Matrix isolation studies of carbonic acid - The vapor phase above the β-polymorph

Article abstract of DOI:10.1021/ja4020925

Twenty years ago two different polymorphs of carbonic acid, α- and β-H₂CO₃, were isolated as thin, crystalline films. They were characterized by infrared and, of late, by Raman spectroscopy. Determination of the crystal structure of these two polymorphs, using cryopowder and thin film X-ray diffraction techniques, has failed so far. Recently, we succeeded in sublimating α-H₂CO₃ and trapping the vapor phase in a noble gas matrix, which was analyzed by infrared spectroscopy. In the same way we have now investigated the β-polymorph. Unlike α-H₂CO₃, β-H₂CO₃ was regarded to decompose upon sublimation. Still, we have succeeded in isolation of undecomposed carbonic acid in the matrix and recondensation after removal of the matrix here. This possibility of sublimation and recondensation cycles of β-H₂CO₃ adds a new aspect to the chemistry of carbonic acid in astrophysical environments, especially because there is a direct way of β-H₂CO₃ formation in space, but none for α-H₂CO₃. Assignments of the FTIR spectra of the isolated molecules unambiguously reveal two different carbonic acid monomer conformers (C_2v and C_s). In contrast to the earlier study on α-H₂CO₃, we do not find evidence for centrosymmetric (C_2h) carbonic acid dimers here. This suggests that two monomers are entropically favored at the sublimation temperature of 250 K for β-H₂CO₃, whereas they are not at the sublimation temperature of 210 K for α-H₂CO₃.

Full text of DOI:10.1021/ja4020925

Article
pubs.acs.org/JACS
Matrix Isolation Studies of Carbonic AcidThe Vapor Phase above
the β‑Polymorph
Jurgen Bernard,† Roland G. Huber,‡ Klaus R. Liedl,‡ Hinrich Grothe,*,§ and Thomas Loerting*,†
̈
†Institute of Physical Chemistry, University of Innsbruck, A-6020 Innsbruck, Austria
‡Institute of General, Inorganic, and Theoretical Chemistry, University Innsbruck, A-6020 Innsbruck, Austria
§Institute of Materials Chemistry, Vienna University of Technology, A-1060 Vienna, Austria
S
* Supporting Information
ABSTRACT: Twenty years ago two different polymorphs of carbonic
acid, α- and β-H2CO3, were isolated as thin, crystalline films. They were
characterized by infrared and, of late, by Raman spectroscopy.
Determination of the crystal structure of these two polymorphs, using
cryopowder and thin film X-ray diffraction techniques, has failed so far.
Recently, we succeeded in sublimating α-H2CO3 and trapping the vapor
phase in a noble gas matrix, which was analyzed by infrared
spectroscopy. In the same way we have now investigated the β-
polymorph. Unlike α-H2CO3, β-H2CO3 was regarded to decompose upon sublimation. Still, we have succeeded in isolation of
undecomposed carbonic acid in the matrix and recondensation after removal of the matrix here. This possibility of sublimation
and recondensation cycles of β-H2CO3 adds a new aspect to the chemistry of carbonic acid in astrophysical environments,
especially because there is a direct way of β-H2CO3 formation in space, but none for α-H2CO3. Assignments of the FTIR spectra
of the isolated molecules unambiguously reveal two different carbonic acid monomer conformers (C2v and Cs). In contrast to the
earlier study on α-H2CO3, we do not find evidence for centrosymmetric (C2h) carbonic acid dimers here. This suggests that two
monomers are entropically favored at the sublimation temperature of 250 K for β-H2CO3, whereas they are not at the
sublimation temperature of 210 K for α-H2CO3.
The crystal structures of both carbonic acid polymorphs still
remain unsolved. Powder X-ray diffraction cryotechniques were
recently employed by us to observe two amorphous forms of
carbonic acid, which then crystallize to the two polymorphs.20
So far our attempts of indexing and refining the Bragg
reflections after crystallization were unsuccessful, though. For
this reason, FTIR and Raman studies of the solid polymorphs
remain to date the only available data providing clues about
symmetry and short-range order. The mutual exclusion of
Raman and IR bands in the case of β-H2CO3 suggests a
centrosymmetric building block, whereas mutual exclusion and
a local inversion center were not found for α-H2CO3.19,20
Examples for possible building blocks of the two polymorphs
are depicted in Figure 1, which can merely be regarded as
working hypotheses in lieu of refined crystal structures.
First indications for the possible existence of carbonic acid in
the gas phase were provided by mass spectrometric observation
of the vapor phase produced after ammonium bicarbonate
(NH4HCO3) thermolysis.21 Later, two H2CO3 conformers with
symmetry C2v (denoted cis−cis, see Figure 1) and Cs (denoted
cis−trans) were produced by using a pulsed supersonic jet
discharge nozzle and studied using microwave spectrosco-
py.22,23 We have studied the vapor phase above α-H2CO3 by
slowly sublimating the crystalline thin film at 210 K in vacuo.
INTRODUCTION
■
Carbonic acid, H2CO3, plays an important role in many fields1
of chemistry and physics, including astrophysics,2−6 and
biological and geochemical carbonate containing systems.
This six-atom molecule commonly found in carbonated drinks
at submicromolar concentration has so far eluded most
attempts of isolation in its pure form and direct detection.
This is mainly because it easily decomposes to carbon dioxide
and water under ambient conditions and even more so in the
presence of water.7 In aqueous solution, detection of its
formation and/or decomposition is only feasible by use of fast8
or ultrafast spectroscopic techniques.9 However, at the
temperature of many extraterrestrial environments, its decom-
position is hindered. Formation of two distinct solid carbonic
acid polymorphs, α and β, was achieved in laboratory
experiments by acid−base chemistry at cryotemperatures.10 β-
H2CO3 is also formed under conditions akin to those
encountered in space. For example, it is formed from 1:1
mixtures of solid carbon dioxide (CO2) and water (H2O) ice by
proton-irradiation,2,3,11 electron irradiation,12 or UV-photo-
lyis.11,13 In the absence of water it may form from solid CO2 ice
by H-implantation3,14 or from carbon monoxide (CO) by
reaction with hydroxyl radicals (OH·).15 It has thus been
suggested that β-carbonic acid may be found on the Martian
surface, on interstellar grains, on comets, especially in the Oort
cloud, or on Jupiter’s icy satellites Europa, Ganymede, and
Callisto.4,16−19
Received: February 27, 2013
© XXXX American Chemical Society
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polymorph of H2CO3.24 The preparation of the starting material (β-
H2CO3) was done in Innsbruck as described in refs 5, 10, and 26 by
layer-by-layer spray deposition of glassy aqueous solutions of acid (2
M HBr) and base (1 M KHCO3) on optical windows kept at 80 K.
Subsequently, this acid−base “sandwich” is heated slowly to 180 K in
order to trigger translational diffusion and acid−base chemistry.
Finally, ice is removed in the high-vacuum chamber by heating to 210
K, which results in the formation of first an amorphous thin film of
carbonic acid, which finally crystallizes to β-H2CO3. The preparation
process of the thin film of carbonic acid is monitored in situ by FTIR
spectra recorded on the Varian Excalibur spectrometer in Innsbruck.
Figure 2a shows the typical FTIR spectrum (recorded at 80 K) of a
Figure 1. Possible basic building blocks in the solid state for (a) β-
H2CO3, which is studied in this work, and (b) α-H2CO3, which was
studied in our previous work.24 The local symmetries of these
polymorphs were inferred by testing the validity of the mutual
exclusion principle from Raman and IR spectroscopic data.19 The
monomers are arbitrarily depicted in the cis−cis conformation (C2v).
In the crystal field of each of the two polymorphs also the cis−trans
conformation (Cs) could be the more stable one. Counterintuitively,
the centrosymmetric dimer can only be detected in the gas phase above
α-H2CO3, but not in the gas phase above β-H2CO3. We attribute this
to the lower temperature of the gas-phase above α-H2CO3 (210 K vs
250 K), which favors dimerization in the gas phase.
Thrillingly, the vapor phase above α-H2CO3 can be
recondensed as α-H2CO3 on cold substrates at a different
location,17 which demonstrates that carbonic acid sublimes at
least partly without decomposition to carbon dioxide and water.
Previously, we succeeded in isolating the vapor phase above α-
H2CO3 in a range of noble gas matrices and analyzed these
matrices by infrared spectroscopy.24 We have interpreted these
infrared spectra in terms of the presence of the C2v and Cs
monomers at a ratio of 10:1, a small fraction of centrosym-
metric (C2h) carbonic acid dimers, and some carbon dioxide
and water mono- and oligomers. After removal of the matrix,
the isolated gas-phase molecules rebuild a hydrogen-bonded
network and condense to a crystalline polymorph. Interestingly,
it is again the α-polymorph that is observed after sublimation of
the α-polymorph, matrix isolation, and removal of the matrix.
In the present paper, we show the isolation of the β-
polymorph in a solid rare gas matrix. The vapor phase above
the β-polymorph is harder to isolate because the vapor pressure
and sublimation rate of this polymorph are even lower than for
the α-polymorph, so higher sublimation temperatures are
required.17 Calculation of the vapor pressure for α-H2CO3 at
200 K by Hage et al. shows the saturation vapor pressure (ps) of
4 × 10−7 mbar for the H2CO3 monomer and 3 × 10−7 mbar for
the dimer in the gas phase.17 Peeters et al. measure the vapor
pressure of β-H2CO3 of about (0.29−2.33) × 10−9 mbar at
240−255 K.6
Figure 2. (a) IR spectrum of a thin film of crystalline β-H2CO3 at 80
K, as prepared by protonation of KHCO3 with HBr in aqueous
solution and removal of the solvent in vacuum at 230 K. (b) IR
spectrum of crystalline β-H2CO3 that re-forms after removal of the
argon matrix containing trapped carbonic acid molecules. Only a small
fraction of carbonic acid re-forms, so the intensity of this spectrum has
to be multiplied by 800 to obtain comparable intensities. Spectra are
shifted vertically for clarity.
thin film of crystalline β-H2CO3 prepared using this procedure on a Si
window. The vertical lines indicate the position of the absorption
bands of crystalline β-H2CO3.26 These thin films on the optical
windows were then stored in liquid nitrogen and transferred to the
matrix isolation chamber in Vienna. In this chamber the ice that has
condensed during the transfer of the window at 77 K from ambient air
was first removed and then the thin film was sublimated. The vapor
above the thin film was then mixed with noble gas (argon or krypton)
and recondensed on a cold mirror as a solid mixture of sublimation
products and noble gas, typically at a ratio of 1:1000. The details of the
matrix isolation procedure can be found in ref 24. FTIR spectra of the
matrix were recorded by a Vertex 80v (Bruker Optic GmbH,
Karlsruhe, Germany) equipped with a liquid N2 cooled narrow band
MCT detector applying a spectral resolution of 0.5 cm−1, adding 512
scans, and using a Ge-coated KBr beamsplitter. The optical path of the
spectrometer was evacuated down to 2 mbar, which minimizes
interferences from CO2 and H2O absorptions of the ambient
atmosphere. Such a sensitive setup is particularly important when
measuring a substance like carbonic acid, which is a very weak absorber
in especially these spectral regions.
EXPERIMENTAL SECTION
■
Matrix isolation spectroscopy is a technique aimed at obtaining pure
vibrational spectra at low temperatures that isolates nonrotating single
molecules by trapping them in a solid matrix of, for example, neon,
argon, or krypton, which are optically transparent in the mid infrared
and are chemically inert. Our matrix isolation study was done in the
ultrahigh-vacuum chamber in Vienna (see Supp.-Figure 1 in ref 24),
which was previously employed for successfully isolating reactive
species such as halogen oxides25 or the sublimation product of the α-
Ab initio quantum mechanical calculations of carbonic acid in
various isotope configurations were performed to obtain reference
frequencies for annotating measured spectra. All calculations were
performed using the Gaussian 09 package.27 Both the C2v and Cs
conformations were employed for geometry optimization, where the
former is the energetically most favorable conformation.28,29
Calculations were performed using second-order Møller−Plesset
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Figure 3. Selected regions of the IR spectra of carbonic acid vapor and isotopologues after sublimation of crystalline β-H2CO3 at 230−260 K and
isolation in solid argon at 6 K: (a) H213CO3, (b) H2CO3, (c) D2CO3. Spectra are shifted for clarity. Bands marked by ★ arise from the ν2 bend of the
HDO monomer and dimer, by # from combination bands of 13CO2, and by o from combination band of CO2.
a
Table 1. Band Positions Assigned to Carbonic Acid Monomers and Isotope Shifts (both in cm−1)
H212CO3
12C/13C shift
H/D shift
Ar
Kr
theor
Ar
theor
Ar
theor
norm. mode
assign.
νs(OH)
molec sym
3617/3614
3611/3607
1833/1829
1792/1788
1446/1438
1392/1385
1255
3805
3801
1880
1834
1467
1409
1289
1270
1166
802
ν(A′)
ν(B2)
ν(A′)
ν(A1)
ν(B2)
ν(A′)
ν(A1)
ν(A′)
ν(B2)
ν(B1)
ν(A″)
Cs
1/−3
42/41
42
0
48
46
34
27
950/949
9/11
1037
13
νas(OH)
C2v
Cs
1828
1787
1443
1390
1254
1226
1134
791
ν(CO)
ν(CO)
νas(C(OH)2)
νas(C(OH)2)
δip(COH)
δip(COH)
δip(COH)
δoop(CO3)
δoop(CO3)
5/7
12
C2v
C2v
Cs
31/30
25/23
72/73
68
189
212
C2v
Cs
1228
1136
6
6
25
23
189
1
204
1
C2v
C2v
Cs
792
24
24
782
781
790
a
Data taken from Figure 2 and Figure S2 (Supporting Information). Values in columns labeled “theor” are calculated at the MP2/aug-cc-pVTZ level
of theory. Normal modes are assigned on the basis of these calculations. Two distinct monomer geometries, namely, in the cis−cis (C2v point group
symmetry) and the cis−trans conformation (Cs point group symmetry), are necessary to explain the spectra.
perturbation theory (MP2) with the augmented correlation consistent
basis sets of Dunning and co-workers.30−32 Initial optimization was
done at the MP2/aug-cc-pVDZ level of theory requiring “very tight”
convergence on displacement and forces. Starting from the resulting
geometry, further optimization at the MP2/aug-cc-pVTZ level of
theory using very tight convergence criteria yielded an energy
minimum for frequency calculations. Subsequently, IR modes were
determined at this minimum geometry using MP2/aug-cc-pVTZ.
Isotope shifts were calculated by performing frequency calculations at
water monomer, also higher water oligomers are found in the
matrix. The bands near 3750 cm−1 correlate with the rotation
and nonrotation mode of H2O monomer and dimer33,34 and
the bands near 1600 cm−1 with the bending mode of H2O
monomer and dimer.33,34 The most intensive bands near 2300
cm−1 and the band around 680 cm−1 appertain to CO2 (with
12C or 13C) molecules.35
During the transfer of the Si window from the liquid N2 to
the high vacuum chamber some air moisture condenses on the
window, which we tried to remove before the matrix isolation
by pumping it off in the vacuum at 210−220 K. During the
matrix production, some ambient water can also be isolated. A
differentiation between water molecules stemming from
moisture or from the decomposition of H2CO3 is shown in
the spectrum of matrix isolated D2CO3 (Figure S1c, Supporting
Information). The band system at 2783, 2771, 2746, 2724, and
2678 cm−1 belongs to the absorption bands of the D2O
monomer and polymer.33,36 Unambiguously, this band can only
result from the decomposition of D2CO3. In the spectrum of
H213CO3 (Figure S1a, Supporting Information), the decom-
position is clearly evident in the strong 13CO2 bands at 2280,
2275, and 2274 cm−1.35 These bands are unequivocally assigned
2
the same minimum for either all H- or 13C-labeled carbonic acid.
Calculated frequencies and isotope shifts were then used to identify
the various signals observed in the experimentally obtained spectra.
RESULTS
■
For β-H2CO3, the same procedure of sublimation and trapping
in solid matrices (Ar or Kr) was applied as for α-H2CO3, with
the exception that higher sublimation temperatures are
required. The β-polymorph (spectrum shown in Figure 2a) is
stable up to at least 230 K. Above this temperature it sublimes
slowly in vacuum. So far it was believed to decompose under
such conditions. Indeed, we do observe the decomposition
products carbon dioxide and water in the spectra (see Figure
S1, Supporting Information). Besides carbon dioxide and the
C
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because the band positions typically agree with literature data to
within 0.5 cm−1.
other spectral ranges shown in Figure 3 and is demonstrated in
the Supporting Information.
After having assigned these bands, all the bands observed in
the whole spectral range are explained. Other possible species,
such as complexes of water with carbonic acid or carbonic acid
dimers, trimers, or higher oligomers, are not present or are at
most trace components producing bands near the noise level of
the spectrum.
In order to hedge our assignments, we have evaporated the
solid noble gas matrices at the end of the spectroscopic
characterization by carefully heating the matrix and checking for
the component remaining on the sample holder. The spectrum
after sublimation of β-H2CO3, matrix isolation of gas-phase
carbonic acid in argon at 6 K, and removal of the argon matrix
by heating to 220 K is shown in Figure 2b. This spectrum is
highly similar to the spectrum of β-H2CO3 before sublimation,
albeit with intensities that are about a factor of 1000 lower.
That is, the isolated carbonic acid monomers start to hydrogen
bond upon removal of the argon and finally produce crystalline
β-H2CO3. For comparison, after isolation of the gas-phase
above α-H2CO3 in argon and removal of argon, α-H2CO3 is
finally observed.24
However, additional bands that cannot be assigned to carbon
dioxide or water are apparent in the spectrum, which we assign
to carbonic acid as outlined below. The intensity of the ν(C
O) mode in carbonic acid amounts to about 10% of the
intensity of the most intense ν(O−H) mode in the water
monomer and to about 5% of asymmetric stretching mode ν3 in
carbon dioxide. Thus, above 230 K a part of β-H2CO3 sublimes
with decomposition, whereas another part does not decom-
pose.
Figure 3 shows the spectral regions that cannot be explained
using CO2 or H2O mono- or oligomers, which we assign to
H2CO3 and its isotopologues after sublimation of crystalline β-
H2CO3 at 230−260 K and isolation in solid argon at 6 K. We
assign all these bands to two conformers of the carbonic acid
monomer (symmetries C2v and Cs; see Scheme 1 in ref 24) on
the basis of selective changes of the experimental conditions:
(a) 12C/13C and H/D isotope shifts, (b) UV radiation of the
matrix, and (c) change of the matrix material (Ar or Kr). Also
theoretical prediction concerning band positions and shifts
between the symmetry of the isotopologues support our
interpretation.
Our band assignment is exemplarily explained here on the
CO-stretching region at 1850−1700 cm−1, which contains two
doublets (see Figure 3b). They appear as doublets because of a
splitting induced by different Ar matrix cages. By contrast, in Kr
matrix two single bands appear at a similar position (see Figure
S2, Supporting Information). This immediately suggests the
presence of two distinct gas-phase carbonic acid species. These
doublets are red-shifted by 5−11 cm−1 for D2CO3/Ar (Figure
3c) and by 42 cm−1 for H213CO3/Ar. These isotope shifts are in
excellent agreement with the theoretical prediction of isotope
shifts for νs(CO) of the H2CO3 monomers (see Table 1).
The assignment of the two carbonic acid species is immediately
evident when looking at the calculated separation of the νs(C
O) between the cis−cis monomer of C2v symmetry and the
cis−trans monomer of Cs symmetry. This amounts to 46 cm−1
at the MP2/aug-cc-pVTZ level of theory (1880 vs 1834 cm−1;
see column labeled 'theor' in Table 1) employed here and is
almost the same also at other levels of theory.37−39 In the
matrix spectrum the two bands are separated by 41 cm−1 (1828
vs 1787 cm−1; see Table 1), which is an excellent match and
allows an unambiguous assignment of the bands.
Upon UV irradiation of the H2CO3/Ar matrix one of the two
doublets increases with time at the cost of the other doublet
(see difference spectra in Figure S3, Supporting Information).
According to the assignment this implies that the Cs isomer
grows at the expense of the C2v isomer. That is, the isomer that
is calculated to be slightly energetically disfavored is formed
from the favored one upon UV irradiation. Most likely this shift
of the equilibrium takes place by a rotation of one H-atom
around the C−O bond from the cis-position to the trans-
position. The ratio of the intensities of C2v and Cs bands before
UV irradiation varies between 5:1 (Figure 3a) and 10:1 (Figure
3c), which is in accordance with the theoretical predictions of
the higher stability of the C2v monomer. The presence of the
two weak internal hydrogen bonds in the C2v monomer
compared to the single internal hydrogen bond in the Cs
monomer results in an increased stability of about 4−8 kJ/
mol.22,28,29,38−41 This interpretation of the presence of these
two monomers at these ratios can also be deduced from all
CONCLUSIONS AND IMPLICATIONS FOR
CARBONIC ACID DETECTION
■
In the past we were successful in the isolation of the α-
polymorph of carbonic acid (α-H2CO3) in a solid matrix.24
Now we show the isolation of the β-polymorph (β-H2CO3) in a
solid noble gas matrix and present our band assignment. The β-
polymorph has a lower vapor pressure and lower sublimation
rates than the α-polymorph at the same temperature. In order
to reach a significant vapor pressure above the β-polymorph, it
is necessary to sublime the β-polymorph at 230−260 K, as
compared to 210 K for the α-polymorph. Similar to our former
experiment, we also find the C2v monomer to be the
dominating species in the gas phase. The ratio between C2v
and Cs of the β-polymorph at 230−260 K is similar to that of
the α-polymorph at 210 K24 and to the calculations of
Schwerdtfeger et al..42 Both polymorphs show the Cs monomer
as the minor species, which occurs at a ratio from about 1:5 to
1:10. In contrast to our earlier matrix isolation study, we now
do not find evidence for the presence of a centrosymmetric
dimer. We attribute this difference to the higher sublimation
temperature, which favors two monomers over the dimer
because of the entropy term. That is, the results suggest that the
enthalpy gain incurred because of hydrogen bond formation in
the dimer is sufficient for the observation of the dimer at a
sublimation temperature of 210 K,24 but not at sublimation
temperatures of 230−260 K used here. There is furthermore no
indication for the presence of a linear oligomer43 in the gas
phase. As a consequence of this difference in the composition
of the gas phase, also the polymorph that crystallizes upon
removal of the matrix is different: α-H2CO3 crystallizes from
sublimed α-H2CO3, and β-H2CO3 crystallizes from sublimed β-
H2CO3.
The finding here that not only α-H2CO3 can sublime and
recondense as α-H2CO3 but also β-H2CO3 can sublime and
recondense as β-H2CO3 is novel and of atmospheric and
astrophysical relevance, especially because it was previously
thought that β-H2CO3 decomposes entirely upon sublima-
tion.2,6,11,12,14−16
In our atmosphere some solid-state carbonic acid may be
present in cirrus clouds or on mineral dust. This possibility was
D
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conjectured 15 years ago,17 but only very recently it could be
shown that indeed β-H2CO3 may form as a bulk species on
mineral dust in the presence of acids and remain stable in the
troposphere even in the presence of high relative humidities up
to 260 K.44 Huber et al.39 have emphasized that the sublimation
temperature of α-H2CO3 of 210 K17,24 is too low for a possible
existence of gas-phase carbonic acid in Earth’s atmosphere. This
is because such low temperatures are only found in the
stratosphere, where cirrus clouds cannot be observed. However,
the sublimation temperature of β-H2CO3 of up to 260 K
reported in this work is of relevance in the troposphere, where
β-H2CO3 is presumed to exist and may sublime and recondense
without decomposition. That is, some gas-phase carbonic acid
may indeed be present in the troposphere, albeit at very low
mixing ratios: the vapor pressure of β-H2CO3 at 260 K is on the
order of 10−8−10−9 mbar, and the atmospheric pressure is
about 200−400 mbar at the relevant altitudes of 5−10 km.
These low mixing ratios will make it very challenging to detect
gas-phase carbonic acid in Earth’s troposphere.
attempt of Philae to land on the comet 67P/Churyumov−
Gerasimenko at the end of 2014 (ESA’s Rosetta mission) or the
plan of an ESA spacecraft visiting the icy Jovian moons
(“Jupiter Icy Moon Explorer”), hold promise for the detection
of carbonic acid.
ASSOCIATED CONTENT
* Supporting Information
■
S
Additional spectra and more details about band assignments.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
grothe@tuwien.ac.at; thomas.loerting@uibk.ac.at
■
Notes
The authors declare no competing financial interest.
Our findings presented here increase the chance for
detection of gas-phase carbonic acid in astrophysical environ-
ments. First, direct routes for the formation of β-H2CO3 in
astrophysical environments are known,2,3,11,12 whereas no
direct route for the formation of α-H2CO3 is known. Typically,
β-H2CO3 is considered in environments containing both H2O
and CO2 ices, which are exposed to radiation, e.g., solar
photons or cosmic rays. This is the case for the icy satellites of
Jupiter and Saturn and also for the polar caps of Mars. The
stability of gas-phase carbonic acid up to 260 K presented here
might then result in a release and accumulation of carbonic acid
in these thin atmospheres. For example, on the Mars surface it
is known that the temperatures may change between 140 and
300 K, so carbonic acid may experience sublimation and
recondensation cycles and reach a steady-state concentration
near the icy caps. However, even with next-generation
telescopes, the remote detection of carbonic acid in the thin
atmospheres of such bodies seems very challenging, as
explained by Huber et al.39 Because of the high angular
resolution required we might need to wait for the European
Extremely Large Telescope (E-ELT), which is planned to be
operative in the early 2020s.39 It might, therefore, be useful to
investigate the gas phase there in the future using micro-
wave22,23 or infrared absorption spectroscopy, e.g., by the
METIS instrument on the E-ELT. Judging from the present
and our earlier work,24 the most intense and characteristic
bands suitable for detection of the most abundant C2v carbonic
acid monomers are the bands at 3608 30 cm−1 (2.77 μm),
1780 10 cm−1 (5.62 μm), 1445 10 cm−1 (6.92 μm), and
ACKNOWLEDGMENTS
■
We are grateful to the Austrian Science Fund (FWF, projects
P18187 and P23027) and the European Research Council
(ERC Starting Grant SULIWA) for financial support. H.G.
̈
acknowledges travel support by the Osterreichische For-
̈
schungsgemeinschaft (OFG).
REFERENCES
■
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