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Home All issues Volume 628 (August 2019) A&A, 628 (2019) A15 Full HTML
Open Access
Issue
A&A
Volume 628, August 2019
Article Number A15
Number of page(s) 15
Section Interstellar and circumstellar matter
DOI https://doi.org/10.1051/0004-6361/201935127
Published online 29 July 2019

© T. Nguyen et al. 2019

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1 Introduction

More than 200 species have been detected so far in the interstellar medium (ISM) and circumstellar shells including complex molecules (COM) from 6 to 13 atoms (McGuire 2018)1. On the other hand, observing isomers is a good way to trace the origin of molecules in the ISM. Indeed excluding diatomic molecules, 30% of all interstellar molecules have isomeric counterparts (Remijan et al. 2005). Therefore, the formation and evolution of interstellar clouds and the stars they generate can be explored through isomer comparison studies.

Many saturated and alkynyl cyanides have been detected in the ISM while only ammonia, methylamine, and aminoacetonitrile were found as amines. For example, there were methyl cyanide (or acetonitrile, CH3CN), ethyl cyanide (CH3CH2CN), n- and iso-propyl cyanide (n-(iso-)C3H7CN), vinyl cyanide (CH2CHCN), and cyanoacetylene (HC3N), which are COMs detected in many regions of the ISM (Solomon et al. 1971; Johnson et al. 1977; Gardner & Winnewisser 1975; Belloche et al. 2009, 2014; Turner 1971; Friedel et al. 2004; Nummelin et al. 1998). Recently a first example of a C ≡ N group conjugated to an aromatic ring, benzonitrile C6H5CN, has been identified (McGuire et al. 2018) in TMC-1, showing a new structural possibility for a hydrocarbon substituent. On the other hand, only three organic isocyanides were observed. For instance, isocyanoacetylene (HC2NC), which is an isomer of cyanoacetylene (HCCCN, a very abundant interstellar species), has been detected in TMC-1 (Kawaguchi et al. 1992); HNC has been found in W51 (Zuckerman et al. 1972) and methyl isocyanide (CH3NC) has been tentatively detected toward Sgr B2 for the first time by Cernicharo et al. (1988). CH3NC was previously possibly detected in the cold dark cloud TMC-1 (Irvine & Schloerb 1984) and has been confirmed in other cold clouds (e.g., L1544) in the hot cores of Orion KL (Remijan et al. 2005; Cernicharo et al. 1988; López et al. 2014; Jiménez-Serra et al. 2016)and in the Horsehead photodissociation region (PDR) by Gratier et al. (2013).

Intuitively proposed, the formation of amines by reduction of nitriles is not obvious. The best examples are the case of abundant CH3CN and CH3CH2CN (Belloche et al. 2016; Margulès et al. 2016, respectively). The bis 13C isotopolog of CH3CN and the three bis 13C isotopologs of CH3CH2CN were found to be ~470 and ~800 times less abundant than their parent compounds, respectively, while attempts to detect ethyl amine (CH3CH2NH2) and propylamine (CH3CH2CH2NH2) failed. On the basis of the nitriles (~4 Debye) and amines (~1 Debye) dipole moments, we can estimate that amines are at least about 30–50 times less abundant than the corresponding nitriles.

To limit the number of possible reactive sites, we consider only the possibility for hydrogenation of a single C ≡ N functional group. The number of such species with saturated sub- stituents is much more limited. It is reduced to CH3CN (Solomon et al. 1971), CH3CH2CN (Johnson et al. 1977), n-C3H7CN (Belloche et al. 2009), and i-C3H7CN (Belloche et al. 2014) for nitriles and only to CH3NC (Cernicharo et al. 1988) for isonitriles. In what follows we focus on the only example with the nitrile/isonitrile group attached to the same hydrocarbon substituent, that is -CH3.

For typical dense cloud conditions, theoretical calculations of Defrees et al. (1985) estimated the ratio of CH3NC/CH3CN to be in the range 0.1–0.4. However, Cernicharo et al. (1988) deduced from their observations an abundance ratio of ~0.03–0.05, showing that CH3CN is an order of magnitude higher than previous estimates. Later, Gratier et al. (2013) found, according to their observations in the Horsehead region, an abundance ratio CH3NC/CH3CN in agreement with estimations of Defrees, at least at the PDR position where a high UV irradiation is at work.

The difference in abundance of stable isomers in the same environment has been tentatively explained by their different intrinsic stabilities (Lattelais et al. 2009) and/or by their different binding energies to solid particles because one of the isomers is more effectively trapped on dust grains, and so depleted from the gas phase (Lattelais et al. 2011). To check this possibility, Bertin et al. (2017) have done a laboratory study showing that the adsorption energy of CH3CN on hydroxylated surfaces is higher than the adsorption energy of CH3NC, which rules out the hypothesis of a lack of CH3NC in the gas phase because of a very high binding energy. So the origin of the low isomeric ratios should have entirely a chemical origin.

Hudson & Moore (2004) bombarded a film of CH3CN with UV photons and energetic protons and found that CH3CN can partially isomerize to CH3NC, indicating a possible direct chemical link between the two isomers in the solid phase. Recently Mencos & Krim (2016) reported that CH3CN may react with ground state N atoms to form CH3NC and CH2CNH in the solid phase at temperatures ranging from 7 to 11 K.

These laboratory results support the now large consensus that gas-phase chemistry alone cannot account for the diversity of complex organic molecules observed in the ISM and that solid-gas reactions, as well as chemical processes involving adsorbed or/and embedded partner, have to be taken into account. In that context interstellar grains play a crucial role. These small particles, most often covered with ices, may protect the molecules from the radiation field and, at the same time, play the role of a third body helping dissipation of the energy excess released by exothermic reactions. These particles may also provide the catalytic conditions to overcome activation barriers impeding reaction paths and directly contribute to the reaction budget.

The most efficient chemical transformation of the molecular mantle during its formation on grains is the hydrogenation. H atoms are always orders of magnitude more abundant than N atoms. According to the astrophysical conditions, the hydrogenation efficiency has been mostly studied at low temperatures (~10 K; Sandford et al. 2006). Hydrogenation drives the formation of many molecules in the solid state, from the simplest (H2, H2O) to the more complex such as formaldehyde (H2CO), methanol (CH3OH), hydroxylamine (NH2OH), formic acid (HCOOH), ethanol (CH3CH2OH), or even formamide (NH2CHO) (Congiu et al. 2012; Minissale et al. 2016; Hiraoka et al. 2005; Watanabe et al. 2004; Ioppolo et al. 2011; Bisschop et al. 2007; Dulieu et al. 2019). Considering C ≡ N containing species, the hydrogenation of HCN/HNC has been looked at in the gas phase (Talbi & Ellinger 1996; Talbi et al. 1996) and on HCN ice (Theulé et al. 2011). Full hydrogenation leads to the same end product, methylamine (CH3NH2), that is the simplest and only aliphatic amine detected in the ISM (Kaifu et al. 1974). To our knowledge the hydrogenation of CH3CN and CH3NC has not yet been undertaken.

Nevertheless, selective destruction pathways of CH3NC relative to CH3CN would help astrochemical modelers to better reproduce observed values. This is the starting point of our study. We compare the hydrogenation of the two isomers on a surface maintained at low temperature (8–40 K), that is the reaction paths leading to the primary amine CH3CH2NH2 (path a) and to the secondary amine CH3NHCH3 (path b) as follows: CH3CN→+4HCH3CH2NH2  patha,CH3NC→+4HCH3NHCH3  pathb.\begin{align*} \textrm{CH}_3\textrm{CN} & \xrightarrow[]{\text{+4H}} \textrm{CH}_3\textrm{CH}_2\textrm{NH}_2 \qquad & \text{path {a}}, \\ \textrm{CH}_3\textrm{NC} & \xrightarrow[]{\text{+4H}} \textrm{CH}_3\textrm{NHCH}_3 \qquad & \text{path {b}} .\end{align*}

Our objective is to understand the details of the chemical network and the possible influence of water, which is the main constituent of molecular mantles on dust grains. The present work reports a concerted experimental/theoretical study coupled with recent observations toward a high-mass star forming region.

The article is organized as follows. In Sect. 2 we present the experimental setup, the protocol of the experiment, and the results obtained from the analysis of the quadrupole mass spectrometer (QMS) spectra as the reaction processes. In Sect. 3, we present the theoretical approach, starting by the benchmarking done to determine the more appropriate level of theory for this study. This is followed by the results of the calculations of enthalpies and activation barriers of the successive steps of the hydrogenation processes in the gas phase and the ice. Section 4 is a first report of the tentative search for molecules relevant to this study in the W51/e2 region, using the IRAM 30 m telescope. In the last section, the theoretical and experimental results are compared and discussed with an emphasis on the possible implications on the abundances of amines in the ISM as shown by the results of the observational campaign.

2 Experimental approach to hydrogenation processes

2.1 Experimental protocol

The experiments were carried out with the VENUS experimental apparatus at the LERMA laboratory of the University of Cergy-Pontoise. It consists of an ultra-high vacuum chamber (UHV) with a base pressure of 2 × 10−10 mbar. The solid sample is a gold-plated surface that is maintained at low temperature using a helium closed-cycle cryostat. The surface temperature is controlled from 7 to 350 K thanks to a resistive heater. Absorbance spectra of adsorbates are recorded by Fourier Transform Reflection Absorption Infrared Spectroscopy (FT-RAIRS) from 750 to 4500 cm−1. Products (or residual reactants) are desorbed from the surface by using temperature programmed desorption (TPD). In our experiments, the surface temperature is raised from 10 K up to 260 K. The heating ramp is kept constant with a value of β = 0.2 K s−1 and simultaneously desorbing species are detected through a QMS positioned in front of the surface.

The injection of atoms or molecules is carried out using a separate system consisting of four atomic or molecular beams. The gas passes through three chambers and into the main chamber via three aligned diaphragms. For this study we used only two beams: the central beam is used to inject CH3NC or CH3CN and the right beam is used to inject H atoms. The H2 flux entering the right beam source is controlled by an automated regulator by Bronkhorst High-Tech and is set to 10 sccm2. A microwave discharge cavity is used to dissociate H2 flowing through a quartz tube (inner diameter 4 mm), cooled with a continuous flow of compressed air at room temperature. The dissociation efficiency of H2 and D2 is 75 and 65%, respectively. The dissociation efficiency, τ, is calculatedvia the formula τ=SOFF−SONSOFF,\begin{equation*} \tau = \frac{S_{\textrm{OFF}} - S_{\textrm{ON}}}{S_{\textrm{OFF}}} ,\end{equation*}(1)

where S is the number of ion counts measured with the QMS in front of the sample when the discharge is switched off or on. H atoms produced in the plasma discharge are thermalized to approximately 300 K via collisions with the walls of the quartz tube, and then pass through the first diaphragm in an effusive regime. We previously determined that atoms are in their ground state (Amiaud et al. 2007; Congiu et al. 2009). Assuming that our flux is about 3 × 1012 molecules cm−2 s−1 (i.e., an adsorbed species receives on average one atom every 300 s from the gas phase) and that the typical diffusion time from one site to another (at 10 K) for H is within the timescale of one millisecond (or faster), the reactivity should be totally dominated by diffusive processes, that is the Langmuir–Hinshelwood mechanism. For the same reason, reacting atoms are mostly thermalized with the surface. It cannot be excluded, however, that the Eley–Rideal mechanism becomes non-negligible, especially when the surface temperature is above 20 K, as H atoms begin to desorb very quickly. Therefore our experimental conditions mainly probe the reactivity of the minimum energy pathways.

CH3NC and CH3CN are liquid at room temperature and their saturation vapor is high enough so that their entrance flux can be manually regulated via a needle valve. The expansion chamber of the central beam is kept constant at a pressure of 1.5 × 10−4 mbar. The length of the mean free flight of molecules in this chamber is about 2 cm before molecules are entering the second stage in which the pressure remains in the 10−7 mbar range. The third stage is kept in the 10−9 mbar range while the main chamber remains in the low 10−10 mbar range to achieve good vacuum conditions for cold surface experiments. Ten grams of CH3NC were synthesized by J.C. Guillemin in Rennes (Bertin et al. 2017) and traveled to Cergy in a frozen state. The experiments were performed twice. CH3NC was prepared performing freeze-thaw cycles in order to eliminate the residual atmospheric gases. We determined that the purity of CH3CN and CH3NC was >98% (our maximum sensitivity), measured with the QMS positioned in front of the cold surface.

Figure 1 shows the FT-RAIRS spectra of CH3CN and CH3NC obtained for different coverages, expressed in monolayers (1 ML = 1015 molecules cm−2). In this work the ML unit is to be intended as an exposed dose, namely the amount of deposited species. In fact, in the case of not-wetting molecules the determination of the coverage is difficult, or in the case of radicals like H atoms the deposited amount either desorbs or reacts. The surface is kept at 10 K. The black curves of Fig. 1 correspond to no exposure, the blue curves to 1.6 ML, the purple curves to 2.4ML, and the red curves to 4 ML. The determination of 1 ML deposition is accomplished within an accuracy of about 20%. This is carried out by scaling the flux used for different species such as CO, which have good wetting properties and allow us to determine the actual coverage (see Nguyen et al. 2018, for further details). Infrared band strength benchmarking confirms the determination of the adsorbate dose. In Fig. 1 we can clearly observe the difference of the stretching vibrations of cyanide (-CN) group at 2250 cm−1 and isocyanide (-NC) groupat 2176 cm−1. Features around 3003 cm−1 for CH3CN are more marked than that of 3008 cm−1 for CH3NC. Infrared spectra are in agreement with those in the literature (Bertin et al. 2017; Murphy et al. 2000).

A TPD analysis completes IR in situ observations. The TPD profiles obtained after the separate deposition of 4 ML of each isomers are shown in Fig. 2. Despite the fact that CH3CN and CH3NC have the same mass (m/z = 41), it is possible to identify the isomer by observing their different cracking patterns consisting of mass 40 and 39, respectively. But the most interesting point is that CH3NC desorbs earlier (its maximum is at 120 K for this specific coverage and heating ramp) than CH3CN, which showsa maximum at 127 K. So it is easy to distinguish the two isomers, thanks to their IR bands or desorption profiles.

For experiments performed in a water ice environment, the water inlet is connected directly to the main chamber and the H2O vapor leak is controlled by a throttling valve. The pressure of the main chamber is few ×10−8 mbar before injecting the reactants and is kept constant. Water deposition is controlled via IR and QMS during all the process. CH3NC, H atoms, and H2O are co-deposited on the surface during 30 min, thus CH3NC is stopped while H and H2O continue to be deposited for 10 extra minutes. In total 6 monolayers of CH3NC and 20 monolayers of H and H2O are deposited. Because the surface is held at 10 K, the water ice network is very porous and does not prevent the access of H to CH3NC efficiently (Minissale et al. 2019). Nonetheless, if we take into account the relative fluxes (water flux is 2.5 higher than the CH3NC flux), most of CH3NC adsorbate should have at leastone H2O neighbor.

thumbnail Fig. 1

FT-RAIRS spectra of pure CH3NC (top panel) and CH3CN (bottom panel) deposited separately at 10 K for different surface coverages.
thumbnail Fig. 2

TPD traces of CH3NC (red curve) and CH3CN (blue curve)obtained after separate depositions of 6 ML of the species on the gold surface held at 10 K.
thumbnail Fig. 3

Evolution of the area of the 2250 cm−1 IR band of CH3CN as a function of deposition time on the substrate held at 10 K. H is co-deposited during the initial and final phases (black squares and blue triangles) and is absent (red circles) in the second phase.

2.2 Probing the reaction paths by TPD-QMS analysis

2.2.1 (Non)reaction between acetonitrile (CH3CN) and H atoms at 10 K

The first experiment consists in monitoring the growth of the 2250 cm−1 IR feature of CH3CN with or without H addition. During the first 16 min CH3CN is co-deposited with H atoms. Area of the IR band is represented as a black dot in Fig. 3. It shows a linear increase. The H beam thus is turned off and the growth of the IR band is plotted in red. After 40 min the H beam is switched ON again. Infrared band areas are plotted in blue. Dots can be fitted by a straight line. Independently from the presence of absence of H atoms, the increase of CH3CN molecules is perfectly linear with time. We conclude that in our experimental conditions, CH3CN does not efficiently react with H atoms on a surface held at 10 K.

thumbnail Fig. 4

Top panel: evolution of the area of the 2176 cm−1 IR band of CH3NC as a functionof deposition time on the substrate held at 10 K. {CH3NC} deposition inblue, and {CH3NC+H} co-deposition in red. Bottom panel: TPD trace (m/z 41) of pure {CH3NC} (blue curve) and the remaining part of CH3NC (red curve) obtained after the same depositions.

2.2.2 Reactivity between methyl isocyanide CH3NC and H atoms at 10 K

Similarly, we can compare depositions of methyl isocyanide (CH3NC) on the golden surface maintained at 10 K in the presence and absence of H atoms co-deposition (Fig. 4, top panel). In contrast to the case of acetonitrile (CH3CN), slopes are very different. For the {CH3NC+H} experiment, the IR bands are under the noise level. Including error bars, we obtain a reduction factor of more than 87% in the amount of consumed CH3NC. Moreover, the TPD profiles of pure CH3NC (blue curve) and remaining CH3NC (red curve) are obtained after completing both depositions. These profiles are shown in the bottom panel of Fig. 4. Similarly to areas of FT-RAIRS band, the amount of CH3NC molecules desorbing are reduced from 875 to 80 counts.K corresponding to a reduction factor of 1 − 80875 ≈ $\,{-}\,\frac{80}{875}\,{\approx}\,$90%. Both FT-RAIRS spectra and TPD profiles provide the same reduction factor for CH3NC indicating its high consumption by H atoms. This demonstrates that the CH3NC molecules are reacting efficiently with H atoms on a 10 K surface.

It is not possible to isolate in the IR spectra some growth of bands that would have been characteristic of products.But the IR spectra inform us that no matter what the origins of the products are, they are not made from the initial reactants during the TPD because they are all consumed at 10 K. Actually, CH3NC remains undetectable and H is known to be steadily transformed in H2 even on a cold surface (Amiaud et al. 2007; Hama et al. 2012; Wakelam et al. 2017).

Figure 5 shows the TPD traces of products that are formed from the reactions between CH3NC and H atoms. There are three successive desorption zones peaking at 105, 120, and 132 K, indicating at least that different molecules are formed. The first peak (87 to 117 K) is dominated by the mass 42 (CH3NCH+− dashed red curve), which corresponds to one extra H to CH3NC and mass 43 (solid red curve), which corresponds to the addition of 2H atoms. Because of the perfect similarity of the temperature profiles, both peaks come from the same mother molecule, which should be CH3NCH2. In mass spectroscopy, methyl groups -CH3 are often ionized in -CH2+$_2^+$ because -CH3+$_3^+$ is a radical and so is less energetically favorable. As examples, the major products of the 70 eV electron impact of methylamine CH3NH2 (m∕z = 31) and methanol CH3OH (m∕z = 32), are m∕z = 30 and m∕z = 31, respectively (Nist webbook).

The second peak around 120 K is a small signal of CH3NC (solid blue curve). It is the same peak shown in Fig. 4, but the vertical scale is very different. The peak is slightly shifted toward higher temperatures. It could be the signature of some isomerization because CH3CN is desorbing at higher temperaturethan CH3NC. However, the molecular environment has also changed because of the presence of others products. As a result, the binding energy and thus the desorption profiles can be affected. As a consequence, we cannot firmly conclude that there is isomerization, but we cannot exclude it.

The last peak from 120 to 150 K is dominated by masses 44 (CH3NHCH2+$_2^+$ – dashed green curve) and 45 (CH3NHCH3+$_3^+$ – solid green curve), which correspond to the reported cracking patterns of dimethyl amine CH3NHCH3, and thus the addition of 4 H atoms. The bottom panel shows the mass 16 (main peak of methane CH4 – solid red curve). Its desorption temperature (40–60 K) is typical and very different from other molecules, so there is no doubt about the methane production. If CH3NC is fragmented during hydrogenation, some correlation with CH4 and HCN or HNC should be found. The corresponding mass (m/z = 27) is also shown in blue in the bottom panel of Fig. 5. The temperature of desorption around 130 K could indicate that it is a cracking pattern of dimethylamine, but it is reported in databases and in Table A.1 that this fragment is only few % of the majorion, so it is not measurable out of the noise in our experimental conditions. The temperature of desorption corresponds to sublimation of HCN (Theulé et al. 2011). Therefore there is no doubt that some fragmentation occurs during the hydrogenation of CH3NC.

We can sum up the {CH3CN/CH3NC + H} first experimental results through the following statements:

  • (i)

    CH3CN does not react with H atoms at low temperatures.

  • (ii)

    CH3NC reacts with H atoms forming: CH3NCH2, CH3NHCH3, CH4, HCN, and possibly CH3CN.

  • (iii)

    We can explain the presence of these products through the following reaction paths:

CH3NC→+2HCH3NCH2→+2HCH3NHCH3,\begin{equation*} \textrm{CH}_3\textrm{NC} \xrightarrow[]{\text{+2H}} \textrm{CH}_3\textrm{NCH}_2 \xrightarrow[]{\text{+2H}} \textrm{CH}_3\textrm{NHCH}_3,\end{equation*}(2) CH3NC+2H→CH4+HCN/HNC,\begin{equation*} \textrm{CH}_3\textrm{NC} + 2\textrm{H} \rightarrow {\textrm{CH}}_4 + \textrm{HCN/HNC}, \end{equation*}(3) CH3NC→?CH3CN.\begin{equation*} \textrm{CH}_3\textrm{NC} \xrightarrow[]{?} \textrm{CH}_3\textrm{CN}. \end{equation*}(4)

thumbnail Fig. 5

Top panel: TPD traces of CH3NCH2+$_2^+$ (m/z 43 – solid red curve) and cracking pattern m/z 42 (CH3NCH+ – dashed red curve) desorb around 90–115 K; remaining CH3NC+ (m/z 41) (solid blue curve) and cracking pattern m/z 39 and 40 (dash blue curves) desorb around 110–125 K; finally, dimethylamine (CH3NHCH3+$_3^+$) (m/z 45 – solid green curve) and cracking patterns m/z 44 (dash green curve), m/z 43 (solid red curve), and m/z 42 (dash red curve) desorb around 120–145 K. Bottom panel: TPD traces of CH4+$_4^+$ (red curve) and HCN+ (or HNC+) (blue curve) are formed from other chemical pathways between CH3NC and H atoms.

2.2.3 CH3NC and its hydrogenation in the presence of H2O

Desorption of CH3NC mixed with H2O

Before performing hydrogenation experiments, it is important to determine if the water and CH3NC react together. Indeed, some stable molecules (such as NH3, CO2, H2CO, H2O2) are known to undergo activated thermal reactions that may be modified by the presence of water (Theulé et al. 2013; Duvernay et al. 2014; Dulieu et al. 2017).

We applied the protocol described in the experimental section. CH3NC is injected using the central beam, whereas H2O is grown by background deposition while the surface is maintained at 10 K. The doses correspond to 6 MLs for CH3NC and 20 MLs for H2O. However, since the surface deposition of the beam is a diameter of about 3 mm and the total diameter of the gold sample is 9 mm, the water signal is much more intense. Most of the surface of the sample is only covered with water. Still, CH3NC is perfectly and homogeneously overlapped by H2O with a ratio of 10 H2O for 3 CH3NC molecules in the mixing area.

The TPD profiles of the {CH3NC} and {CH3NC + H2O} systems are shown in Fig. 6. In the absence of H2O, CH3NC (blue curve) desorption is finished at a lower temperature than the desorption of water (measurable in our experimental conditions above 130 K), meanwhile the CH3NC desorption (red curve) is split in two parts when co-deposited with H2O. A fraction desorbs before H2O and the other part is co-desorbing with H2O. More precisely, CH3NC mostly desorbs during the amorphous desorption phase (Speedy et al. 1996), which occurs before the change in the slope of desorption of water at around 140 K. A smallest and last fraction of CH3NC is desorbing just after the crystallization. We do not observe an obvious volcano effect, nor desorption after the water, as can be seen for others species and/or substrates (Collings et al. 2004; Chaabouni et al. 2018). It means that a part of CH3NC is trapped inside H2O and then it desorbs when released from H2O. We calculated the desorption areas of pure {CH3NC} and {CH3NC + H2O} and the ratiogives a factor of 1. This means that CH3NC does not react with H2O, although CH3NC is trapped inside H2O. No indication of reactivity can be addressed for this system.

To sum up the experimental results of the interaction between CH3NC and H2O, we give the statements:

  • (iv)

    CH3NC does notreact with H2O.

  • (v)

    CH3NC is trapped and released with H2O.

Table 1

Percentages (%) of the remnants (CH3NC) and products after the deposition of pure CH3NC, {CH3NC + H2O}, {CH3NC + H}, and {CH3NC + H + H2O} co-depositions on surface held at 10 K.

thumbnail Fig. 6

Desorption profiles of pure CH3NC (blue curve) and {CH3NC + H2O} (red and green curves).
Reactionof CH3NC and H atoms in thepresence of H2O

For this experiment both H (via the right beam) and H2O (via background deposition) are deposited during 40 min, while CH3NC is only injected for 30 min. The doses are 6 ML for CH3NC and 20 ML for H and H2O.

Figure 7 shows the TPD profiles obtained after the reactions between CH3NC and H atoms in the presence of H2O. Remnants and products present broad and less featured desorption profiles. This is due to the necessity to diffuse out of the water ice. The dynamic of the desorption of mixed ice is not the purpose of this article. The key observation is that the ratio in the products have changed. Especially, CH3NHCH3 (solid green curve) is clearly enhanced, dominating all the other products or remnants. We propose in Eq. (2) that the hydrogenation occurs in two main steps, first CH3NCH2 and second CH3NHCH3. It is clear that in absence of water the hydrogenation is less complete, CH3NCH2 being the major product at the end, whereas in the presence of water the second step is easier and complete hydrogenation is more often reached. The fragmentation is still occurring.

The quantitative amounts of the different products or remnants are listed in Table 1. These amounts are obtained by calculating the area under the TPD profiles (including cracking patterns as shown in Table A.1). They are normalized to the amount of CH3NC injected in the first experiment. First of all, we stress that the efficiency of QMS detection is not exactly identical from one molecule to another. It can vary by a few tens of percent for these molecules. It is due to the different ionization cross sections (which can be calculated), but also to the different spatial and kinetic distributions of the different species that desorb (unknown). Even though cracking patterns are included in the estimation, the ratios presented in the table do not exactly represent the species ratiobut rather give a good indication. In the same column, the relative error is similar, so if we underestimate or overestimate one species, we do the same way for all the experiments. Therefore, it is possible that the total, which is less than 100% in case of reactivity experiments, does not represent a real lost of reactants, but more the inaccuracy of the method. However, it could also be due to losses investigated by the chemical desorption process (Dulieu et al. 2013). This aspect is discussed later. No matter what the source of uncertainties are, the table makes clear that the hydrogenation in presence of water is more efficient. There are fewer remaining products (12% instead of 15%) and the first measurable step of hydrogenation (CH3NCH2) is largely transferred in the second, CH3NHCH3. We can give the following statement from the experiment of CH3NC and H atoms in the presence of H2O:

(vi) H2O has a catalytic role for the hydrogenation of CH3NC.

thumbnail Fig. 7

TPD profiles of the reactivity between CH3NC and H atomsin the presence of H2O. CH3NHCH3 (MCH 3NC + 4H)(solid green curve) is enhanced after completing the deposition. A part of CH3NC (MCH 3NC – solid blue curve) and CH3NCH2 (MCH 3NC + 2H – solid red curve) desorb before H2O and the other part is trapped and desorbed with H2O.

2.2.4 Reactivity of CH3NC and D atoms on the golden surface at 10 K

We performed deuteration of CH3NC with D atoms in the same conditions (i.e., surface temperature and molecular dosage). CH3NC and D atoms co-deposited during 30 min and an additional 10 min of D atoms exposure is applied. In contrast with H experiments, only ~44% ± 6% of CH3NC is consumed by D atoms. Figure 8 shows the comparison between pure CH3NC (blue curve) and the remaining CH3NC (red curve) after completing the reaction (top panel). Thus, CH3NC is less reactive with D at low temperature and the first step of the reaction is not complete.

The bottom panel of Fig. 8 shows the products obtained after deuteration of CH3NC. The first step of the deuteration can be seen through the parent mass 45 corresponding to CH3NCD2 and is desorbing around 115 K. Cracking patterns of m∕z 43 and 44 are also present and are of about the same intensity. On the contrary the trace for m ∕ z 46 and 47 corresponding to dimethylamine (CH3NDCD3) and its fragmentations are undetectable. Moreover, the fragmentation pathway is observed and some CH3D (m/z 17) and DCN (m/z 28) are detected. The quantitative analysis is listed in Table 2.

We can describe the chemical network of {CH3NC + D} atomswith the reactions CH3NC+2D→CH3NCD2,\begin{equation*} \textrm{CH}_3\textrm{NC} + 2\textrm{D} \rightarrow {\textrm{CH}}_3\textrm{NCD}_2,\end{equation*}(5) CH3NC+2D→CH3D+DCN/DNC.\begin{equation*} \textrm{CH}_3\textrm{NC} + 2\textrm{D} \rightarrow {\textrm{CH}}_3\textrm{D} + \textrm{DCN/DNC}.\end{equation*}(6)

From the present set of the {CH3NC + D} experiment, we summarize our findings as follows:

(vii) deuteration of CH3NC is uncompleted at low temperatures, and

(viii) D atoms have an entrance activation barrier.

Table 2

Percentages (%) of remnants (CH3NC) and products after completing the co-deposition of CH3NC and H/D atoms at 10 K.

thumbnail Fig. 8

Top panel: pure CH3NC (m/z 41 – blue curve) and remaining CH3NC (m/z 41 – red curve) after completing the reaction with D atoms at 10 K. Bottom panel: TPD traces of mass 45 (MCH3NC$_{\textrm{CH}_3\textrm{NC}}$ + 2D-solid red curve) and cracking pattern (m/z 43, and 44) (dashed red curves) corresponding to CH3NCD2. No traces of CH3NDCD3 are detected (MCH3NC$_{\textrm{CH}_3\textrm{NC}}$ + 4D-solid green curve).

2.2.5 Reactivity of CH3NC and H atoms at various temperatures.

We have seen earlier that CH3NC can be almost fully consumed by H atoms at 10 K whereas this is not the case with D atoms, suggesting that the CH3NC + D reaction has an entrance barrier and is slower than the CH3NC + H reaction. Surface reactivity is actually a kinetic competition between many different reactive pathways, the most important in the case of hydrogenation being the self reaction H + H → H2. Thus by changing the surface temperature, we can change the kinetics and see how the products are affected.

The top panel of Fig. 9 shows the remaining CH3NC at different surface temperatures after completing the same {CH3NC + H} co-depositions. The remaining CH3NC increases with the surface temperature. The products are shown in the middle and bottom panels. The CH3NCH2 yield decreases when the surface temperature increases, which is also the case for CH3NHCH3 which becomes undetectable above 20 K on TPD profiles. Table 3 lists the quantitative values.

We point out that the consumption of reactants is different at 30 and 40 K (respectively 58 and 48%, respectively), whereas the product yield is about the same (9 and 8%).

We can give the following statement for these experimental results:

(ix) At higher temperatures, the reaction between CH3NC and H atoms is reduced because of the activation barriers.

3 Theoretical approach to hydrogenation processes

3.1 Strategy and computational methods

In view of the number of calculations to be performed in this work and of the complementary work to come on similar subjects we found it necessary to reach a compromise between level of theory and computational effort. We took the first two steps of reaction paths a and b for case studies, namely

CH3CN + H• → CH3CHN•3

and

CH3NC + H• → CH3NCH•4,

and carried out benchmark studies.

First, calculations at the coupled-cluster single and double excitation (CCSD) level (Purvis & Bartlett 1982) and at the CCSD level augmented with a noniterative triple excitation correction (CCSD(T)) (Raghavachari et al. 1989) allowed us to establish reference data.

Second, several methods based on the density functional theory (DFT) were then confronted to CCSD and CCSD(T) treat- ments to determine which level of theory had to be employed for the whole study. We started with the B3LYP functional (Becke 1993; Lee et al. 1988) originally adjusted to reproduce molecular geometries of a panel of representative organic species. Then we tested six functionals developed more recently to account better for weak and long range interactions, belonging to three different families: two meta-GGA functionals, MPWB1K (Zhao & Truhlar 2004) and M06-2X (Zhao & Truhlar 2008); two double-hybride functionals, B2PLYP (Grimme 2006) and B2PLYP-D3 (Grimme et al. 2011); and two long-range corrected functionals, LC-ωPBE (Vydrov & Scuseria 2006), and ωB97XD (Chai & Head-Gordon 2008). These functionals all include a percentage of exact Hartree–Fock (HF) exchange, known to be required for radical systems.

More precisely, the stationary points (reactants, products, and transition states) along paths a and b were first located at the CCSD level. Geometry optimizations were also performed at the Møller-Plesset (MP2) level (Møller & Plesset 1934), which is often used as a standard in molecularcalculations. In this case, energies were refined by single point calculations at the CCSD(T) level. Geometry optimizations were then repeated at the DFT level. At each level of theory (MP2, CCSD, or DFT), the geometries of the stationary points were fully optimized and verified to be true minima for reactants or first order saddle points for transition states, which allowed us to determine the zero-point vibrational energy correction (ZPE). Moreover, at the temperature of the ISM, entropic contributions to the Gibbs free energies can be neglected so we provide only reaction or activation enthalpies, which reduce to energetic quantities at 0 K. Anticipating a possible situation in case the co-deposition of H2 and nitrile might modify the environment compared with isolated species, the embedded molecules (in ice) were calculated using the polarizable continuum model (PCM), in which the environment is described as a homogeneous medium characterized by an ɛ dielectric constant (Tomasi & Persico 1994). The value used is 600, which is very different from that at room temperature; this value was extrapolated to the (10–50 K) range characteristic of the ISM from the results obtained in a study of ɛ at decreasing temperature of water ice (Johari & Jones 1976). All these calculations were done in conjunction with the aug-cc-pVTZ basis set (Dunning 1989), as implemented in the Gaussian package (Frisch et al. 2009).

thumbnail Fig. 9

Top panel: remaining CH3NC (from red to green curves) after completing the reaction with H atoms on the surface with the temperature ranging from 20 to 40 K. Middle panel: TPD traces of CH3NCH2 (MCH3NC$M_{\textrm{CH}_3\textrm{NC}}$ + 2H) (from red to green curves) are gradually reduced with the surface temperature. Bottom panel: the trace of dimethylamine (CH3NHCH3 − MCH3NC$M_{\textrm{CH}_3\textrm{NC}}$ + 4H) is not detectable at high temperatures.

3.2 Benchmarking the DFT approaches

Taking the CCSD level of calculation as our reference, we found that MP2 leads to an overestimated value of the activation barrier (by a factor of almost 3 for CH3CN) and an underestimation of the exothermicity (by a factor of almost 5 for CH3CN). It is noteworthy that errors are larger for the nitrile than for the isonitrile (Table 4). The perturbative treatment of correlation at MP2, including only doubly excited configurations on top of HF, does not seem to be able to account properly for the interaction with the incoming H• atom. However, applying CCSD(T) on top of optimized MP2 geometries helps rectify the energy profile. It should be noted that the CCSD geometries of the transition states are significantly different from those of the MP2 transition states, where the C–H distances are underestimated by about 0.15 Å.

Contrary to post HF treatments, using DFT provides well-grouped energies of within ~ 3.5 kcal mol−1 for ΔH‡ and within ~5 kcal mol−1 for ΔH. The standard B3LYP with only ~20% of HF exchange appears as an exception. It gives the lowest activation barrier, that is ~2.5 kcal mol−1 below the average of the other functionals, and the highest exothermicity, that is ~3.0 kcal mol−1 above the average of the other functionals. The other six functionals with HF exchange contributions in the range ~40–50% provide energy values close to those obtained at the CCSD level. The origin of this behavior lies in the fact that introducing the HF exchange in the exchange-correlation functional corrects the known over-delocalization of the electron density at the DFT level.

In view of the aforementioned results we focused on how embedding in the ice may affect the hydrogenation process by considering only the first step of reaction a (Table 5). The first conclusion to be drawn is that, by comparing with CCSD(T) results, the errors observed for MP2 and B3LYP remain in the same proportion for both the gas phase and icy environment. Thus, ignoring the B3LYP and MP2 calculations, the general effect of embedding in the ice is to increase the activation barrier within ~1.5 kcal mol−1 and to make the reaction less exothermic by ~2.5 kcal mol−1.

The same general conclusions can be drawn for all the DFT computational treatments. The best compromise is obtained using the M06-2X functional. But for the chemical aspects, CH3CN is very different from CH3NC. Indeed, at this level the activation barrier on path b is found to be nearly three times smaller than for path a.

In the end, we present the reactions profiles of path a and b as obtained in the M06-2X/aug-cc-pVTZ framework. In addition, this treatment can be applied to a chemical problem of large dimensions. This approach has the further advantage of being extended to gas-solid catalytic interactions if needed.

Table 3

Percentages (%) of the remnants (CH3NC) and productsafter completing the depositions at different surface temperatures.

Table 4

Reaction (ΔH) and activation (ΔH‡) enthalpies, corrected for ZPE, of the hydrogen addition reaction to CH3CN and CH3NC, calculated at 0 K with different post-HF and DFT methods (kcal mol−1), in conjunction with the aug-cc-pVTZ basis set.

3.3 Probing the reaction paths

Formation of ethylamine and dimethylamine requires additions of four hydrogen atoms, which means four successive elementary steps for each reaction path. It should be mentioned that only the thermodynamically a priori most probable reaction paths were considered, i.e., those for which each step gives rise to the more stable product (Table 6).

On the way to CH3CH2NH2 on path a, the first addition leads to the formation of CH3CH=N•, a free radical of the same type as H2C=N detected by Ohishi et al. (1994). This is an exothermic process over approximately −20 kcal mol−1, which is however opposed by an activation barrier of ~8 kcal mol−1. Addition on nitrogen leading to CH3C=NH is also exothermic, but only by 11 kcal mol−1, and presents an even greater activation barrier (9.3 kcal mol−1); thus, this process and those stemming from are not considered. The second elementary step is a radical-radical reaction without activation barrier leading to (E)-CH3CH=NH similar to methanimine (Godfrey et al. 1973). The third hydrogenation leads to a second free radical, (E)-CH3CHNH2•$_2^{\bullet}$ centered onthe carbon. There is an activation barrier of ~4 kcal mol−1. The fourth step completes the reaction path to ethylamine without any transition state. Both the second and fourth steps are strongly exothermic, by about 90 kcal mol−1. The height of the first activation barrier should be considered as a real obstacle.

The presence of ice does not modify the exothermicity of the radical-radical steps, but slightly decreases it in the first and third steps (by ~3 and ~2 kcal mol−1, respectively). Moreover both activation barriers increase, the first by 1.4 cal mol−1, thus rendering the hydrogenation reaction of CH3CN even less favorable.

On the way to CH3NHCH3 on path b, we find the same sequence of reactions. The radical formed in the first step, CH3N=CH•, can exist in the forms of (E) and (Z) isomers, both being stable intermediates. They are both produced (Table 6) in exothermic processes, ΔH ~ −27 kcal mol−1 and ΔH ~ −23 kcal mol−1 for (E) and (Z), respectively. Reaching the transition state requires only ~3 kcal mol−1, that is a small activation energy compared to that of the first step in path a. Therefore, hydrogenation of CH3NC is probable, at least by tunneling through a small barrier, while hydrogenation of CH3CN is not. This result is consistent with the so-called Fukui indices that rationalize the rules of frontier molecular orbital theory (Fukui et al. 1952), which is relevant to interpret radical reactivity. Indeed, following the procedure of Bulat et al. (2004) we obtain, for the carbon atom in the CN (or NC) group, 0.17 and 0.70 in CH3CN and in CH3NC, respectively (the greater the index, the faster the reactivity with H atoms).

A detailed study of the reaction path using the intrinsic reaction coordinate technique showed that the transition state connects preferentially to the less stable (Z) isomer, which can transform easily into (E). Indeed, the activation barrier of the transformation (Z) → (E) is only a hidden barrier at this stage.

The second elementary step is a radical-radical reaction without activation barrier leading to CH3N=CH2. Further addition of a hydrogen atom produces a new radical intermediate CH3NH-CH2•$_2^{\bullet}$ that is exothermic (approximately − 35 kcal mol−1) and also opposed by a small barrier of ~4.5 kcal mol−1. In the end the last step is barrier free and leads to dimethylamine, CH3NHCH3. As in path a radical–radical reactions are strongly exothermic, by about 90 kcal mol−1.

The presence of ice has the same effects as for path a, though attenuated. In particular, the first barrier to hydrogenation increases slightly, reaching 3.8 kcal mol−1. If tunneling is still possible, our calculations predict that it should be less facile, which does not seem to be in agreement with the experiments.

Table 5

Reaction enthalpy (ΔH) and activation barrier (ΔH‡), corrected for ZPE, of the hydrogen addition reaction to CH3CN, calculated at 0 K at the MP2, CCSD(T) and DFT level with the aug-cc-pVTZ basis set, in vacuum and in ice (kcal mol−1).

Table 6

Energy profiles of reaction paths a and b in vacuum and in ice.

4 Comparison with astronomical observations

We investigate the presence of CH3NH2 along with that of the C2H5NH2 amine and that of the possible parent molecules, CH3CN and CH3NC in the ISM; spectroscopic data of (CH3)2NH are not available at the present time in the (sub)millimeter wavelength. More specifically, we search for these species toward the high-mass star forming region W51/e2 (located at 5.4 kpc, Sato et al. 2010), in which the presence of methyl cyanide has been reported by Remijan et al. (2004). In the present study, we make use of IRAM 30 m observations that we have available (PI C. Favre) and that cover the following frequency ranges: 99.70–106.3 GHz and 238.2–246.0 GHz. We refer to Lykke et al. (2015) for a full description of the observations and data reduction.

In this paper, we focus on CH3CN and CH3NC transitions with an upper energy level, Eup, up to 660 K, and search for C2H5NH2 and CH3NH2 transitions with an Einstein coefficient, A, greater than 5 × 10−5 s−1. Tables B.1–B.4 give the spectroscopic and observational line parameters for CH3CN, CH3NC, C2H5NH2, and CH3NH2, respectively. We detect five lines and observed two partially blended and two blended lines for CH3CN. The mean velocity is around 55.6 km s−1, which is consistent with the systemic velocity of the source, and the mean line width, Δv1∕2, is about 8 km s−1 (see Table B.1). We do not detect CH3NC, C2H5NH2 nor CH3NH2: their respective lines are either not detected or are blended with the emission from another species. The observed W51/e2 spectra for the targeted molecules are shown in Fig. 10.

From the rotational population diagram method (see Turner 1991; Goldsmith & Langer 1999), we can estimate the physical properties (T, N) of the detected CH3CN molecule. This analysis hinges upon the assumptions that local thermodynamical equilibrium (LTE) is reached (i.e. the kinetic, excitation, and rotational temperatures are equal) and that the lines are optically thin. Following Goldsmith & Langer (1999), we estimate that the CH3CN emission is optically thin with τ ≤ 0.2. In addition, in the present study, we assume that the emission comes from a source size that is equal to the beam size (~10′ at 1 mm). The rotational population diagram analysis yields to a rotational temperature for CH3CN of Trot = (343 ± 43) K. The derived beam-averaged total CH3CN column density5, NCH3NC$N_{\textrm{CH}_3\textrm{NC}}$, is (1.5 ± 0.2) × 1015 cm−2.

Regarding the nondetected species, assuming the same rotational temperature (Trot = 300 K) as that derived from the CH3CN study, we estimate the following upper limits on their beam-averaged column densities: N(CH3NC)$N_{(\textrm{CH}_3\textrm{NC})}$ ≤ 9.8 × 1013 cm−2, N(C2H5NH2)$N_{({\textrm{C}_2}{\textrm{H}_5}\textrm{NH}_2)}$ ≤ 1.8 × 1015 cm−2, and N(CH3NH2)$N_{(\textrm{CH}_3\textrm{NH}_2)}$ ≤ 4 × 1015 cm−2.

To summarize, we find that CH3NC is at least 15 times less abundant than CH3CN, while ethylamine and methylamine could have an abundance of the same order of magnitude as CH3CN.

5 Concluding remarks

Experiments have shown that CH3CN does not react with H atoms. Calculations have confirmed that the hydrogenation barrier of the first hydrogenation is high and makes starting the chemical process leading to the primary amine unlikely. It should be noted that the reduction of CH3CN, and therefore the reactivity with hydrogen atoms, has been observed in UV photolysis experiments of this molecule (e.g., Hudson & Moore 2004). Our experiment differs from previous approaches because we use atoms thermalized with the surface and because most of the highest energy reactive paths are not efficient under our experimental conditions. The calculated barrier is too high for our experimental conditions, whether by direct thermal crossing or by tunnel effect; this barrier, is reachable by H atoms, which would have a higher initial kinetic energy, for example, after the photolysis of CH3CN.

On the contrary, CH3NC reacts with H and forms CH3NCH2, CH3NHCH3, which have been identified as the most stable products along the hydrogenation process. The detection of CH3NCH2 is in agreement with the decrease of the calculated activation barrier of the first hydrogenation going from CH3CN to CH3NC, showing that the starting of the chemical process is easier. Methane (CH4) and HCN have been also identified but only as collateral products of much lower abundance.

The different reactivity of CH3NC and CH3CN is a possible explanation for the observed ratio of CH3NC and CH3CN factor of 0.03–0.05 (Cernicharo et al. 1988), which is less than expected by pure gas-phase routes. Indeed, CH3NC and CH3CN have a different fate on the surface of grains. CH3NC likely evolves under hydrogenation, whereas CH3CN is unaffected by hydrogenation, which is the main chemical driving force for surface reactivity in molecular clouds. So, if part of the mantle is feeding the gas phase, as occurs in shocks or hot cores, thus chemical differentiation on grains can induce differences in the gas phase. As highlighted by TPD experiments CH3NC does not react with H2O but is trapped in H2O.

In addition, hydrogenation of CH3NC is enhanced in the presence of H2O. More specifically, when CH3NC and H atoms mix with the H2O molecules, the chemical reactions take place inside the pores or in the bulk of the ice. The water molecules therefore may increase the efficiency of quantum tunneling in the hydrogenation process of CH3NC.

At low temperatures, CH3NC should react with H atoms via quantum tunneling. At high temperatures, the reactions of CH3NC take place with temperature effect adding to tunneling. However, a temperature dependence out of the Arrhenius regime would be the final argument to ensure that the first step of the reaction is actually achieved by quantum tunneling. We base our conclusions on the observed isotope effect, temperature dependence,and calculation of the height of the reaction barrier. We note that taken separately our arguments might appear weak. However, once combined together they are stronger and represent the best we can currently do to build our conviction.

To summarize, the hydrogenation is enhanced with the H2O presence, although there is no evidence of a chemical participation in the calculations. In this context, the fact that our calculations show an increase of the first activation barrier is not compatible with the experimental facts, even if the barrier of the first hydrogenation of CH3NC remains lower than that of the third hydrogenation. Modeling solid water by an implicit solvent, originally proposed for disordered liquid media, is therefore questionable. The answer is to be found in the neglect of local interactions with the well-defined structure of ice; porous does not mean disordered at the local scale. Taking into account well-oriented local interactions, namely the hydrogen bonds in the solid, can be done in solid state calculations (Lattelais et al. 2011; Bertin et al. 2017). Such a treatment as presented for the trapping of small molecules in cometary ice (Mousis et al. 2016, 2017) and Europa ice crust (Ozgurel et al. 2018) is well beyond the aim of the present work.

In the presence of H2O, the easier hydrogenation of CH3NC may be due to different electronic properties. In terms of quantum chemistry, it may affect the geometry of the reaction and thereby lower the reaction barrier. Water can also slightly change the geometry of the molecule CH3NC itself, and/or displace slightly charges changing the heights of the reaction barriers. Water molecules could also participate by helping proton transfer (Rimola et al. 2014). It should also be stressed that the above developments are valid for all the R-CN nitriles (as quoted in Sect. 1), but limited for R-NC isonitriles since the hydrogenation can only provide N-methyl substituted secondary amines.

Under astrophysical conditions, the flux of H atoms is orders of magnitude lower than in our experimental conditions. Therefore, the H concentration on the surface is also lower, and therefore the H + H → H2, is less competitive than in our experimental conditions. We have seen that even under our “high” flux conditions, hydrogenation of CH3NC is efficient, so it should also be the case in dense molecular clouds. On the contrary, if the temperature of the grain rises above 20 K, thus the hydrogenation efficiency drops and dimethyl amine has probably very little chance to be formed this way.

In the end, the present interdisciplinary study mixing experimental and theoretical aspects of laboratory astrophysics together with tentative detection of simple primary and secondary amines leads to some suggestions for observation proposals. Finding CH3NHCH3 would need more refined spectra. The detection of CH3CH2NH2 seems much less plausible if, as considered in this work, it comes from nitrile hydrogenation. Thus, another route to the formation of amines has to be found. For example we could consider the hydrogenation of imines, assuming the latter are obtained by a mechanism different from hydrogenation of nitriles.

thumbnail Fig. 10

Portion of the W51/e2 spectra as observed at 1 and 3 mm with the IRAM 30 m telescope. Line assignmentd of CH3CN (red), CH3NC (blue), C2H5NH2 (cyan), and CH3NH2 (magenta) are indicated on top of the spectra.

Acknowledgements

This work was supported by the Programme National “Physique et Chimie du Milieu Interstellaire” (PCMI) of CNRS/INSU with INC/INP co-funded by CEA and CNES and the DIM ACAV+, a funding program of the Region Ile de France. T.N. thanks the LabEx MICHEM for her PhD grant. This work is also supported by the French National Research Agency in the framework of the Investissements d’Avenir program (ANR-15-IDEX-02). C.F. acknowledges support from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme, for the Project ”The Dawn of Organic Chemistry” (DOC), grant agreement No 741002. I.F. is greatful to P. Chaquin and B. Silvi for fruitful discussions about reactivity.

Appendix A New estimation of the cracking patterns of CH3NCH2

Since the National Institute of Standards and Technology (NIST) Chemistry Webbook has no records for the mass specta of CH3NCH2, we list our experimental values in the Table A.1.

Table A.1

Signal intensity of QMS normalized to the highest peak of CH3NC, CH3CN, and productsafter completing the reactions with H atoms.

Appendix B Spectroscopic and observational line parameters

Tables B.1–B.4 give the spectroscopic and observational line parameters for the observed CH3CN, CH3NC, C2H5NH2, and CH3NH2 transitions.

Table B.1

Spectroscopic and observational line parameters for CH3CN.

Table B.2

Spectroscopic and observational line parameters for CH3NC.

Table B.3

Spectroscopic and observational line parameters for C2H5NH2.

Table B.4

Spectroscopic and observational line parameters for CH3NH2.

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2

1 sccm = 592 m3 Pa s−1 in SI units.

3

More stable than CH3C•NH.

4

More stable than CH3NHC•.

5

We obtain NCH3NC$N_{\textrm{CH}_3\textrm{NC}}$ assuming a rotational temperature of 300 K as the Jet Propulsion Laboratory (JPL) catalog provides a line partition function for this species that is only valid up to 300 K. Nonetheless, we note that the assumed value is commensurate within the error bars with the value we derived in this study.

All Tables

Table 1

Percentages (%) of the remnants (CH3NC) and products after the deposition of pure CH3NC, {CH3NC + H2O}, {CH3NC + H}, and {CH3NC + H + H2O} co-depositions on surface held at 10 K.

In the text
Table 2

Percentages (%) of remnants (CH3NC) and products after completing the co-deposition of CH3NC and H/D atoms at 10 K.

In the text
Table 3

Percentages (%) of the remnants (CH3NC) and productsafter completing the depositions at different surface temperatures.

In the text
Table 4

Reaction (ΔH) and activation (ΔH‡) enthalpies, corrected for ZPE, of the hydrogen addition reaction to CH3CN and CH3NC, calculated at 0 K with different post-HF and DFT methods (kcal mol−1), in conjunction with the aug-cc-pVTZ basis set.

In the text
Table 5

Reaction enthalpy (ΔH) and activation barrier (ΔH‡), corrected for ZPE, of the hydrogen addition reaction to CH3CN, calculated at 0 K at the MP2, CCSD(T) and DFT level with the aug-cc-pVTZ basis set, in vacuum and in ice (kcal mol−1).

In the text
Table 6

Energy profiles of reaction paths a and b in vacuum and in ice.

In the text
Table A.1

Signal intensity of QMS normalized to the highest peak of CH3NC, CH3CN, and productsafter completing the reactions with H atoms.

In the text
Table B.1

Spectroscopic and observational line parameters for CH3CN.

In the text
Table B.2

Spectroscopic and observational line parameters for CH3NC.

In the text
Table B.3

Spectroscopic and observational line parameters for C2H5NH2.

In the text
Table B.4

Spectroscopic and observational line parameters for CH3NH2.

In the text

All Figures

thumbnail Fig. 1

FT-RAIRS spectra of pure CH3NC (top panel) and CH3CN (bottom panel) deposited separately at 10 K for different surface coverages.
In the text
thumbnail Fig. 2

TPD traces of CH3NC (red curve) and CH3CN (blue curve)obtained after separate depositions of 6 ML of the species on the gold surface held at 10 K.
In the text
thumbnail Fig. 3

Evolution of the area of the 2250 cm−1 IR band of CH3CN as a function of deposition time on the substrate held at 10 K. H is co-deposited during the initial and final phases (black squares and blue triangles) and is absent (red circles) in the second phase.
In the text
thumbnail Fig. 4

Top panel: evolution of the area of the 2176 cm−1 IR band of CH3NC as a functionof deposition time on the substrate held at 10 K. {CH3NC} deposition inblue, and {CH3NC+H} co-deposition in red. Bottom panel: TPD trace (m/z 41) of pure {CH3NC} (blue curve) and the remaining part of CH3NC (red curve) obtained after the same depositions.
In the text
thumbnail Fig. 5

Top panel: TPD traces of CH3NCH2+$_2^+$ (m/z 43 – solid red curve) and cracking pattern m/z 42 (CH3NCH+ – dashed red curve) desorb around 90–115 K; remaining CH3NC+ (m/z 41) (solid blue curve) and cracking pattern m/z 39 and 40 (dash blue curves) desorb around 110–125 K; finally, dimethylamine (CH3NHCH3+$_3^+$) (m/z 45 – solid green curve) and cracking patterns m/z 44 (dash green curve), m/z 43 (solid red curve), and m/z 42 (dash red curve) desorb around 120–145 K. Bottom panel: TPD traces of CH4+$_4^+$ (red curve) and HCN+ (or HNC+) (blue curve) are formed from other chemical pathways between CH3NC and H atoms.
In the text
thumbnail Fig. 6

Desorption profiles of pure CH3NC (blue curve) and {CH3NC + H2O} (red and green curves).
In the text
thumbnail Fig. 7

TPD profiles of the reactivity between CH3NC and H atomsin the presence of H2O. CH3NHCH3 (MCH 3NC + 4H)(solid green curve) is enhanced after completing the deposition. A part of CH3NC (MCH 3NC – solid blue curve) and CH3NCH2 (MCH 3NC + 2H – solid red curve) desorb before H2O and the other part is trapped and desorbed with H2O.
In the text
thumbnail Fig. 8

Top panel: pure CH3NC (m/z 41 – blue curve) and remaining CH3NC (m/z 41 – red curve) after completing the reaction with D atoms at 10 K. Bottom panel: TPD traces of mass 45 (MCH3NC$_{\textrm{CH}_3\textrm{NC}}$ + 2D-solid red curve) and cracking pattern (m/z 43, and 44) (dashed red curves) corresponding to CH3NCD2. No traces of CH3NDCD3 are detected (MCH3NC$_{\textrm{CH}_3\textrm{NC}}$ + 4D-solid green curve).
In the text
thumbnail Fig. 9

Top panel: remaining CH3NC (from red to green curves) after completing the reaction with H atoms on the surface with the temperature ranging from 20 to 40 K. Middle panel: TPD traces of CH3NCH2 (MCH3NC$M_{\textrm{CH}_3\textrm{NC}}$ + 2H) (from red to green curves) are gradually reduced with the surface temperature. Bottom panel: the trace of dimethylamine (CH3NHCH3 − MCH3NC$M_{\textrm{CH}_3\textrm{NC}}$ + 4H) is not detectable at high temperatures.
In the text
thumbnail Fig. 10

Portion of the W51/e2 spectra as observed at 1 and 3 mm with the IRAM 30 m telescope. Line assignmentd of CH3CN (red), CH3NC (blue), C2H5NH2 (cyan), and CH3NH2 (magenta) are indicated on top of the spectra.
In the text

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