Reactivity of HNCO with NH3 at low temperature monitored by FTIR spectroscopy: Formation of NH4+OCN-

Article abstract of DOI:10.1016/S0009-2614(02)01919-X

The reactivity of isocyanic acid (HNCO) with solid ammonia (NH3) was first studied at 10 K, using FTIR spectroscopy. The ammonium isocyanate (NH₄⁺OCN^-) is formed from a reaction between HNCO and NH₃. Vibrational band assignments for NH₄⁺OCN^- have been given. On the other hand, when HNCO is adsorbed on amorphous NH₃ film, the reaction does not occur. Warming up of this sample at 90 K induces the NH₄⁺OCN^- formation. Quantum calculations showed that the solvation of NH₃ directly bonded to HNCO by at least three NH₃ molecules plays a major role in the NH₄⁺OCN^- formation process and confirmed the spontaneous character of this reaction.

Full text of DOI:10.1016/S0009-2614(02)01919-X

Chemical Physics Letters 368 (2003) 594–600
www.elsevier.com/locate/cplett
Reactivity of HNCO with NH3 at low temperature
monitored by FTIR spectroscopy: formation of NHþ4 OCNÀ
1
S. Raunier , T. Chiavassa , F. Marinelli, A. Allouche, J.P. Aycard
*
Physique des Interactions Ioniques et Moleꢀculaires, UMR 6633, Bo^ıte 252, Universiteꢀ de Provence,
F-13397 Marseille Cedex 20, France
Received 16 October 2002; in final form 27 November 2002
Abstract
The reactivity of isocyanic acid (HNCO) with solid ammonia (NH3) was first studied at 10 K, using FTIR spec-
troscopy. The ammonium isocyanate (NHþ4 OCNÀ) is formed from a reaction between HNCO and NH3. Vibrational
band assignments for NHþ4 OCNÀ have been given. On the other hand, when HNCO is adsorbed on amorphous NH3
film, the reaction does not occur. Warming up of this sample at 90 K induces the NHþ4 OCNÀ formation. Quantum
calculations showed that the solvation of NH3 directly bonded to HNCO by at least three NH3 molecules plays a major
role in the NHþ4 OCNÀ formation process and confirmed the spontaneous character of this reaction.
Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction
cmÀ1 is in excellent agreement with the one, known
as ÔXCNÕ band, observed on ISO spectra and lo-
cated at 4.62 lm [6,7]. This assignment is sup-
ported by isotopic labeling experiments [8,9]. The
NHþ4 OCNÀ formation can be explained from an
acid–base reaction occurring between NH3 and
HNCO formed during the CO/NH3 UV irradia-
tion [9–11]. The reactions proposed by Grim et al.
[12] are as follows:
HNCO has been detected in the interstellar me-
dium, in gas phase [1–3] but never in the cold (10/
100 K) interstellar dust grains. Space-based obser-
vations by the infrared space observatory (ISO)
have revealed the presence of many species in these
grains as H2O, CO, CO2; CH3OH and NH3 [4].
Laboratory studies, relative to the UV irradia-
tion of CO/NH3 and CO/NH3/H2O mixtures [5]
at 10 K, have shown that NHþ4 OCNÀ could be
formed in the grains. The vibrational band as-
signed to the mOCNÀ mode located about 2165
NH3 þ hm ! NHÃ3 ! NH2 þ H
NH2 þ CO ! H þ HNCO
HNCO þ NH3 ! NHþ þ OCNÀ
4
* Corresonding author. Fax: +491288605.
Here, using the tools of FTIR spectroscopy and
quantum calculation, we focus on the conditions
of NHþ4 OCNÀ formation and the ionization pro-
E-mail addresses: sebastien.raunier@piimsdm.univ-mrs.fr
(S. Raunier), thierry@piimsdm.univ-mrs.fr (T. Chiavassa).
1
Also corresponding author.
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved.
PII: S0009-2614(02)01919-X

S. Raunier et al. / Chemical Physics Letters 368 (2003) 594–600
595
cess when HNCO is co-deposited with NH3 (1/10
mixture) or adsorbed on NH3 ice film.
2. Experimental
Pure HNCO is synthesized from the thermal
decomposition of cyanuric acid (Aldrich Chemi-
cal, 98%) at 650 °C, under a primary vacuum
[13,14]. HNCO is condensed and conserved in a
tube cooled by liquid nitrogen. HNCO is degassed
before each experiment. Ammonia is supplied by
Air Liquide (N36, H2O P 200 ppmv).
To study the reactivity of HNCO into the NH3
solid, the sample HNCO/NH3 (1/10) was deposited
at a rate of 2 Â 10À2 mol/min from two separate
inlets onto a gold-platted mirror held at a constant
pressure of 10À7 mbar at 10 K. The concentration
was estimated from standard manometric tech-
niques. Previously, and under the same conditions,
we undertook a study of pure HNCO and pure
NH3 spectra.
The thermal activation of the samples was
achieved through gradual warming up of the
mirror from 10 to 180 K, and using a heating rate
of 1 K/min.
Fig. 1. Co-deposition of HNCO and NH3 in a ratio 1/10 at
10 K: (a) NH3 10 K; (b) HNCO 10 K; (c) HNCO/NH3 10 K;
(d) 30 K; (e) 120 K; (f) 160 K.
When HNCO and NH3 are co-deposited at 10
K. The HNCO IR spectrum shows three bands,
which are slightly shifted, compared with the val-
ues observed for the solid HNCO or HNCO in an
H2O environment (Table 1). The most striking
feature is the NHþ4 OCNÀ spontaneous formation
due to an acid–base reaction in the bulk between
HNCO and NH3 (Fig. 1c). The absorption bands
relative to NHþ4 OCNÀ species are reported in
Table 2 with their assignments. OCNÀ is charac-
terized by an intense band at 2151 cmÀ1 which is
well correlated with and two weaker bands at 1212
and 630 cmÀ1, which are, respectively, relative to
the asymmetric stretching and symmetric bending
modes [16]. The band at 2151 cmÀ1 cannot be due
to the CO formation because this latter can be
characterized in NH3 environment by a band at
2145 cmÀ1. Moreover, Co is expected to sublimate
near of 40 K whereas the 2151 cmÀ1 band is not
reduced in intensity before the temperature of 200
K reached. The frequency (2151 cmÀ1) of the
OCNÀ asymmetric stretching mode is different
from that observed in a H2O environment (2170
cmÀ1) [17]. NHþ4 can be characterized by a weak
band at 1495 cmÀ1 assigned to NH bend and by a
broad and intense band in the region near 3000
cmÀ1, which is the superposition of bonded NH
stretch and combination modes [18]. As the sample
is heated above 30 K (Fig. 1d), due to a possible
Infrared spectra were recorded with a FTIR
Spectrometer (Nicolet series II Magna system 750)
in the range 4000–500 cmÀ1, with a resolution of
0.5 cmÀ1 and 100 scans.
3. Experimental results
Regarding NH3 [15], the infrared spectrum of
solid HNCO at low temperature (10–145 K) has
already been reported [10]. The solid NH3 infrared
spectrum (Fig. 1a) can be characterized by four
fundamental modes noted m1, m2, m3 and m4, whose
frequencies are 3210, 1075, 3375 and 1625 cmÀ1
respectively. The HNCO infrared spectrum (Fig.
1b) displays six vibrational modes noted m1–m6 and
the frequencies are listed in Table 1 with their
corresponding assignments. The HNCO IR spec-
trum is characterized by an intense vibrational
band located at 2252 cmÀ1 and a broad feature
split into two bands at 3200–3365 cmÀ1, which are
relative to the masNCO and mNH stretching modes.

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S. Raunier et al. / Chemical Physics Letters 368 (2003) 594–600
Table 1
Experimental frequencies (cmÀ1) of HNCO solid, in H2O and in NH3 environments at 10 K
Modes
Assignments
HNCO solid
at 10 K
HNCO in H2O solid
at 10 K [28]
HNCO in NH3 solid
at 10 K
a
m1
m2
m3
m4
m5
m6
NH stretch
3365–3245
2252
–
2242
3218
2259
NCO asymmetric stretch
NCO symmetric stretch
HNC bend
b
1322–1252
862
597
1321–1261
1313–1254
–
–
–
–
–
–
NCO in plane bend
NCO out of plane bend
657
a Two components are observed for this mode. Such a splitting is due to hydrogen bond interaction.
b Fermi resonance of m3 and 2m6.
Table 2
Experimental frequencies (cmÀ1) of NH4þOCNÀ at 10 K in NH3 environment and NHþ4 OCNÀ solid at 160 K
Assignments
NHþ4 OCNÀ in solid NH3 at 10 K
NHþ4 OCNÀ solid at 160 K
m1 þ m5
Combination mode
NH stretch
3225
–
3200
3170
m3
NHþ4
m2 þ m4
2m4
Combination mode
First overtone of NH bend
Combination mode
NH bend
3030
2800
2080
1630
1495
2151
3034
2853
2080
m2 þ m6
m2
m4
m3
NH bend
OCN asymmetric stretch
1477–1441a
2165
OCNÀ
2m2
m1
m2
First overtone of OCN bend
OCN symmetric stretch
OCN bend
1300
1212
630
1335–1317a
1244–1227a
645
a Splitting probably due to a crystallization effect.
excessive NH3 diffusion in the mixture, the reac-
tion continues up to 120 K with HNCO, which has
not yet reacted in the solid (Fig. 1e). After NH3
sublimation at 125 K, we observe only NHþ4 OCNÀ
which displays a vibrational spectrum (Table 2,
Fig. 1f) different from the one previously recorded
at 10 K. This change shows that NH3 presence
(T < 120 K) significantly disturbs the NHþ4 OCNÀ
environment, which in its crystalline form displays
NHþ4 cation surrounded by eight NCOÀ anion
[19]. The frequency of the asymmetric OCNÀ
stretching mode is 2163 cmÀ1 at 160 K. This value
is consistent with the experimental data obtained
from UV irradiation of CO/NH3 (1/1) ice and
heated up to 100 K [8].
split into two components as in the metastable (77
K) or cubic (150 K) phases [20] (Fig. 2b). More-
over, the lattice band at 530 cmÀ1 expected for the
metastable or cubic phase is not present. X-ray
study of solid ammonia at )102 °C shows that the
lone electron pair of NH3 is used and that each
NH3 molecule is involved in six hydrogen bonds
[21]. When HNCO is adsorbed on NH3 ice (Fig.
2a), no reaction occurs at 10 K. A warming-up the
sample to a temperature circa 70 K induces a solid
NH3 transformation, probably from amorphous
to a metastable phase. Thus, a splitting into two
components of the m2 and m3 modes and, at 530
cmÀ1, emergence of a lattice band not observed in
the amorphous film, are now observed for solid
NH3. At 90 K (Fig. 2c), however, a new weak band
is observed at 2163 cmÀ1, which is assigned to the
masOCNÀ stretching mode. After sublimation of
NH3 at 125 K, the IR spectrum shows (Fig. 2d)
We shall now compare these results with those
obtained from HNCO adsorbed on NH3 solid.
The NH3 film prepared at 10 K is an amorphous
film [20]. The m2 and m3 bands are broad and not

S. Raunier et al. / Chemical Physics Letters 368 (2003) 594–600
597
Each term is evaluated using the entire orbital set
as usual in the BoyÕs counterpoise method (BSSE
correction) [24].
With one NH3 only (n ¼ 0), HNCO is bonded
to NH3. The system features a strong hydrogen
ꢁ
bond (H-bond) (r2 ¼ 1:825 A in Fig. 3a), between
the hydrogen of HNCO and the nitrogen of NH3,
forming an angle (N–H. . .N) of 177.2°. The sta-
bilizing energy of the system is )46 kJ/mol. In this
configuration, HNCO displays a proton donor
character and the covalent bond length NH (r1) of
ꢁ
ꢁ
HNCO (H–NCO) is 1.040 A (1.008 A for isolated
HNCO).
(1) n ¼ 1. The optimized structure (Fig. 3b)
presents three H-bonds, forming a quasi-planar
cycle. In this configuration, HNCO is in the time
proton donor to the first NH3 and proton acceptor
by its lone electron pair with the second NH3. The
Fig. 2. Adsorbtion of HNCO on NH3 surface: (a) 10 K; (b) 70
K; (c) 90 K; (d) 130 K.
similar features to those observed for NHþ4 OCNÀ
in the co-deposition experiment (Fig. 1f). The
small amount of NH4þOCNÀ detected can be ex-
plained because it is only formed at the solid in-
terface HNCO/NH3.
Since no reaction occurs at 10 K when HNCO
is adsorbed on NH3 ice film but only when HNCO
is co-deposited with NH3, we suggest that
NHþ4 OCNÀ is produced by solvation-induced
dissociative ionization process. In order to confirm
this hypothesis, we performed calculations on
HNCO ionization in direct interaction with NH3
molecules.
ꢁ
H-bond (r2) length is now 1.721 A, indicating a
strong reinforcement. Simultaneously, r1 increases
ꢁ
to 1.060 A. The H-bond (r3), between the nitrogen
ꢁ
of HNCO and hydrogen of NH3 is 2.405 A, which
denotes a rather weak H-bond. The bond length
ꢁ
(r4), between the two NH3, is 2.076 A. The stabi-
lizing energy is now )67 kJ/mol, the gain between
the systems (3a) and (3b) is about )21 kJ/mol.
Adding extra solvent molecules reinforces r2
ꢁ
(1.825–1.721 A). One consequence is to weaken the
ꢁ
involved covalent bond r1 (1.040–1.060 A) as al-
ready frequently reported [25,26].
(2) n ¼ 2. The new additional NH3 presents a
ꢁ
H-bond (r5 ¼ 2:094 A in Fig. 3c) with the
HNCO:ðNH3ÞðNH3Þ system. The other H-bonds
ꢁ
4. Computational section
are altered, (r3) decreased to 2.366 A and (r4) in-
ꢁ
creased to 2.169 A. The position of the third NH3
To explain the formation of NHþ4 OCNÀ, we
investigated the interaction between HNCO and
the reactive NH3, and then the solvation of HNCO
by one, two and three NH3 molecules. These sys-
tems are noted HNCO:ðNH3ÞðNH3Þn ðn ¼ 0–3Þ
and were optimized at B3LYP/6-31G** basis set
level of theory [22]. The quantum calculations
were carried out using GAUSSIAN 98 [23]. The
interaction energy of the different system, DE, was
calculated using the following equation:
is out of plane and does not involve any interac-
tion with HNCO. The stabilizing energy increases
to )89 kJ/mol. The new NH3 contributes, by hy-
ꢁ
perconjugation, to weakening r1 down to 1.088 A
ꢁ
and reinforcing r2 to 1.617 A, without interacting
with HNCO.
(3) n ¼ 3. After optimization, the HNCO:NH3
ðNH3Þ system spontaneously developed into
OCNÀ3:ðNHþÞðNH3Þ (Fig. 3d). There was forma-
4
3
tion of NHþ4 and OCNÀ ions by proton transfer
Â
Ã
ꢁ
from HNCO to NH3 (r1 ¼ 1:715 A). The different
DEðnÞ ¼ E HNCOðNH3Þnþ1 À E½HNCOꢀ
NH bonds of NHþ4 are bonded directly to three
NH3 and OCNÀ molecules via hydrogen bonding.
Â
Ã
À E ðNH3Þnþ1
ð1Þ

598
S. Raunier et al. / Chemical Physics Letters 368 (2003) 594–600
Fig. 3. Optimized structures of HNCO:ðNH3ÞðNH3Þn for n ¼ 0–3, with interaction energies DE(BSSE) given in kJ/mol. r1 and r2 are
ꢁ
distances between N(1) and H(2), and between H(2) and N(3) given in A (values in parentheses).
The isocyanate displays three H-bonds, r1, r3 and
r7, with 2.137, 1.715 and 2.046 A, respectively. The
roughly equal to a covalent bond energy. As a
consequence, for n ¼ 3, the H–NCO bond break-
ing, is completely offset by the solvation contri-
bution and the resulting system is energetically
stable. These results are consistent with the cal-
culations of Daigoku et al. [27] and Wang et al.
[28]. Both found a stabilization effect of NHþ4 with
full solvation of NH bonds in a symmetry Td for
ꢁ
lengths of the different H-bonds, r4, r5 and r6, be-
tween the NH3 and NHþ4 are 1.952, 1.892 and
ꢁ
1.945 A.
The stabilization effect of the system increases
with the number of NH3. The gain between every
addition of NH3 is about 20 kJ/mol from n ¼ 0 to
2 [27]. When n ¼ 2, the total gain in energy is
NHþ4 ðNH3Þ . The bond lengths of NH in NH4þ
4

S. Raunier et al. / Chemical Physics Letters 368 (2003) 594–600
599
decreased while the H-bond distances NH. . .N
with the NH3 solvents increased, continuously
and HNCO. The more typical OCNÀ (2165 cmÀ1
)
and NHþ4 (1495 cmÀ1) IR absorptions are shifted
versus those recorded by the ISO spectra of pro-
tostellar objects.
from NHþ4 ðNH3Þ to NH4þðNH3Þ [28]. In this case,
4
ꢁ
the H-bond distances with all NH3 are 1.920 A.
This is good agreement with our values 1.952,
ꢁ
Thus, the two bands assigned to NHþ4 OCNÀ in
the ISO spectra are generally reported at 2165
cmÀ1 (mOCNÀ) and 1460 cmÀ1 (dNHþ4 ) [17]. We
think that these differences could be explained
from the environment of NHþ4 OCNÀ. In this study
we did not consider H2O, which is the major
constituent of the interstellar grains. NH3/H2O ice
could modify the frequency of the ions, we also
consider studying the behavior of HNCO co-de-
posited with such a mixture. For example, the
mOCNÀ frequency changes from 2151 cmÀ1 in an
NH3 environment to 2170 cmÀ1 in an H2O envi-
ronment. The solid NHþ4 OCNÀ frequencies at
T ¼ 160 K are also very much altered modified
compared to NHþ4 OCNÀ frequencies in an NH3
environment (T < 125 K).
1.892 and 1.945 A for the system (n ¼ 3), where
the symmetry is broken.
Quantum calculations confirm the spontaneous
character of the reaction between HNCO and
NH3 at 10 K. Due to the proton donor character
of the HNCO molecule, we confirm that the
ionization process occurs if one NH3 molecule is
in interaction with HNCO via its lone electron
pair, and if this NH3 is solvated enough. Three
NH3 molecules are at least required to induce the
proton transfer.
5. Discussion and conclusion
We showed that when HNCO is co-deposited
with the NH3 (1/10) mixture, an acid–base reaction
occurs between HNCO and NH3 as soon as 10 K
to produce NHþ4 OCNÀ. The more typical vibra-
tional bands of NH4þOCNÀ, which serve in prob-
ing its identification, at 2151 and 1495 cmÀ1 are in
excellent agreement with the values reported for
NHþ4 OCNÀ induced during the photolysis of CO/
NH3 ices [29]. However, the behavior of HNCO is
different when it is co-deposited with NH3 and
when it is adsorbed on NH3 amorphous films at 10
K. In the latter case, the system is not reactive.
Nevertheless, upon sample warm-up at a temper-
ature of 90 K, NHþ4 OCNÀ can be formed but in a
weak proportion because it is only formed at the
interface of the two solid phases. The NHþ4 OCNÀ
formation can be explained from a dynamical
reconstruction of the NH3 film. The changes ob-
served on the NH3 infrared spectrum at 70 K
suggest a transformation from an amorphous to
metastable phase [20]. This transformation is fa-
vorable to solvation of HNCO by NH3 molecules
and it follows the emergence of a reaction as
confirmed by the quantum calculations.
In the same way, the mNCO frequency of HNCO
changes significantly according to its environment:
from 2259 cmÀ1 in the HNCO/NH3 system to 2242
cmÀ1 in the HNCO/H2O [17] system compared
with the 2252 cmÀ1 expected for HNCO solid at 10
K. We believe that the non-detection of HNCO in
the interstellar grain, nevertheless produced in
laboratory during the photolysis of CO/NH3 or
CO/NH3/H2O mixtures [5], can be due both to its
great reactivity to NH3 and to its fast photode-
composition [30].
Further study concerning HNCO adsorbed on
solid NH3 and HNCO in NH3/H2O ices must be
envisaged.
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