© ESO, 2010

1. Introduction

In molecular clouds, the chemical composition of the interstellar material evolves as clouds collapse to form protostars, and eventually main-sequence stars with planets and comets. As infrared absorption studies of protostellar environments have shown, many molecules are frozen on the icy mantle of dust grains Whittet et al. (1996). Their heating by the protostar infrared photons, cosmic rays, and ultraviolet radiation changes the ice composition and triggers chemical reactions. Newly formed molecules can eventually be released into the gas phase or be incorporated into comets and planets van Dishoeck et al. (1998). Changes in the ice molecular composition during the star formation process can be monitored by studying the infrared absorption bands of molecules and they provide data to constrain chemistry models. Among all chemical reactions occurring in the interstellar medium, in this paper, we decided to focus more particularly on thermal reactions. To observe these reactions, they have to be thermodynamically favored and possess a low activation barrier in order to occur on a reasonable timescale. Several thermal reactions that are expected to occur in interstellar grains have already been studied and can be classified into two types. The first type contains acid-base reactions. Main examples of which are the reactivity between HNCO and NH₃, which leads to NH₄⁺OCN⁻Raunier et al. (2003, 2004), or the reaction between HCOOH and NH₃ which forms HCOO⁻NH₄⁺Shutte et al. (1999). These reactions have a very low activation barrier and are expected to occur at temperatures lower than 50 K in interstellar ices. The second type of thermal reaction is addition reactions involving aminolysis or hydrolysis reactions. For example, it has been shown that amine RNH₂ and CO₂ can react in a water ice environment to produce carbamates in relevant astrophysical conditions and timescales (Bossa et al. 2008, 2009a). The simplest aminoalcohol, aminomethanol NH₂CH₂OH, has been recently thermally produced in interstellar ice analogs at temperatures lower than 100 K from a reaction between NH₃ and H₂CO (Bossa et al. 2009b).

In this paper, we experimentally study the reaction between ammonia NH₃ and acetaldehyde CH₃CHO in a water ice environment. Gas phase acetaldehyde was detected in the Sagittarius B2 molecular cloud by observing rotational transitions and is suspected to be present into ice mantles. We demonstrate that it reacts with ammonia to produce a chiral molecule: the alpha-aminoethanol NH₂CH(CH₃)OH. The formation of this molecule within ices is very interesting because, except in meteorites (Cronin & Pizzarello 1997, 1999), no chiral molecule has been detected in either the interstellar medium, comets, or planetary environments. Thus, the detection of a chiral molecule and a possible enantiomeric excess provide evidence of a link between interstellar molecules and life as we know it on Earth. Among the chiral molecules produced by the living world, aminoacids play an important role and alpha-aminoethanol NH₂CH(CH₃)OH can be considered as a possible aminoacid precursor (Charnley et al. 2001; Courmier et al. 2005).

In this work, we use Fourier transform infrared (FTIR) spectroscopy and mass spectrometry to study the formation of alpha-aminoethanol NH₂CH(CH₃)OH in H₂O: NH₃: CH₃CHO ice mixtures. Isotopic substitution with ¹⁵NH₃ and ab-initio calculation are used to confirm the identification of alpha-aminoethanol. The vacuum ultraviolet (VUV) photolysis of alpha-aminoethanol gives acetamide CH₃CONH₂ as a photoproduct, which provides additional proof of alpha-aminoethanol identification. The possible formation of alpha-aminoethanol NH₂CH(CH₃)OH in different interstellar environments is then discussed. The kinetic, thermodynamic, and infrared data provided in this work suggest its formation in warm interstellar environments such as protostellar envelopes or in cometary environments.

2. Experimental and theoretical

Ammonia is commercially available as a 99.9995% pure gas from Air Liquide. Acetaldehyde is bought under its liquid form from Aldrich and is gently heated up to be evaporated. The H₂O:NH₃:CH₃CHO gas mixtures are prepared at room temperature into two primary vacuum pumped mixing lines, the first one for the H₂O:NH₃ mixture and the second one for CH₃CHO, to prevent any gas phase reaction occuring. They are then co-deposited with a chosen ratio onto a gold-plated metal surface cooled to 10 K by a Model 21 CTI cold head within a high vacuum chamber (ca 10^-8 mbar). The IR spectra are recorded between 4000 and 650 cm^-1 in the transmission mode using a FTIR spectrometer. A typical spectrum has a 1 cm^-1 resolution and is averaged over one hundred interferograms. The sample is warmed using a heating resistance and the temperature is controlled using a Lakeshore Model 331 temperature controller. The mass spectra are recorded using a MKS quadrupole mass spectrometer as the products are being desorbed. The ionization source is an 70 eV impact electronic source and the mass spectra are recorded between 1 and 80 amu. The VUV radiations (λ > 120 nm) are generated from a microwave discharge hydrogen flow lamp (Opthos instruments). The fluence of the hydrogen lamp is estimated to be ca 10¹⁵ photons cm^-2 s^-1. The concentration ratio of the different mixtures is first set before deposition using standard manometric techniques and then derived from the IR spectra by integrating vibrational bands to estimate the column density of NH₃ and CH₃CHO according to their band strengths provided by the literature. Moreover, the values of the band strengths depend on the nature, composition, and temperature of the ice in which they are found, and this dependence is a major source of uncertainties when evaluating the column densities of frozen molecules. For NH₃, the band strengths of the wagging mode at 1070 cm^-1 varies from 1.3 × 10^-17 (Kerkhof et al. 1999) to 1.7 × 10^-17 cm molecule^-1d’Hendecourt et al. (1986) in its pure solid form. We use the value given by Kerkhof et al. of 1.3 × 10^-17 cm molecule^-1, as usually done in the literature. For CH₃CHO, we use the values given by Schutte et al. (1999) and Wexler (1967) of 1.3 × 10^-17 cm molecule^-1 for the CO stretch mode at 1715 cm^-1 and 1.5 × 10^-18 cm molecule^-1 for the CH bending mode at 1350 cm^-1. The thickness of the deposited solid films, assuming a density of 0.73 g cm^-3 and 0.92 g cm^-3 for NH₃ and H₂O, respectively, is estimated to be around 0.1 μm, which is consistent with interstellar ice mantle thickness. We also use the band integration strengths to monitor the decrease in the NH₃ and CH₃CHO reactant IR-band intensities as they are consumed in the reaction, and to estimate how much of each product is formed. We also perform a vibrational analysis of aminoethanol to compute the harmonic vibrational frequencies for both the ¹⁴N and ¹⁵N isotopomers. Nevertheless, to simplify this analysis we do not take into account the effect of environment on the calculations. Calculations were performed using the Gaussian 98 package (Lee 1988; Frish 1998) at the B3LYP/6-311G++(2d, 2p) level, which is known to supply reliable predictions of vibrational wavenumbers.

3. Results

3.1. Aminoethanol formation in a NH₃:CH₃CHO ice mixture

Pure NH₃ and acetaldehyde CH₃CHO are deposited independently at 10 K and their respective IR spectra are displayed in Figs. 1b and c.

The assignment of each band is well known and reported in Table 1 in both pure solids and the 4:1 mixture. When the pure solid is heated with a temperature ramp of 4 K min^-1, acetaldehyde begins to sublimate at 120 K. On the basis of its IR signal, acetaldehyde fully disappears at 135 K. Pure NH₃ begins to sublimate at the same heating rate at 120 K and fully disappears at 125 K (Sandford & Allamandola 1993). A binary NH₃:CH₃CHO ice mixture is then deposited at 10 K within a 4:1 concentration ratio. No reaction occurs at this temperature and the IR spectrum of this mixture is simply the superimposition of the NH₃ and acetaldehyde spectra as seen in Fig. 1a. Assignments of the observed bands are listed in Table 1. When the sample is warmed above 110 K, NH₃ and CH₃CHO are consumed, their IR features diminish, and new features are observed as seen in Fig. 2.

After complete sublimation of the reactants, around 130 K, the new species, the alpha-aminoethanol NH₂CH(CH₃)OH, formed by the reaction between acetaldehyde and ammonia remains on the gold surface. It fully sublimates at 200 K. Its infrared spectrum at 185 K (Fig. 2d) provides evidence of a NH₂ group at 3347 and 3285 cm^-1 (Shutte et al. 1993) both -CH and -CH₃ groups at 2978, 2934, and 2865 cm^-1, and C-O/C-N groups at 1134, 1101, and 925 cm^-1 (Shutte et al. 1993). An OH group may be present around 3300 cm^-1 but covered by broad NH₂ and CH features. The observed bands are listed in Table 2 with their corresponding assignments. This result is comparable to the thermally promoted reaction of formaldehyde with ammonia at low temperature which leads to the formation of aminomethanol NH₂CH₂OH (Courmier et al. 2005; Bossa et al. 2009b; Woon 1999). The infrared spectrum of alpha-aminoethanol is similar to that of aminomethanol, as seen in Fig. 3, especially in the NH stretching and bending regions. The upshift observed for the intense C − O stretching mode of alpha-aminoethanol (1101 cm^-1) with respect to the aminomethanol frequency (1040 cm^-1) is in agreement with the difference expected between primary and secondary alcohols. The proposed mechanism for this reaction is a nucleophilic attack by the lone electronic pair of the ammonia N atom on the acetaldehyde C atom, followed by a proton transfer from N to O (Courmier et al. 2005). To confirm alpha-aminoethanol formation, we perform an isotopic substitution experiment using a ¹⁵NH₃:CH₃CHO ice mixture in a 4:1 concentration ratio and compare these results with those obtained from our ab-initio calculation using the B3LYP 6-311G++(2d,2p) basis set. The theoretical and experimental frequencies for ¹⁴N alpha-aminoethanol are listed in Table 2 along with theoretical and experimental frequency shifts between ¹⁴N and ¹⁵N isotopomers. Although harmonic calculations overestimate frequencies corresponding to X-H stretching modes (Table 2), we observe good agreement between the experimental and calculated frequencies of ¹⁴N alpha-aminoethanol (Table 2) especially at frequencies lower than 1600 cm^-1. We also compare experimental frequency shifts between ¹⁴N and ¹⁵N isotopomers with theoretical values. Calculated frequency shifts are small (smaller than 10 cm^-1). The larger calculated shift of 10 cm^-1 is for the NH stretching mode, which is in good agreement with the observed experimental shift of 14 cm^-1 (Table 2). Small differences between experimental and theoretical values are observed, particularly in the region related to the C-O/C-N stretching mode. From the calculations, three strong bands are expected in this region at 1158, 1048, and 953 cm^-1 (Table 2). The calculated isotopic shifts are 2, 4, and 2 cm^-1, respectively. Experimentally, three bands are observed in this region for ¹⁴NH₂CH(CH₃)OH located at 1134, 1101 cm^-1 and 925 cm^-1 (Fig. 2d).

The isotopic substitution induces frequency shifts of 0, 1, and 5 cm^-1 respectively (Table 2) in the same order of value as the theoretical values. The alpha-aminoethanol formation is also confirmed by mass spectrometry as shown in Fig. 4. A clear peak at m/z 46 is observed when alpha-aminoethanol desorbs around 200 K. It is assigned to the NH₂CHOH⁺, which corresponds to the loss of a CH₃ fragment from an alpha fragmentation of the alpha-aminoethanol. When the reaction is performed with ¹⁵NH₃, this peak is observed at m/z 47. This result confirms that the newly formed species contains only one nitrogen atom. Unfortunately we have never observed the molecular ion at m/z 61. It is probably insufficiently stable to be seen, either because alpha-aminoethanol is not stable in the gas phase or because it undergoes collisionally induced unimolecular dissociation in our experimental conditions. This is consistent with the mass spectrum of aminomethanol reported by Bossa et al. (2009a). The molecular ion of aminomethanol (NH₂CH₂OH) at m/z 47 was 15 times lower in intensity than the base peak at m/z 46 corresponding to the loss of one hydrogen atom in the alpha position. If we were to apply the same fragmentation pattern to alpha-aminoethanol, the molecular ion would have an intensity lower than the detection threshold of our spectrometer. Moreover in molecule-bearing heteroatoms such as amines and alcohols, the most common means of fragmentation is the C − C bond cleavage in alpha position of the heteroatom. In addition, the peak at m/z 43 (Fig. 4) corresponding to the loss of H₂O (M-18) is consistent with a molecule-bearing OH group. The peak at m/z 28 for ¹⁴NH₂CH(CH₃)OH is assigned to the loss of CH₄ and OH. The peak at m/z 29 for ¹⁴NH₂CH(CH₃)OH is assigned to the loss of CH₃ and NH₃. In the ¹⁵N isotopic experiment, they are both observed at m/z 29, which explains the strong intensity of this peak in the ¹⁵N isotopic experiment. The assigned mass spectra for both ¹⁴N and ¹⁵N experiments are shown in Fig. 4

3.2. Alpha-aminoethaol formation in a water ice environment

To form alpha-aminoethanol in a more relevant astrophysical environment, we use a water-ice-dominated H₂O:NH₃:CH₃CHO mixture. Figure 5 shows the thermal evolution of a ice mixture in a 10:5:0.5 concentration ratio deposited at 10 K. At this temperature, the spectrum is dominated by strong water absorption bands at 3225, 1628, and 790 cm¹, assigned to the OH stretching mode, OH bending mode, and libration mode, respectively. Acetaldehyde is characterized by two strong bands located at 1721, and 1351 cm¹ which can be assigned to the C=O stretching mode and CH bending mode, respectively. Finally, ammonia is characterized by its strong band at 1120 cm¹ assigned to the inversion mode. All the observed bands are listed in Table 1.

As previously, when the ice mixture is warmed above 100 K, new bands at 1377, 1248, 1101, and 922 cm¹ corresponding to alpha-aminoethanol are observed, while the bands corresponding to ammonia and acetaldehyde decrease as can be seen in Fig. 5. The new features continue to increase in intensity until all acetaldehyde is consumed. This indicates that alpha-aminoethanol is formed by a reaction involving acetaldehyde and ammonia during the warming up. At 200 K, both unreacted starting material (NH₃ and CH₃CHO) and alpha-aminoethanol desorb with H₂O. In these conditions, it is impossible to obtain the IR spectrum of the pure product. Nevertheless, the direct comparison of the product at 185 K in a water environment (Fig. 5d) with the spectrum of pure alpha aminoethanol recorded at 185 K allow us to conclude to its formation in a water ice environment.

3.3. The rate of alpha-aminoethanol formation

We measure the NH₂CH(CH₃)OH formation rate in a H₂O:NH₃:CH₃CHO ice mixture, where NH₃ and CH₃CHO are diluted into H₂O. The reaction rate can be written as v = k[NH₃]^α[CH₃CHO]^β, where α and β are the partial orders of the reaction related to NH₃ and CH₃HCO, and [NH₃] and [CH₃CHO] the molar fraction of NH₃ and CH₃CHO, respectively. We use a H₂O:NH₃:CH₃CHO mixture in a 10:5:1 concentration ratio and measure the reaction rate as a function of temperature. The H₂O:NH₃ CH₃CHO mixture is as quickly as possible brought to a fixed temperature, and the evolution with time of the species IR bands is recorded at this fixed temperature. From both the disappearance of CH₃CHO and the formation of NH₂CH(CH₃)OH along with time, we derive a reaction rate k′, where k′ = k[NH₃]^α, considering that [NH₃] is constant since NH₃ is in excess within the mixture. The single exponential decay of CH₃CHO indicates a first partial order for CH₃CHO (β = 1). As for NH₃, the partial order α is derived to be 0.2 ± 0.01 (Fig. 6), from reaction rate measurements at identical temperature (T = 120 K) and different NH₃ molar fractions, as seen in Table 3. Finally, the reaction is found to have a rate law v = k[NH₃]^0.2[CH₃CHO] and Table 3 gives the reaction rates measured for different temperatures.

This set of reaction rates as a function of temperature is fitted to an Arrhenius law k(T) = Aexp(−E_a / RT), where E_a is the activation energy and A is the pre-exponential factor, as shown in Fig. 7. We find that E_a = 33 ± 2   kJmol^-1, A = (7 ± 1) × 10¹⁰ s^-1. Introducing a temperature dependence as a power law of the pre-exponential factor does not improve the fit. These two parameters E_a and A can later be incorporated into kinetic databases for astrochemistry and used to extrapolate the rate of the reaction to lower temperature, assuming that Arrhenius law remains valid.

3.4. VUV irradiation of alpha-aminoethanol

We form NH₂CH(CH₃)OH from a NH₃:CH₃CHO ice mixture in a 4:1 concentration ratio as described previously. We then desorb the unreacted starting material (CH₃CHO and NH₃) at 130 K to obtain pure alpha-aminoethanol. We then irradiate it at 10 K with a VUV hydrogen lamp and follow its photolysis by IR spectroscopy as seen in Fig. 8. The VUV flux is estimated to be around 10¹⁵ photons cm^-2 s^-1. After 120 min of VUV irradiation, 80% of alpha-aminoethanol is consumed as shown in Fig. 9 and its decay is fitted by a first order kinetic rate. The corresponding kinetic constant is found to be 4.5 × 10^-4 s^-1. With our flux of photons, the photodissociation cross-section can be estimated to be σ_photo = 4.5 × 10^-19 photon^-1 cm². Besides the IR signatures of alpha-aminoethanol (see Fig. 8), the irradiated sample displays the presence of new bands at 1674 and 1400 cm¹ due to the formation of acetamide CH₃CONH₂ (Sankar et al. 1987), as shown in Fig. 8. The formation of acetamide after irradiation is also confirmed by mass spectroscopy. After the irradiation of aminoethanol at 10 K, the sample is warmed to 250 K. The desorption of the product at 220 K shows a peak at m/z 59 corresponding to the molecular ion of acetamide, CH₃(CO)NH₂. This peak is not present when NH₂CH(CH₃)OH is not irradiated. Its formation from alpha-aminoethanol may be the result of a dehydrogenation process as follows:

$\mathrm{N}{\mathrm{H}}_{\mathrm{2}}\mathrm{CH}\mathrm{\left(}\mathrm{C}{\mathrm{H}}_{\mathrm{3}}\mathrm{\right)}\mathrm{OH}\mathrm{+}\mathrm{h}\mathit{\nu }\mathrm{\to }\mathrm{C}{\mathrm{H}}_{\mathrm{3}}\mathrm{CON}{\mathrm{H}}_{\mathrm{2}}\mathrm{+}{\mathrm{H}}_{\mathrm{2}}\mathit{.}$The same dehydrogenation mechanism has been observed in the VUV photolysis of methanol and aminomethanol that give formaldehyde and formamide, respectively (Gerakines et al. 1996; Bossa et al. 2009b). The direct production of acetamide from the photolysis of our product is additional evidence of the alpha-aminoethanol formation. The 2160 cm^-1 band is easily assigned to OCN⁻ (Hudson et al. 2000; Raunier et al. 2004) a secondary photoproduct formed by acetamide photolysis. It results from the reaction at 10 K between HNCO and CH₃NH₂, but also with the non-desorbed NH₃ (Raunier et al. 2003) by means of two major channels (Duvernay et al. 2007):

$\mathrm{C}{\mathrm{H}}_{\mathrm{3}}\mathrm{CON}{\mathrm{H}}_{\mathrm{2}}\mathrm{+}\mathrm{h}\mathit{\nu }\mathrm{\to }\mathrm{HNCO}\mathrm{+}\mathrm{C}{\mathrm{H}}_{\mathrm{4}}$

$\mathrm{C}{\mathrm{H}}_{\mathrm{3}}\mathrm{CON}{\mathrm{H}}_{\mathrm{2}}\mathrm{+}\mathrm{h}\mathit{\nu }\mathrm{\to }\mathrm{CO}\mathrm{+}\mathrm{C}{\mathrm{H}}_{\mathrm{3}}\mathrm{N}{\mathrm{H}}_{\mathrm{2}}\mathit{.}$

4. Discussion

The NH₃:CH₃CHO ice mixture at low temperature leads to the formation of alpha-aminoethanol NH₂CH(CH₃)OH. This product identification is supported by (1) the agreement between the experimental and theoretical IR band shifts of ¹⁴NH₂CH(CH₃)OH and ¹⁵NH₂CH(CH₃)OH; (2) mass spectrometry results, which are consistent with alpha-aminoethanol formation; and (3) the detection of acetamide CH₃CONH₂ as VUV photolysis product. This new mechanism for acetamide production is interesting from an astrophysical point of view. Acetamide has been already detected in the interstellar medium in the gas phase and several others mechanisms have been proposed to explain its gas phase formation (Quan & Herbst 2007; Hollis et al. 2006). Here we propose a new mechanism in the solid phase, starting with the thermal reaction between acetaldehyde and ammonia followed by VUV photolysis of the product:

Thermal synthesis of aminoethanol NH₂CH(CH₃)OH, is the first synthesis of a chiral molecule (i.e. carbon with four different substituents) in interstellar ice analogs without any non-thermal energetic processes (i.e. photon irradiation or particle bombardment). However, we note that in our conditions the alpha-aminoethanol will be produced as a racemate.

Chiral molecules have an essential role in prebiotic chemistry and many biologically active molecules, including aminoacids (the building blocks of proteins) and sugars, have the same chirality. Thus, the possibility of alpha-aminoethanol formation in interstellar ices is interesting to consider as it may be an intermediate in the formation of more complex chiral molecules such as alanine, one of the most common aminoacids. Alanine NH₂CH(CH₃)COOH would be the product of the Strecker synthesis that can be summarized by two steps: (i) a thermal reaction between CH₃CHO and NH₃ to form alpha-aminoethanol; and (ii) an acid hydrolysis of alpha-aminoethanol in the presence of HCN to produce successively aminoacetonitrile and finally alanine. Here alanine would be again formed as a racemate. The first question is in which astrophysical environments can we envisage the presence of aminoethanol. NH₃ has been detected in interstellar ices through different lines of sight (mainly high-mass young stellar objects YSOs, such as W33A) with a 10% maximum abundance relative to H₂O. It has also been detected in the gas phase of the interstellar medium or in cometary ices. Acetaldehyde has been detected in the gas phase of the interstellar medium and in comets but its detection still requires confirmation in interstellar ices. The presence of this molecule in ices should not exceed 1% (Shutte et al. 1999; Gibb et al. 2004; Dartois 2005).

Alpha-aminoethanol may be formed thermally in objects that are mildly warm, such as YSOs, or comets and we roughly estimated its production in these environments using a simple model. To determine the alpha-aminoethanol abundance as a function of time in different environments such as cold molecular clouds, comets, or YSOs we needed to numerically solve a system of first-order differential equations to model the formation and destruction of each species, the abundance of water being kept constant. These equations can be sorted into three different classes: thermal formation, thermal desorption, and photo-destruction. The chemical reaction network and the ordinary differential equation system are shown in Fig. 10. For two-body thermal reactions (k₁) and thermal desorptions (k₅, k₆, k₇), the rate coefficients are given under the form k = A.exp(−E / RT), where T is the ice temperature, A the pre-exponential factor, and E the energy (activation or desorption). Photodissociation rates (k₂, k₃, k₄) are written as k = σ.f, where σ represents the photodissociation cross-section and f the interstellar ultraviolet radiation field (Woodall et al. 2007). We used kinetic data as well as the photodissociation cross-section obtained in this present work to estimate the kinetics rates of the formation (k₁) and photodissociation reactions (k₂) of alpha-aminoethanol. We also took into account the photodestruction rates of NH₃ (k₃) and CH₃CHO (k₄) reactants using the kinetic rates of the UMIST database (Woodall et al. 2007). We included the desorption of the NH₃ (k₅) and CH₃CHO (k₆) reactants, and the NH₂CH(CH₃)OH product (k₇). For the k₅ and k₆ we used the value given by Sandford & Allamandola (1993). The desorption rate of aminethanol (k₇) was estimated from the measured desorption energy of aminomethanol (Bossa et al. 2009a), since they both desorb at the same temperature. The initial abundance for NH₃ and CH₃CHO are 10% and 1% with respect to H₂O respectively. All thermal formation, photodestruction, and desorption kinetic rates are given in Table 4.

Figure 11 shows the solid alpha-aminoethanol abundance relative to H₂O evolution as a function of time, temperature, and UV flux. With a flux of 10⁴ photon cm^-2 s^-1, typical of attenuated field within a dense molecular cloud, the amount of alpha-aminoethanol is maximal (around 1% with respect to H₂O) within a month and 10⁵ years in the range 90–110 K, and within 1000 years at 130 K. The maximum amount of alpha-aminoethano occurs at 90 K. With a flux of VUV photons around 10⁸ photon cm^-2 s^-1 (Fig. 12), typical of the average interstellar radiation field in the diffuse medium, alpha-aminoethanol cannot be observed after 1000 years regardless of the temperature because of the photodissociation process. Therefore, the best conditions to observe solid-state alpha-aminoethanol would be a warm (T > 80 K) and blended environment (f < 10⁵ photon cm^-2 s^-1) as in the warm layer of a protoplanetary disk or in a hot corino. A cometary nucleus may be exposed to a wide range of temperatures anda VUV radiation field, depending on its distance to the star, so a cometary nucleus could also be a suitable environment for alpha-aminoethanol formation. NH₃ and CH₃CHO have already been detected in cometary coma, so the thermal reaction between them may lead to the formation of alpha-aminoethanol in this environment. However approximate this reduced model may be, it provides a rough idea of what the favorable conditions are to produce alpha-aminoethanol. It can be produced on a reasonable time-scale in relevant interstellar conditions, and thus may be a plausible candidate for further investigation of cometary material. Moreover, if it is stable in the gas phase, alpha-aminoethanol may be observed from its rotational transitions in hot core regions when the solid material is desorbed in the gas phase.

5. Conclusion

In this work, we have investigated the thermal reaction of solid NH₃ and acetaldehyde CH₃CHO at low temperature. This reaction leads to the formation of a chiral molecule, the alpha aminoethanol NH₂CH(CH₃)OH. For the first time, we report its infrared and mass spectra. We have also investigated its photochemical behavior under VUV irradiation. The main photo-product is acetamide (CH₃CONH₂). The data provided in this work demonstrate that the alpha-aminoethano is formed at low temperature on a one-hour time-scale in the laboratory, suggesting that its formation in warm interstellar environments, such as protostellar envelopes or in cometary environments, is likely. The alpha-aminoethanol should be analyzed in more detail by future studies of cometary material or the gas phase by radioastronomy. The mass spectrum of the alpha-aminoethanol obtained in this study is of interest for the forthcoming analysis of the Rosetta mission: the observation of a peak at m/z 46 from the COSAC gas analyzer maybe the signature of an aminoalcohol.

Acknowledgments

This work has been founded by the French national program Physique Chimie du Milieu Interstellaire (P.C.M.I) and the Centre National des Etudes Spatiales (C.N.E.S).

References

All Tables

All Figures