 1. FTIR spectra of CO2 in Ar matrix (1:500) at (a) 10 K, (b) after annealing at
36 K and then recorded at 10 K. m: monomer, D: dimer, X: unknown.
(Table 1, Fig. 1a). In the bending mode region m2, a strong band is
due to the bending mode of monomeric CO2 in different matrix
sites [11,12].
observed at 662 and a doublet at 663, 664 cmÀ1 (Table 1, Fig. 1a)
Table 1
Observed infrared frequencies for different NH3:CO2 mixtures in an argon matrix.
NH3:CO2 mixture in Ar matrix (cmÀ1
1/8/500
)
Species
Assignment
8/1/500
10 K
10 K
10 K after 32 K
3460
10 K after 32 K
3460
3460
3400
3460
3400
3390
3377
3346
3310
3305
3242
3231
3204
NH3
NH3
NH3
NH3
NH3
NH3
NH3
NH3
NH3
NH3
CO2
CO2
CO2
CO2
m1
Dimer
Trimer
Multimer
m3
Dimer
Trimer
Dimer
Multimer
Multimer
Dimer
m3
Dimer
m3
1:1
2:1
m3
m3
1:1
2:1
3346
3346
3346
3310
3242
2346
2345
2342
2339
2338
2345
2345
2345
2339
2338
2339
2338
2339
2338
2337
2280
2274
2272
2271
1647
1646
1645
1640
1639
1630
1616
1047
NH3:CO2
NH3:CO2
13CO2
13CO2
NH3:CO2
NH3:CO2
NH3
NH3:CO2
NH3
NH3:CO2
NH3
NH3 and NH3:CO2
NH3
NH3
NH3:CO2
NH3:CO2
NH3
NH3:CO2
NH3
2280
2274
2272
2280
2274
2272
2280
2274
2272
Multimer
2:1
Trimer
2:1
1645
1639
1639
1639
1630
m4
Dimer and 1:1
P(1)
Multimer
1:2
2:1
Trimer
2:1
Dimer
1:1
1032
1028
1017
1003
1000
997
1000
997
975
997
997
NH3:CO2
NH3
975
975
975
m2
669:667
669/667
669/667
669/667
668
664/663
662
NH3:CO2
NH3:CO2
CO2
1:1
2:1
m2
m2
664/663
662
664/663
662
664/663
662
CO2
660
CO2
D
646/645
646/645
646/645
646/645
639
NH3:CO2
NH3:CO2
1:1
2:1
In bold types are indicated the NH3:CO2 complex features.

J.B. Bossa et al. / Chemical Physics 354 (2008) 211–217
213
Important changes are observed when the sample is heated at
36 K and then re-cooled at 10 K. The relative band intensities of
the CO2 monomer trapped in different sites vary and new bands as-
signed to the CO2 dimer appear (Table 1, Fig. 1b) [11]. These bands
(labeled D in Fig. 1) are located at 2346, 2342 and 2334 cmÀ1 on
the m3 region and at 660 cmÀ1 in the m2 region.
asymmetric and symmetric NH stretching modes are respectively
observed at 3446 and 3460 cmÀ1 [14]. Beside the monomer bands
(m), several bands assigned to the dimer (D) and trimer (T) are also
observed in the spectrum of NH3:Ar in a 8:500 ratio deposited at
10 K. Annealing this sample to 36 K shows the increase of the
amount of ammonia trimers and multimers (M) at the expense
of the monomers (i.e. m bands decrease while D, T and M bands in-
crease) (Fig. 2) due to the diffusion of ammonia at 36 K inside the
Ar matrix. All the observed bands are listed in Table 2.
4.2. NH3 isolated in an Ar matrix
NH3 has been the subject of several studies in rare gas matrices
[13,14]. Fig. 2 displays the spectra of NH3:Ar (8:500 mixture ratio)
at 10 K (Fig. 2a) and after annealing at 36 K and recorded at 10 K
(Fig. 2b). The spectrum is consistent with those already reported
in the literature and shows the presence of the NH3 monomer, di-
mer and trimer (labeled m, D, T in Fig. 2) trapped in the argon ma-
trix [14]. The observed bands are reported with their assignments
in Table 1. A strong band located at 975 cmÀ1 and assigned to the
wagging NH mode m2 dominates the spectrum of the NH3 mono-
mer (Fig. 2a). The d(NH) mode m4 is located at 1639 cmÀ1 and the
4.3. NH3:CO2 complexes
Fig. 3c shows the infrared spectrum of NH3 and CO2 diluted in
argon with an excess of NH3 (NH3:CO2:Ar = 8:1:500), deposited at
10 K. This spectrum is compared with the spectra of CO2:Ar = 1:500
and NH3:Ar = 8:500 in Fig. 3a and b, respectively. New features are
observed with respect to the spectra of the isolated species. In the
m2 region of NH3 (1100–900 cmÀ1) a new band is observed at
997 cmÀ1 in the vicinity of the NH3 (D) band (Fig. 3c). This band
is also clearly visible in the NH3:CO2:Ar spectrum where CO2 is
in excess (1:8:500 ratio) (Fig. 5). This band was already assigned
in a previous study, to the m2 mode of NH3 in the NH3:CO2 complex
[15]. In the m2 region of CO2 (700–600 cmÀ1) when ammonia is
present, two doublets centered at 668 and 645.5 cmÀ1 are ob-
served. This mode, which is degenerated for the monomer CO2
(Pu symmetry), splits into two components, and this mode is no
longer degenerated, as observed in the complex. The origin of the
doublets could be due to a site effect, as observed for the monomer.
In the m
3 region of 13CO2 (2290–2270 cmÀ1) a new band due to the
molecular complex is assigned for the first time at 2272 cmÀ1. The
corresponding band for the 12CO2 is observed at 2338 cmÀ1 (Table
1). Fredin and Nelander in their study could not observe any
absorption for the complex in this region because of the compli-
cated structure of this band [15]. All the bands observed for the
complex are listed in Table 3.
Matrix annealing induces important changes on the spectrum
(Fig. 4). The temperature of the matrix was kept at 32 K during
15 min and then the spectrum was recorded at 10 K. As men-
Fig. 2. FTIR spectra of NH3 in Ar matrix (8:500) at (a) 10 K, (b) after annealing at
36 K and then recorded at 10 K. m: monomer, D: dimer, T: trimer, M: multimer.
Table 2
Calculated B3LYP/6-311++G(2d,2p) and experimental frequencies (cmÀ1) for the NH3:CO2 molecular complex.
Calculated Frequencies (cm-1)
B3LYP/6-311++G(2d,2p)
Experimental Frequencies (cm-1)
Assignment Monomer
Complex
3596 (<1)
3594 (<1)
3481 (<1)
1674 (3)
ν
0
Monomer Complex
ν
νasNH
νsNH
δNH
3596 (2)
3596
334
-2
1
3480 (2)
NH3
1674 (10)
0
1638
1630
1673 (3)
-1
26
-3
/
-8
23
ωNH
νasCO
νas13CO
νsCO
1039 (100)
1065 (24)
974
2345/2239
2280/2274
/
997
2338
2400 (100) 2397 (100)
-7/-1
-8/-2
/
/
/
2272
1364 (0)
1363 (<1)
679 (4)
649 (10)
-1
4
/
CO2
663/661
663/661
669/667
646/645
5/6
δOCO
675 (4)
-26
-17/-16
The relative intensities are given between brackets.
D
m
=
m
(complex) À m(monomer). m: stretching; d : bending ; x : wagging.

214
J.B. Bossa et al. / Chemical Physics 354 (2008) 211–217
Fig. 3. FTIR spectra deposited and recorded at 10 K of CO2:Ar 1:500, (b) NH3:Ar 8:500, (c) NH3:CO2:Ar 8:1:500. D: dimer, T: trimer, a:b refers to (NH3)a:(CO2)b molecular
complex.
tioned before, all the absorption bands assigned to the dimer (D)
and trimer (T) of NH3 grow while the bands due to NH3 monomer
decrease (Fig. 4b). At the same time, bands previously assigned to
the molecular complex between NH3 and CO2 increase. New
bands are also observed after annealing at 2272, 1028, 1003
and 639 cmÀ1 (labeled 2:1 in Fig. 4). These bands cannot be as-
signed to dimers or higher aggregates of neither NH3 nor CO2;
consequently, they are assigned to a (NH3)2:CO2 molecular com-
plex. Indeed the annealing increases the mobility of the NH3
which is in excess in the matrix and allows the formation of
molecular complex with more than one NH3 molecule. Further-
more, the presence of two bands at 1028 and 1003 cmÀ1 in the
m2 mode region of NH3 is also consistent with a (NH3)2:CO2
molecular complex. All the observed bands for this new complex
are listed in Table 3. Inversely, when a NH3:CO2:Ar matrix with
CO2 excess (1:8:500 ratio) is annealed, a new band is observed
at 1032 cmÀ1 (Fig. 5b, Table 1), which can be assigned to a
NH3:(CO2)2 molecular complex.
Table 3
Calculated B3LYP/6-311++G(2d,2p) and experimental frequencies (cmÀ1) for the (NH3)2:CO2 molecular complex.
Calculated Frequencies (cm-1)
B3LYP
Experimental Frequencies
(cm-1)
B3PW91a
ν a
-17
-65
-84
-10
-17
-9
Assignment Monomer Complex
3590
ν
-6
-38
-53
0
Monomer Complex
ν
νasN1H
νsN1H
νasN2H
νsN2H
δN1H
3596 (<1)
3480 (<1)
3596 (<1)
3480 (<1)
1674(10)
3446
3345
3446
3345
3558
3427
3596
3590
3479
1699
1677
1681
1665
1101
1075
2396
-6
-1
33
3
NH3
30
1638
1640
0
2
8
7
5
1646
1638
δN2H
1674 (10)
-19
62
36
-4
-13
89
1618
-20
54
29
-8
ωN2H
ωNH
νasCO
1039(100)
1039(100)
2400
974
974
1028
1003
2337
52
-9
2345
νas13CO
/
/
/
-3
7
2280
/
2271
/
-9
/
CO2
νsCO
1364
1361
668
638
-5
5
668
639
7
δOCO
675
663/661
-37
-44
-24
aCalculated vibrational shifts at B3PW91/6-311++g(d,p) level of theory, see Ref. [21].
(complex) À m: stretching; d: bending ; x: wagging.
The relative intensities are given between brackets. (monomer).
D
m
=
m
m

J.B. Bossa et al. / Chemical Physics 354 (2008) 211–217
215
gen matrix (Fig. 6a) [15,16]. CO2 and NH3 geometries are not af-
fected by the interactions, which is typical for van der Waals
interactions [19]. The distance between the N–CO2 framework of
the complex, which has a Cs symmetry 3.02 Å, is in good agreement
with the experimental value (2.99 Å) [16]. The OCO angle is bent
from the linearity (177°) due to the interaction with NH3. With re-
spect to the free components, the whole system is stabilized by
10 kJ molÀ1 according to DFT calculation while 12 kJ molÀ1 are
found experimentally [20]. Table 2 lists the calculated and theoret-
ical vibrational frequencies along with the frequency shifts (
Dm)
and the assignments.
The molecular interaction causes a change in the symmetry
point group from C3v to Cs. The asNH and dasNH modes that are
m
both degenerated in the NH3 monomer, split into two different
components for each mode. No absorption bands were observed
experimentally for those modes, as predicted by the calculations.
Indeed the calculated shifts are pretty small, À1 and À2 cmÀ1 for
Fig. 4. FTIR spectra of NH3:CO2:Ar matrix (8:1:500) deposited at 10 K and then
recorded at (a) 10 K, (b) 10 K after annealing at 36 K. D: dimer, T: trimer, a:b refers
to (NH3)a:(CO2)b molecular complex.
the masNH and dasNH modes, respectively. So the bands from the
complex should overlap the monomer bands. Moreover, intensities
of these modes are too low to be observed in the IR spectra (Table
2). The same is also true for the
msNH stretching mode of NH3 (Ta-
ble 2). The
m
2 mode of NH3 is theoretically upshifted by 26 cmÀ1, in
good agreement with the frequency shift observed in argon matrix
(23 cmÀ1). In nitrogen matrix this frequency shift is 33 cmÀ1 while
in gas phase a 37 cmÀ1 shift is observed [15,16]. The asymmetric
stretching mode of CO2 is downshifted by 3 cmÀ1 with respect to
the monomer value, as observed in argon matrix (7 cmÀ1). The
deformation mode of CO2 splits into two components, as the com-
plex has a Cs symmetry. One component is upshifted by 4 cmÀ1
(5 cmÀ1 experimentally) and the other one is downshifted by
26 cmÀ1 (17 cmÀ1 experimentally). The symmetric stretching
mode of CO2 becomes IR active, as the OCO angle in the complex
is bent away from linearity. Nevertheless its intensity is still too
low to be observed in our spectra (Table 2). Thus all the observed
frequencies for the NH3–CO2 molecular complex in argon matrix
are in good agreement with a T-shaped structure (Table 2) and sug-
gest that the NH3–CO2 molecular complex is isolated in argon ma-
trix with this structure.
The structure and IR spectra of the (NH3)2:CO2 complex are cal-
culated at the B3LYP/6-311++G(2d,2p) level of theory. The opti-
mized geometry (Fig. 6b) is in good agreement with a previous
calculation performed by Borowiak and co-workers [21]. This com-
plex can be described as a T-shaped (NH3:CO2) complex with an
extra NH3 molecule involved in two hydrogen bonds HaÁ Á ÁO
(2.49 Å) and HbÁ Á ÁN2 (2.27 Å) as shown in Fig. 6. The (NH3)2:CO2
complex also exhibits an electron donor–acceptor interaction
N1. . .C (2.97 Å), as in the complex 1:1. The OCO angle is also tilted
Fig. 5. FTIR spectra of NH3:CO2:Ar matrix (1:8:500) deposited at 10 K and then
recorded at (a) 10 K, (b) 10 K after annealing at 36 K. m: monomer, a:b refers to
(NH3)a:(CO2)b molecular complex.
4.4. Structure of the (NH3)n:(CO2)m molecular complexes
The structure of the gas phase NH3:CO2 (1:1) complex was
determined by Klemperer and co-workers [16]. They report a T-
shaped geometry in which the nitrogen of NH3 bonds with the car-
bon of CO2. Fredin and Nelander [15] suggest as well, the same
structure for the NH3:CO2 complex isolated in a nitrogen matrix.
This assumption is based on the interpretation of their infrared
spectrum. The small frequency shift they observed between the
m
NH of free and complexed ammonia (3 cmÀ1) rules out any hydro-
gen bonding between hydrogen of NH3 and CO2. This T-shaped
structure is also supported by Hartree–Fock calculations but no
valuable information on infrared frequencies have been given in
these studies [17,18].
In an argon matrix no absorption band was observed that could
be assigned to the NH3:CO2 complex in the mNH region. In these
conditions, the structure of the NH3:CO2 molecular complex iso-
lated in argon matrix has been elucidated by comparison of the
experimental frequencies with the theoretical frequencies ob-
tained from DFT calculations.
One stationary point was found on the potential energy surface
of NH3 interacting with CO2. This optimized structure is consistent
with the T-shaped complex observed in both gas phase and nitro-
Fig. 6. The optimized structures of the NH3:CO2 and (NH3)2:CO2 complex at the
B3LYP:6/311++g(2 d,2p).

216
J.B. Bossa et al. / Chemical Physics 354 (2008) 211–217
Table 4
away from linearity like in the complex 1:1 with an angle of 176°.
Nevertheless the intensity of the sOCO stretching mode is still too
Vibrational bands observed after VUV irradiation of the NH3:CO2 complex in argon
matrix at 10 K.
m
low to be observed in our spectra (Table 3). The complex is stabi-
lized by 23.9 kJ molÀ1 in good agreement with the B3PW91/6-
311++G(d,p) calculation performed by Jamróz et al. [21]. However,
the frequency shifts calculated in their study are overestimated in
comparison with the experimental shifts observed in argon matrix
(Table 3) while our calculations at the B3LYP/6-311++G(2d,2p) le-
vel fit them nicely (Table 3). For example in the wagging region of
NH3, two bands are observed for the (2:1) complex, upshifted by 54
and 29 cmÀ1 while 62 and 36 cmÀ1 shifts are calculated using the
B3LYP method. The corresponding shifts in Jamróz’s study are 89
and 52 cmÀ1 (Table 3) [21].
The good agreement observed between the theoretical and
experimental shifts allows us to assign the new bands observed
after annealing the 8:1:500 matrix to a (NH3)2:CO2 molecular com-
plex. Nevertheless the single band observed for the NH3:(CO2)2
complex do not allow us to draw any conclusion about the struc-
ture of this complex.
(cmÀ1
)
Assignment
3273
3184
3167
3152
3126
2149
2147
2145
2139
2138.4
2042
2037
1863
1843
1523
1498
1388
1281
1122
HNC+?
HCN+?
?
NHÀ2
NH
CO:H2O
NH2OH:CO
CO:NH3
CO:NH
CO
HNC:?
CO3
HCO
HCOO
NHÀ2
NH2
HOO
HN@NH
NH2OH:CO
4.5. VUV Photolysis of the NH3:CO2 molecular complex
VUV photolysis of the CO2 monomer in an argon matrix is
known to yield CO and CO3, while VUV photolysis of NH3 produces
tion of atomic hydrogen and atomic oxygen coming from the
photolysis of NH3 and CO2, respectively. In the mCH, mNH stretching
regions, strong bands are also observed at 3273 and 3183 cmÀ1
(Fig. 7) and could be due to complexed HNC and HCN. In a previous
NH2 radical [22,23]. In the present work, irradiation of
a
NH3:CO2:Ar matrix decomposes NH3 and CO2 on the same way
as well but slowly (Fig. 7, Table 4). We observed bands due to
NH2 and to the NH radical at 1498 and 3126 cmÀ1, respectively
study on HCN and HNC associated with H2O, the
m
m
CH mode of
NH mode at
HCN:H2O complex was found at 3181 cmÀ1 and its
À
3283 cmÀ1 [30]. The photodissociation mechanisms will be dis-
cussed in the next part.
[23,24]. The absorption bands of NH2 are also observed at 3152
and 1453 cmÀ1 while there is a band of the CO monomer at
2138 cmÀ1 [25,26]. In contrast with the monomer photolysis we
observe an efficient decomposition of the NH3:CO2 complex.
Simultaneously with the decrease of NH3:CO2 complex, new bands
appear (Table 4). After irradiation several features are observed in
4.6. Photolysis mechanisms
The complexity of the spectrum recorded after irradiation can
be explained by the fact that the photolysis decomposes the com-
plex as well as the CO2 and NH3 monomers (Scheme 1).
This photolysis also provides hydrogen and oxygen atoms,
which can escape from the parent cage and undergo further reac-
tions, then forming new products (Schemes 1 and 2). For example
oxygen atom recombination with CO2 leads to CO3 and hydrogen
atom recombination with CO2 and CO forms HCOO and HCO radi-
cals, respectively (Scheme 2).
The presence of H2O:CO molecular complex, seems to be more
complicated to explain. Nevertheless this complex is known to be
one of the formic acid HCOOH [31] photoproducts. So we suppose
that in our experiment HCOOH is produced from hydrogen recom-
bination with the HCOO radical and then photo-dissociated into
the H2O:CO molecular complex (Scheme 2). The NH3:CO molecular
complex is formed from the photolysis of CO2 in the NH3:CO2 com-
plex followed by the escape of oxygen atom from the matrix cage
(Scheme 3).
the mCO stretching region in the vicinity of the CO monomer at
2149, 2148, 2145 and 2139 cmÀ1 (Fig. 7, Table 4). This suggests
that molecular complexes involving CO are produced. In the NH2
wagging region a strong band is observed at 1122 cmÀ1 assigned
to NH2OH, which is upshifted from the monomer value at
1117 cmÀ1 [27]. The bands located at 2148 and 1122 cmÀ1 are as-
signed to a NH2OH:CO molecular complex [28]. These bands are in
good agreement with a previous study on this complex [28]. The
bands at 2149, 2145 and 2139 are assigned to CO:H2O [26], CO:NH
[29], CO:NH3 [29] complexes, respectively. Several radicals are also
observed such as HCO (1863 cmÀ1) and HCOO (1843 cmÀ1). Forma-
tion of these species can be explained by diffusion and recombina-
Moreover, we can explain the formation of the NH2OH:CO molec-
ular complex which has already been observed in the photolysis of
Scheme 1. Photolysis of NH3 and CO2.
Fig. 7. Difference spectrum of a NH3:CO2:Ar matrix at 10 K (8:1:500) obtained by
subtracting the spectrum recorded after 300 min of irradiation at 10 K from the
spectrum recorded before irradiation. The formed species give positive bands while
the species consumed during the irradiation give negative bands.
Scheme 2. Possible mechanism for HCO, HCOO and H2O:CO2 formation.

J.B. Bossa et al. / Chemical Physics 354 (2008) 211–217
217
of the 1:1 complex leads the NH2OH:CO complex and carbamic
acid is not observed in our experimental conditions.
Acknowledgements
Scheme 3. Possible photochemical mechanism for NH2OH:CO formation.
This work was supported by the PCMI (Physique et Chimie du
Milieu Interstellaire) program and The CNES (Centre National
d’Etudes spatiales).
formohydroxamic acid [HCONHOH] an isomer of carbamic acid
[H2NCOOH] [28].
References
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and then formation of H2NCOOH. However, in argon matrix car-
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This fundamental study on NH3:CO2 complexes in rare gas ma-
trix is of great help to understand the basic mechanisms involved
in a NH3:CO2 binary ice. Indeed, the IR bands are much narrower
in a weakly interacting rare gas environment, and different com-
plexes structures and frequencies are much easier to infer. More-
over, it is also much easier to assign photolysis products. In this
study, we characterized 1:1 and 2:1 molecular complexes between
NH3 and CO2 in an argon matrix. The 1:2 molecular complex has
been tentatively identified from a single vibrational band at
1032 cmÀ1. The structures of the complexes were established from
B3LYP calculations. Moreover, we showed that the VUV irradiation
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