The Structure and Spectroscopy of Cyanate and ... - ACS Publications

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The Structure and Spectroscopy of Cyanate and Bicarbonate Ions. Astrophysical Implications M. A. Moreno,† B. Maté,† Y. Rodríguez-Lazcano,† O. Gálvez,† P. C. Gómez,‡ V. J. Herrero,† and R. Escribano*,† †

Instituto de Estructura de la Materia, IEM-CSIC, Serrano 123, 28006 Madrid, Spain Departamento de Química Física I, Universidad Complutense, Unidad Asociada UCM-IEM-CSIC, 28040 Madrid, Spain

‡

ABSTRACT: Cyanate and bicarbonate are two ions that play active roles in many fields of physics and chemistry, including biological sciences and astrochemistry. We present here a comprehensive study of these species covering a range of phases and methodologies. We have performed theoretical calculations on the isolated ions and their hydrates with one to four water molecules, and in clusters with 15 water molecules. The predicted infrared spectra are compared with observed spectra from experiments where liquid droplets of their solutions are frozen at 14 K on a substrate, to mimic some astrophysical conditions. Crystals of cyanate and bicarbonate sodium and potassium salts are also studied experimental and theoretically. As well, the spontaneous decomposition of cyanate into bicarbonate is documented from the spectra of an aged solution. Finally, the possible astrophysical observation of bicarbonate in watercontaining particles is discussed.

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INTRODUCTION Cations, whether atomic or molecular, have been considered traditionally to be the main species in the ion chemistry of interstellar environments and other extra-terrestrial media, but in the terrestrial atmosphere ions are also of well-known relevance. The nature and properties of the ionosphere, mostly due to cations, and the role of both anions and cations in nucleation processes in troposphere and stratosphere are clear examples of the above said. As far as Astrochemistry is concerned, cations were the first charged species to be identified in the interstellar medium (ISM) as well as in laboratory studies. It is worth mentioning here, as a way of example, the case of CH+ identified first by Douglas and Herzberg1 and that of H3+, which is at the origin of the synthesis of many interstellar molecules, whose IR spectrum was found first by Oka.2 However, a number of anions, such as OCN−, CnH− (n = 4, 6, 8) and CnN− (n = 3, 5) have also been observed recently3−5 and constitute one of the most promising subjects in this field. Especially relevant is the case of those species containing the CN group, because of their possible occurrence in early stages of planetary formation and comets and its subsequent role in prebiotic processes. In this paper, the study of two anions, cyanate (OCN−) and bicarbonate (HCO3−), in association with a number of water molecules is envisaged by means of both theoretical and experimental methods. Both anions are known to have important implications in many areas of Chemistry and Astrochemistry, particularly in relation with their association with water in a variety of forms. In the astrophysical context there is direct reference to OCN− in the interstellar solid phase3,4 since it is thought to be responsible of the IR 2165 cm−1 absorption band observed © 2013 American Chemical Society

toward certain young stellar objects, matching laboratory measurements. Observed OCN− is believed to be formed from interstellar ices containing H2O, CO, and NH3 plus surrounding UV radiation or by thermal processing involving solid isocyanate HNCO plus NH3.6−8 However, whereas gas phase isocyanic has been identified in ISM,9,10 there is no direct observation of its solid phase on interstellar ices, and some aspects of the formation mechanisms of this anion seem to be still under study. On the other hand, OCN− can also be viewed as the product of dissociation of the two isomeric acids, cyanic and isocyanic, thus being a link between them. Finally, it must be taken into account that this anion and its interactions with water are also relevant in a wide variety of scenarios far from Astrochemistry, including adsorption on boron nanotubes,11 being a marker in biological molecules,12 the degradation process of cyanide to the less harmful cyanate carried out on mining waste waters,13 photodecomposition of urea,14 the hydrolysis of cyanate to ammonia and bicarbonate (a process that is also studied in this work; see below) by the enzyme cyanase,15 and others. Bicarbonate, on the other hand, is one of the intermediate key species in the formation of H2CO3. Gerakines et al.16 studied this process from 1:1 mixtures of H2O and CO2 exposed to UV radiation and/or proton bombardment Special Issue: Oka Festschrift: Celebrating 45 Years of Astrochemistry Received: December 13, 2012 Revised: January 29, 2013 Published: January 29, 2013 9564

dx.doi.org/10.1021/jp3122616 | J. Phys. Chem. A 2013, 117, 9564−9573

The Journal of Physical Chemistry A

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For this reason we have selected a correlation-consistent Dunning basis, aug-cc-pvTZ, for all calculations involving negative ionic species, i.e., the cyanate and bicarbonate ions, and their hydrates with up to four water molecules. For the calculation of frequencies, the density functional theory (DFT) method usually gives reliable values. We have chosen DFT with B3LYP functionals. These calculations have been performed using the Gaussian09 suite.27 Periodic, neutral systems have been calculated using the CASTEP package,28 also with DFT, and with gradient-corrected functionals and RPBE parametrization, with a plane wave cutoff of 830 eV. These calculations include, on one hand, those of a single molecular species, NaOCN or NaHCO3, surrounded by an amorphous water cluster of 15 H2O molecules, with densities of 0.929 and 1.137 g/cm3, respectively, and on the other hand, the solid crystals of the cyanate and bicarbonate anions with Na+ and K+ cations. Initial structures for these crystals were taken from crystallographic references.29−32 Infrared spectra were recorded with a Bruker Vertex 70 FTIR spectrometer on a normal transmission configuration using a liquid nitrogen refrigerated MCT detector. The standard FTIR setup was used to record spectra of KBr pellets of ground crystals of NaOCN, KOCN, NaHCO3 and KHCO3, and water solutions of these salts. The spectrometer can be linked to a low-temperature, high-vacuum chamber in a configuration that has been described in detail before.33,34 We have used the socalled hyperquenching (HQ) technique, a sudden freezing of droplets of diluted solutions of the salts impinging on a cold substrate placed inside the vacuum chamber. The chamber background pressure is ∼10−8 mbar, and the substrate temperature was 14 K. The ices formed in this way retain the ionic structure from the liquid state. In a subsequent step, the substrate was annealed to 240 K to evaporate the water content. Spectra were recorded at all stages of this process.

simulating different astrophysical environments ranging from interstellar icy grains to cometary ices. The biological role of bicarbonate has been well-known since long ago and continues to be a topic of relevance, mostly in relation to human health,17,18 due to its buffering capacity operating in various contexts. Also bicarbonate in water environment is the key species involved in many natural process related to solution of CO2 that have implications in atmospheric sciences, marine response to the amount of CO2 in the atmosphere, and geological karstic cycles among others. Special mention is deserved by some of the very interesting mechanisms of sequestration of CO2 with NH3 that have been proposed so far19 and by mineralization of calcium carbonate from the reaction with Ca-silicate minerals.20 With respect to carbonic acid, it is interesting to note that, in spite of the wellknown difficulties in assessing the stability of the acid as a separate molecule, its existence has been proved both experimentally and theoretically,21 and its capability to form very stable dimers contributes to prevent decomposition into CO2 plus H2O.22 Finally, the interest of a combined study of the two anions, cyanate and bicarbonate, is enhanced by their interrelation, bicarbonate being a subproduct in the decomposition of cyanate, as already mentioned above for biological species. In spite of so many implications of both anions, there is not an abundance of theoretical studies, but special mention must be made of the interesting study of IR spectroscopy and quantum chemistry of bicarbonate−water clusters,23 and those of refs.24−26 Thus, whereas previous works on cyanate and bicarbonate deal with specific aspects of these ions in some of the contexts introduced above, we present here a theoretical and experimental comprehensive study of these anions focusing on three main fields: (1) Isolated ions and hydrates with one to four water molecules; these species are very interesting, as they adopt several structures with different properties, and constitute an initial step to dilution; also, ionic ices formed by sudden freezing of droplets of OCN− and HCO3− solutions, which can be taken as models for astrophysical systems. (2) Solid state crystals and water solutions of neutral salts with Na+ and K+ cations; the crystals present quite different structures depending on the cation, which are reflected in their IR spectra and in their calculated properties. (3) Cyanate to bicarbonate conversion reaction; this process takes place slowly at room temperature, and can be followed by IR spectroscopy. A comparison of the corresponding spectra can be valuable to guide a possible search for bicarbonate in astrophysical media. Our experiments and calculations span a variety of techniques described in the following section. In general terms, we make use of our theoretical results to help interpret the observations. The Results section is divided into three subsections for the different fields mentioned above. In a final section, we summarize the main conclusions and present the astrophysical implications of this work.

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RESULTS 1. Hydrates and Solution. a. OCN−. Our results for the cyanate anion and its hydrates are summarized in Table 1 and Figures 1 and 2. For simplicity, Table 1 reports the energies as relative values with respect to the monomers: ΔE = E(OCN−(H 2O)n ) − E0(OCN−) − n × E(H 2O) (1)

using as references the values of the isolated ion and the water molecule, E0(OCN−) = −168.1998503 hartree and E(H2O) = −76.4663923 hartree, respectively (1 hartree = 2626.5 kJ/mol = 627.503 kcal/mol).35 The values listed in the table describe an increase in stability as the number of water molecules grows by an average amount of ∼12 kcal/mol per water molecule addition. Cyanate is special because it has a possible proton acceptor center at each end of the molecule, and in fact we have found stable structures when water molecules are attached on either side. This effect is clearly visible in the hydrates with one and three water molecules. In both cases, the species where water linking to the N-end is prevalent are more favorable energetically by 2.6 kcal/mol for the monohydrate and by 1.6 kcal/mol for the trihydrate. If a thermally averaged mixture of hydrates were produced, either in the lab, e.g., in a supersonic expansion, or in the ISM, the N-end species would be expected to be present in a slightly larger abundance. Additionally, a lengthening of the C−N bond and a shortening of the O−C bond is predicted for these N-end favored hydrates, with the reverse effect for the O-end favored species. We have found one

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METHODOLOGY From the theoretical point of view, anions always pose a challenge for ab initio calculations, since the extra charge increases correlation and requires heavily polarized basis sets. 9565


Calculated at the B3LYP/aug-cc-pvTZ level of theory for n = 1,4 (Gaussian); GGA PBE for n = 15 (CASTEP). Distance in Å and energies in kcal/mol, except where indicated. E0(OCN−) = −168.1998503 hartree, E(H2O) = −76.4663923 hartree, taken as reference. bThis energy value, in hartree, lies between the absolute values of the 2 and 3 water molecules (−321.18 and −397.66 hartree, respectively).

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Figure 1. Schematic view of cyanate hydrates with 1, 2, 3, 4, and 15 water molecules. Hydrates with one and three water molecules present two structures each, depending on which end of the cyanate ion is favored for water molecules attachment. The four-water hydrate also presents two structures.

stable structure for the two-water hydrate, and two different ones for the four-water one, although we cannot discard the existence of other stable species. The ring structure of the fourwater hydrate is slightly more stable (1.8 kcal/mol) than its counterpart, which has a kind of scissors or crab-like geometry, and where two water molecules are attached to the O. When we jump to the cluster where the ion is surrounded by 15 water molecules in a periodic cell, we find that the O−C and C−N bonds tend to converge to an intermediate value between those of the individual bonds in the smaller hydrates, yielding a structure where the excess negative charge is distributed evenly on either side of the C atom. Table 1 reports the calculated energy for this species, whose value lies between those of the two- and three-water clusters. These results can be interpreted as follows. The bonding between cyanate and water in the small clusters is fairly strong, yielding stable molecular species, whereas in the large cluster the ion is surrounded by an amorphous cloud of water molecules without creating strong bonds with any one of them, and interacting only with the nearest neighbors. Figure 2 presents a comparison of calculated spectra of these hydrates, in the left column, and recorded spectra at two stages of our HQ experiment,34 on the right. These correspond to the deposit at 14 K after the HQ experiment (lower panel) and the remains after warming to 240 K to eliminate water from the samples (upper panel). In the HQ deposit spectrum, two strong, broad bands dominate, corresponding to the C−N (∼ 2200 cm−1) stretch (ν3) on top of a water combination band, and the water bending mode (∼1600 cm−1), respectively, with a shoulder on the right-hand side of the latter assigned to the O−C stretch (ν1), at ∼1250 cm−1. The large width of these bands has been associated to the very inhomogeneous water environment that these ions present in the deposit. This effect is well matched on the predicted spectrum, which shows how

a

1.205 1.215 −330.04b 1.179 1.222 −49.0 1.188 1.210 −50.8 1.184 1.221 0.0 C−N O−C ΔE

1.185 1.218 −15.9

1.181 1.233 −13.3

1.187 1.218 −27.5

1.183 1.217 −39.5

1.176 1.230 −37.9

OCN−···(H2O)4 open chain H2O···OCN−···(HOH)2 OCN−···(H2O)2 HOH···OCN− OCN−···HOH OCN− parameter

Table 1. Cyanate Ion and Cyanate Hydrates OCN−(H2O)n Main Structural Parametersa

(HOH)2···OCN−···H2O

OCN−···(H2O)4 ring

NaOCN···(H2O)15


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Figure 2. Infrared cyanate spectra. (a) Predicted isolated OCN− ion, (b) observed KOCN at 240 K, (c) predicted OCN− hydrates (offset for clarity), and (d) observed HQ deposit of KOCN at 14 K.

Table 2. Bicarbonate Ion, Dimer, and Bicarbonate Hydrates HCO3−(H2O)n Main Structural Parameters, for Calculations at the B3LYP/aug-cc-pvTZ Level of Theory, n = 1−4 (Gaussian), and GGA PBE for n = 15 (Castep)a parameter

HCO3−

(HCO3−)2

HCO3−···H2O

HCO3−···(H2O)2

HCO3−···(H2O)3

HCO3−···(H2O)4

NaHCO3···(H2O)15

C−O C−O (acid) O−H COH ΔE

1.250, 1.234 1.448 0.963 102.4 0

1.264,1.250 1.390 0.990 109.5 −45.9b

1.260,1.237 1.423 0.964 103.2 −16.5

1.249,1.244 1.412 0.964 103.5 −30.7

1.263,1.236 1.402 0.963 104.1 −43.5

1.252,1.246 1.399 0.964 104.2 −55.4

1.261, 1.264 1.376 1.011 109.43 −352.439c

a Distances in Å and energies in kcal/mol, except where indicated. E0(HCO3−) = −264.5733683 hartree, E(H2O) = −76.4663923 hartree, taken as references. bΔE(dimer) = Edimer − 2 × E0. cThis energy value, in Hartree, lies between the absolute values of the 1 and 2 water molecules (−341.07 and −417.56 hartree, respectively)

the ν3 band shifts its position for the different hydrates, with the species with predominant water attachment to O peaking always at higher wavenumber than the N-attachment counterparts. This broadening effect is less apparent but still appreciable for the water bending and the O−C stretching modes. After water has been evaporated, the cyanate modes stand out in the observed spectrum (top right), where the strong Fermi resonance between ν1 and 2ν2, at ∼1200 and 1300 cm−1, respectively, is a characteristic feature that also appears in the first overtone of these vibrations, in the 2400− 2500 cm−1 region. On the other hand, our predicted harmonic spectrum, top left, does not include these anharmonic modes, and the large strength of ν3 makes ν2 barely visible at ∼600 cm−1. The nearest theoretical investigation on these hydrates that we are aware of is that by Park and Woon,25 focused on chargetransfer effects in complexes of OCN− with NH3 and H2O. They also studied these effects on the HNCO and HOCN precursors. Whereas the ν3 mode of OCN− is not altered in their calculations for the neutral species, they do predict a shift of ∼20 cm−1 when a water molecule is attached to either end, in the same sense as our calculations, i.e., with a higher

wavenumber for the N-attachment species. They report two values for the anion complexes with 15 water molecules, with a small redshift for the O−C mode. In our case, for the 15 water amorphous structure, we predict a larger displacement, of ∼130 cm−1, probably a consequence of an overweakening of this bond in favor of the O−C bond, as mentioned above. b. HCO3−. Our results for the bicarbonate anion and its hydrates are summarized in Table 2 and Figures 3 and 4. Several structures with stable minima were found for most of these hydrides; for simplicity, we collect those of the lowest energy for each set of clusters only. In all these cases, the water molecules are linked to the CO2-end of the bicarbonate, creating hydrogen bonds where water acts always as donor, with a trend to form ring-like structures. As could be expected, in clusters with odd number of water molecules, the C−O bond bearing larger water attachment is longer than the other C−O bond of the CO2 moiety. The C−O of the acid part is considerably longer in all cases, but shortens progressively with increasing number of water molecules. Bicarbonate is a very weak acid that does not easily release a proton, as shown by an O−H distance close to that in neutral water. Only in the 15H2O limit does the structure tend toward a bianionic form, 9567



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CO2 group. We have included in Table 2 the calculated values for the bicarbonate dimer, a very stable species formed spontaneously in diverse media, including the K+ salt as described below, the monomers being interlinked by two hydrogen bonds (see Figure 3). Relative energies calculated as in eq 1 are also listed. There is a stable progression along the series with decreasing energy interval between one species and the next one with one more water. The 15-H2O species yields a calculated energy close to the absolute value of the single H2O species. This gives an estimation of the very weak bonding between the ion and the amorphous surrounding in the large cluster. The observed (right) and predicted (left) spectra of these species are displayed in Figure 4. The main bicarbonate vibrations are the C−O asymmetric stretching, and a mixture of C−O symmetric stretching and COH bending, appearing as a strong peak at ∼1660 cm−1 and a plateau at ∼1300 cm−1 in the spectrum of the HQ deposit (lower panel, right). Note that the broad feature at ∼2200 cm−1 is due to a water combination band, and a good part of the feature peaking at 1660 cm−1 corresponds to the water bending mode. The predicted spectra for the hydrates present a disparity of wavenumber peaks for the different species, responsible for the large width of the corresponding observed bands. The upper panels present spectra of water-free bicarbonate; in the experimental spectrum of the annealed sample stands out the large width of the strongest bicarbonate bands, much wider than those of cyanate in the similar experiment, displayed in Figure 2. Additionally, there appear some features above 2500 cm−1, which are not

Figure 3. Schematic view of bicarbonate ion, dimer, and hydrates with 1, 2, 3, 4, and 15 H2O molecules.

OCO22−···H+, characterized by the shortest acid C−O, the longest O−H and approximately equivalent C−O bonds in the

Figure 4. Infrared bicarbonate spectra. (a) Predicted isolated HCO3− ion, (b) observed NaHCO3 at 240 K, (c) predicted HCO3− hydrates (offset for clarity), and (d) observed HQ deposit of NaHCO3 at 14 K. 9568



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found in the predicted spectrum, where the O−H vibration is calculated at 3800 cm−1. These two characteristics, the width of some lines and the broad features above 2500 cm−1, are found again in the spectra of the KBr pellets of the Na+ salt, discussed below. The theoretical calculations for the crystal do yield spectral bands in the 2500 cm−1 region (see Figure 6), attributed to O−H vibrations with marked H-bonding character. Consequently, we interpret these observed spectral characteristics as arising from inhomogeneous bicarbonate microcrystals that are grown in the annealing process.36 Our results are in good agreement with those of Garand et al.23 for the hydrates with up to four water molecules, calculated using a different theoretical methodology, MP2/6-311+G(d,p) in their case. The concord with their experimental measurements is particularly pleasing, as our calculations predict a similar splitting of the ∼1700 cm−1 band for the hydrates with three and four water molecules. 2. Crystalline Solids. We collect in Table 3 and Figures 5 and 6 our main results for the crystalline solids of the cyanate Figure 5. Schematic view of unit cells of cyanate and bicarbonate Na+ and K+ salts. Crystals systems are indicated in italic after the salt’s name (see Table 3).

Table 3. Cyanate and Bicarbonate Na+ and K+ Salts Main Structural Parameters, Calculated at GGA PBE with CASTEPa NaOCNb parameter unit cell a = b, c ß C−N O−C

(exp.)

(exp.) unit cell a,b,c ß C−O C−O (acid) O−H COH a

(calc.)

R3m 3.5679, 15.123 120 1.192 1.186 1.227 1.212 NaHCO3d (calc.)

P21/c 3.51, 9.71, 8.05 111.51 1.264, 1.263 1.275, 1.261 1.346 1.353 1.052 109.29

fairly similar, and are well reproduced by the calculations, with the exception of the missing Fermi doublet in the theoretical harmonic predictions. On the other hand, the spectrum of NaHCO3 holds two peaks not present in that of KHCO3, at 1925 and 1290 cm−1, plus a better resolved structure of the 1450 cm−1 region. The band at 1925 cm−1 is probably an overtone of the strong 995 cm−1 mode, not due to appear in the harmonic calculations, whereas that at 1292 cm−1 should correspond to a strong feature at 1240 cm−1 in the predicted spectrum, assigned to a mixture of asymmetric C−O stretch plus COH in-plane bending, a motion that does not occur in the bicarbonate dimers of KHCO3. This band is indicated by gray arrows in the corresponding panels. The O−H stretching modes when H-bonding is involved are known to appear as broad spectral features with large red-shifts from the free O−H modes. Those modes are identified as the large bands in the 2500 cm−1 region of the observed spectra. In the calculation these vibrations are well predicted in frequency for both cations, with strong intensity, which would resolve in broad spectral features if information on band widths were available. We are not aware of previous calculations of the spectra of these crystals. Experimentally, the spectroscopy of these salts has more often been quoted using nujol as support,37 yielding spectra similar to those reported here. 3. Spontaneous Cyanate-to-Bicarbonate Conversion. A spontaneous process of bicarbonate formation in cyanate solutions has been reported before, but different works yield diverse results.38−40 The present investigation, centered of both ions, afforded us a good opportunity to document this process. We prepared a fresh cyanate solution of 3:100 KOCN/H2O number of molecules concentration. A sample of this solution was used for the experimental procedure described above, i.e., HQ expansion and deposit at 14 K, followed by warming to 240 K. Spectra were taken at all steps, and are shown on the left column of Figure 7. A similar process was carried out for a 1:100 NaHCO3/H2O solution, the corresponding spectra of which was collected on the right column of the same figure for reference. The cyanate solution was kept at room temperature in a sealed flask for 3 days. After that time, the flask was opened,and a sample was extracted to be subjected to the same

KOCNc (exp.)

(calc.)

I4/mcm 6.0909, 7.052 90 1.199 1.196 1.199 1.224 KHCO3e (exp.)

(calc.)

P21/a 15.173, 5.628, 3.711 104.631 1.274, 1.240 1.275, 1.256 1.337 1.362 1.09 1.023 111.5 109.96

Distances in Å and angles in degrees. bFrom ref 29. cFrom ref 30. From ref 31. eFrom ref 32.

d

and bicarbonate ions, with both Na+ and K+, the most frequently found cations for them. It is interesting to see how different the configuration of the salts is depending on the cation, although the internal structure of the anions is fairly similar in both their Na+ or K+ crystals. Perhaps the most outstanding difference is for bicarbonate: in the Na+ salt, the anions are linked to each other forming chains where each HCO3− acts as H donor from one neighbor and acceptor to the next, whereas in the K+ salt, the anions form dimers. The respective structures are shown in the bottom graphs of Figure 5. In general, the calculations carried out using the CASTEP program yield good agreement with the observed X-ray data. The largest discrepancy is for the O−H bond in KHCO3, but the observed value is reported with a fairly large uncertainty. Crystals of the salts were ground with KBr to record their spectra. It was, however, difficult to obtain homogeneous pellets and the observed spectra are afflicted with this problem, which is especially noticeable in the C−N stretch band of cyanate at ∼2200 cm−1. The spectra of both OCN− salts are 9569



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Figure 6. Predicted spectra of Na+ and K+ cyanate and bicarbonate solid crystals, and experimental spectra of KBr pellets of the corresponding salts. Absorbances truncated in calculated spectra for better visualization of weaker bands: NaOCN, max(absorb.) = 959; KOCN, max = 4593; NaHCO3, max = 10459; KHCO3, max = 11824.

dashed lines, and bicarbonate, dotted lines, in the 1000−1800 cm−1 range. The best witnesses for cyanate are the Fermi doublet at 1200−1300 cm−1, and for bicarbonate, the bands at 1360 and 1660 cm−1. The latter is obscured by the water bending mode in the spectra of the solutions and of the HQ

experimental procedure. The resulting spectra are collected in the central column. A smell of NH3 was noticed at the opening of the flask. To help in the visualization of the process, we have drawn vertical broken lines marking the specific vibrations of cyanate, 9570



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Figure 7. Comparison of spectra of freshly prepared samples of OCN− and HCO3− solutions with those of the cyanate solution after being allowed to evolve spontaneously at room temperature for 3 days.

transformation from cyanate to bicarbonate is evident. The reaction must have been faster for the more concentrated solution of Brooker and Wen, as they had only traces of cyanate left after 73 h, whereas in our case, cyanate is still a predominant component of the aged solution.

deposits, but not on those of the annealed samples. By comparing the relative intensity of cyanate, bicarbonate, and water modes in the fresh and aged samples, a rough estimate of 50 ± 10% of the original cyanate concentration appears to be transformed into bicarbonate. The chemical reactions taking place have been assumed to be38 KOCN + 3H 2O → KHCO3 + NH4OH

(1)

2KOCN + 4H 2O → K 2CO3 + (NH4)2 CO3

(2)

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CONCLUSIONS AND ASTROPHYSICAL IMPLICATIONS This paper presents an overview of results on the cyanate and bicarbonate ions in a variety of forms and phases, using IR spectroscopy and theoretical calculations of different types as research tools. The main conclusions are summarized below. The cyanate ion, OCN−, offers two active ends for water acceptance, one at either side of its linear structure, which results in several stable hydrate species. The isomers where water attachment to the N-end predominates are always more stable energetically. In the low temperature deposits formed by HQ experiments, probably many of these hydrates exist, which explains the broad IR bands found in their spectra, according to the theoretical predictions. When the ion is embedded in amorphous water, as in our model with 15 water molecules, the O−C and C−N bonds tend to acquire an equal length, and the interaction with surrounding water molecules is equivalent to that released when the ion is linked to between two and three H2O molecules. In crystals, the anion keeps the same structure as in its isolated form. Bicarbonate, HCO3−, also presents several isomers with various water units, the structures where water is linked to the O atoms of the CO2-end being the most stable. The OH-end of the ion forms a weak acid, with a bond distance almost similar to that in water. Only in the limit of inclusion of the ion in amorphous water does this O−H distance increase, while the corresponding acid C−O shortens, showing the trend toward ionization as CO32−, where all three C−O bonds would become homogeneous. Again, the predicted spectra of the hydrates show a range of wavenumber values for the main bicarbonate modes, consistent with the broad bands observed in HQ experiments. Crystals with Na+ and K+ have two different

3OCN− + 3H 2O → H 2NCO2− + (NH 2)2 CO + CO32 − (3) 38

Brooker and Wen made a very detailed study of the hydrolysis of cyanate. Starting from a 2.0 M KOCN solution, they concluded that urea, carbamate, and carbonate are the products of this process, which takes place very slowly at room temperature, being accelerated by the presence of ammonium, whereas, in an open sample, the escape of ammonia would decrease the formation of urea and carbamate. Their results are supported by evidence of Raman spectra taken at different times after the fresh solution was prepared. In our case, bicarbonate is a clear reaction product as shown by our IR spectra (compare especially the aged solution plots at 14 and 240 K, in the central column, with the corresponding ones of bicarbonate, on the right). Urea should present a strong IR peak at 1150 cm−1,37,41 in a fairly uncongested area in our spectra, plus other peaks in the 1400 and 1600 cm−1 more crowded regions. The absence of the former leads us to discard the presence of urea. Similar train of thought drives to the elimination of carbamate, which ought to produce a medium intensity peak at ∼1150 cm−1, with other stronger bands at ∼1400 cm−1.42 This case is less conclusive, as the expected intensity of the 1150 cm−1 is smaller than that of urea. Finally, the shoulder at 1437 cm−1 (pointed by an arrow on the middle column, bottom graph) can be assigned to carbonate. We therefore conclude that in our time span, no urea was formed and only traces of carbonate may be found, whereas a strong 9571



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structures. In the potassium salt, bicarbonate ions forms dimers, bound by two H bonds from the respective O−H bond of each monomer. In the sodium crystal, each bicarbonate shares its proton with a neighbor making chains. The spectrum of the Na+ crystal thus displays one strong peak, not seen in the K+ crystals, assigned to a vibrational mode of the chain. The HQ 14K deposits constitute models for possible compact astrophysical ices,33 allowing to study their spectroscopic properties in laboratory samples free from interactions or blurring caused by species other than water and the ion under study. The apparent intensity loss when compared to spectra of solutions is interpreted as due to inhomogeneous environment of water molecules around the ions, as supported by the present calculations. Thus, the astronomical observation of cyanate in compact water ice dominated samples would be hampered by this effect, but the 2165 cm−1 peak may still provide a detection signal. A comparison with other ions in water ices has recently been presented in ref 43. In water solutions, cyanate slowly transforms into bicarbonate in a process that releases ammonia. While this reaction is spontaneous at room temperature, it would probably be very slow, or totally inactive, at the temperatures of molecular clouds, but it could be favored by incoming energy or rising temperature, as it may happen in diffuse interstellar clouds hot cores or comets close to their perihelion. Therefore, the possibility of existence of bicarbonate in media where cyanate is present cannot be totally discarded. The detection of this ion is problematic, though, since its main modes may overlap those of water or cyanate. If water is suspected, thus obscuring the 1600 cm−1 region, bicarbonate may be detected by its 1360 cm−1 band, if enough resolution to discriminate against the 1300−1200 cm−1 cyanate modes is available. Even if urea were present, the 1360 cm−1 band would still be a good marker, as beautifully illustrated in Figure 1 of ref 44 for biochemical media. If there is no water in the source under observation, then the best candidate would be the strong 1660 cm−1 mode, which would still be discernible in cyanate containing lines of sight.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research has been carried out with funding from the Spanish Ministry of Education, Projects FIS2010-16455 and CTQ-2008-02578/BQU, and within the frame of the Unidad Asociada UCM-CSIC. O.G. and Y.R.-L. acknowledge financial support from the Ramón y Cajal Program and CSIC, JAE-Doc Program, respectively.

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REFERENCES

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