Article http://pubs.acs.org/journal/aesccq
Electron-Induced Synthesis of Formamide in Condensed Mixtures of Carbon Monoxide and Ammonia Jan H. Bredehöft,* Esther Böhler, Fabian Schmidt, Tobias Borrmann, and Petra Swiderek Institut für Angewandte und Physikalische Chemie, Fachbereich 2 (Chemie/Biologie), Universität Bremen, Leobener Strasse 5, Postfach 330440, D-28334 Bremen, Germany ABSTRACT: The formation of the prebiotically relevant molecule formamide under electron exposure of ammonia and carbon monoxide was studied at cryogenic temperatures of 30−35 K. Postirradiation thermal desorption spectroscopy was used to study the energy dependence of the reaction. A resonant process centered around ∼9 eV and a threshold type increase of the yield above ∼12 eV were observed. On the basis of the absence of particular side products such as urea and ethanediamide and supported by quantum chemical calculations, reaction mechanisms related to the two observed energy regimes of formamide production are proposed. Below the ionization threshold, electron attachment to ammonia and the subsequent dissociation of the radical anion trigger the reaction sequence. At higher energies, electron impact ionization and addition of the formed radical cation to a neutral molecule ultimately lead to the formation of formamide. KEYWORDS: Low-energy electrons, cryogenic chemistry, thermal desorption spectrometry, ice grain chemistry, prebiotic chemistry
1. INTRODUCTION Formamide (NH2CHO) is a small molecule that has recently attracted great interest in the context of astrochemistry. Its presence in the gas phase around young stellar object W33A,1 comet C/1995 O1 Hale-Bopp,2 cloud core NGC 7538 IRS9,3 as well as in the interstellar medium4 seem to suggest its widespread abundance. It is the smallest molecule to contain the structural element of the peptide bond, which makes it an interesting molecule to study as a model for the (photo-) stability of proteins under astrochemical conditions. On top of that, it is known that formamide is a possible precursor in the prebiotic synthesis of nucleic acid bases5−8 and their subsequent condensation into polymers.9 It has been found that formamide is formed by UV photoprocessing of carbon monoxide (CO) and ammonia (NH3) in the gas phase10 as well as in condensed phase ice layers.11−13 Formamide also forms during the irradiation of CO/NH3 ices by high energy (keV) electrons14 or protons13 and through a proposed multistep process during the irradiation of mixtures of methanol and NH315 with high energy electrons. Ion bombardment of mixtures of H2O/CH4/ N2, H2O/CH4, NH3, and CH3OH/N2 has also been observed to produce formamide.16 Furthermore, there have been theoretical studies that predict a gas phase reaction route to formamide from OH radicals with methanimine (CH2NH) and NH2 radicals with formaldehyde (H2CO).17 The work by Kaiser et al.14 is particularly intriguing because it proposes a reaction mechanism for the formation of formamide from CO and NH3. Unfortunately, the high © 2017 American Chemical Society
electron energy E0 of 5 keV used in this study complicates the interpretation of the results, because electrons with keV energies trigger a great number of concurring processes. The interpretation provided relies on uncharged presumably radical species combining to yield the final product. Electron irradiation, however, favors the creation of charged particles.18 A revisit of these previous findings with tools to discriminate between all the chemically active species that are created under electron irradiation is thus highly relevant. It is well recognized nowadays that chemical processes occurring under high-energy irradiation are in fact driven by the large number of secondary low-energy electrons that are released when energetic photons or particles interact with a dense medium such as an ice layer.19,20 Recently, we provided evidence that low-energy electrons in fact induce reactions that can even couple two molecules to form a product incorporating all atoms of the two reactants. For example, irradiation of a condensed mixture of ethene (C2H4) and NH3 with electrons having E0 just above the ionization threshold of the reactants induces the formation of ethylamine (C2H5NH2).21 In this case, ionization of one of the reaction partners removes the electrostatic repulsion between the lone pair of NH3 and the electron-rich double bond of ethene and thus sufficiently lowers the existing activation barrier of the reaction between the two Received: Revised: Accepted: Published: 50
December 9, 2016 January 20, 2017 January 31, 2017 February 1, 2017 DOI: 10.1021/acsearthspacechem.6b00011 ACS Earth Space Chem. 2017, 1, 50−59
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ACS Earth and Space Chemistry molecules22 so that the adduct formation can take place21 (Figure 1). The neutral product is then formed by recapture of
on the Au substrate) revealed that the stoichiometric ratio between NH3 and CO was not 1:1 as expected but rather closer to 8:1. The experiments were repeated on another instrument (with Ta substrate) with better control of gas inlet parameters and a slightly colder sample, yielding a true 1:1 ratio of reactants. Qualitatively, the results are very similar, so a combined data set from both sets of experiments was used in the analysis and is presented here. Film thickness in the first set of experiments was 12−18 monolayers, whereas in the second set it was 10−14 monolayers. The condensed films on Au were exposed to low-energy electrons from a commercial flood gun (SPECS FG15/40). The second set of experiments on Ta that aimed in particular at discriminating between possible isomers of formamide was performed with a STAIB NEK150-1 electron source. The estimated energy resolution is 1 eV. Incident electron flux was kept constant during all experiments. Significant charging of the condensed films was not observed, because the current measured at the sample was also constant throughout experiments. Desorbing neutral molecules were monitored using a quadrupole mass spectrometer (QMS) residual gas analyzer (Stanford Research Systems RGA 200). The QMS is equipped with an electron impact ion source operating at an electron energy of 70 eV. Desorption was monitored after irradiation upon resistive heating of the substrate by thin Ta ribbons spot-welded to the substrate. The heating rate was typically 1 K/s but 2 K/s was chosen in the experiments to distinguish between possible isomers of formamide. Four different masses were monitored simultaneously during a typical experiment. The mass spectra of NH3, CO, and formamide were recorded with our instrument to provide a comparison baseline. As was the case for many prior studies, it was observed that fragmentation patterns in our mass spectrometer agree very well with those found in the NIST database.27 Substance amounts in TDS were quantified by integrating the desorption peaks. 2.2. Calculations. The energetics of reactions of selected fragments produced by electron impact in the condensed molecular layers were investigated by quantum chemical calculations to support the experimental findings. The calculations were performed at the B3LYP/6-311++G(d,p) level of theory using the Gaussian09 package,28 using RB3LYP and UB3LYP methods for closed shell and open shell species, respectively. The Berny algorithm was used for geometry optimizations. All minima and transition states were confirmed from analyses of their vibrational frequencies. Calculations were performed for isolated fragments, intermediates, and transition states. The absolute energies from calculations are thus not accurate for the condensed phase studied here. The ion energies usually differ by about 2 eV, which is in line with the experimentally observed difference in, for instance, ionization energies when comparing gas phase values to condensed phase systems.19,29,30 When studying the energy barriers of ionic reactions, however, only relative energies on the ionic potential energy surface are relevant, which we believe to be accurate enough for the purpose of discussing reaction mechanisms.
Figure 1. Proposed mechanism of an electron-induced hydroamination reaction between NH3 and ethene.21
a thermalized electron present within the molecular layer while electron exposure continues. This electron-induced hydroamination reaction was recently used to functionalize a selfassembled monolayer (SAM) terminated by CC double bonds23 and has also been investigated in mixtures of different olefins and amines, yielding evidence for the production of larger amines.24 Also, ethanol was obtained in an analogous reaction between ethene and water but its formation was surprisingly also induced by electron attachment presumably to ethene.25 Considering that much insight has been obtained concerning the reaction mechanisms in these examples of chemical syntheses driven by low-energy electrons, it is highly appealing to revisit the example of formation of formamide under these well-defined conditions of controlled electron energy and film thickness. Therefore, the aim of the present study is to provide evidence that formamide can indeed be formed via similar reactions as described above for ethylamine, larger amines, and ethanol. The reaction scheme presented in Figure 1 suggests that formation of formamide could proceed by a similar mechanism when ethene is replaced by CO. To obtain evidence for such a reaction, we have investigated by thermal desorption spectrometry the production of formamide in thin layers of mixed ices of CO and NH3 as a function of electron incident energy (E0). A study of the E0 dependence of the reaction uncovers which reactive species initiate the reaction and thus provides important insights into the reaction mechanisms at work.
2. EXPERIMENTAL SECTION 2.1. Postirradiation Thermal Desorption Spectrometry. Electron-induced reactions in condensed multilayer molecular films of CO and NH3 produced from vapors of both reactants were investigated by postirradiation thermal desorption spectrometry (TDS). Two sets of experiments were performed in different ultrahigh vacuum chambers evacuated by turbomolecular pumps to a base pressure of 10−10 mbar.26 In these experiments, product formation was monitored after electron exposure of the condensed films. The films were deposited at ∼35 K on a polycrystalline gold (Au) foil or at ∼30 K on a tantalum (Ta) foil by leaking onto the substrate defined amounts of the gases as monitored from the pressure drop (measured in mTorr) in a gas handling manifold. The film thickness and stoichiometric ratio of reactants was estimated by carefully observing at which pressure drop in the inlet manifold the transition from monolayer to multilayer desorption signals occurs.26 A reanalysis of the first set of experiments (performed
3. RESULTS AND DISCUSSION 3.1. Possible Structural Isomers of Formamide. Products formed during electron exposure of condensed mixed ices of CO and NH3 were investigated by monitoring during postirradiation thermal desorption experiments characteristic mass over charge ratios as known from literature mass 51
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ACS Earth and Space Chemistry spectra.27,31 Here, the desorption temperature of a given product provides further support for the assignment deduced from the selected mass spectrometric signals. The anticipated product formamide can be detected by monitoring its dominant signal at m/z 45 M•+ during a TDS experiment. A second characteristic fragment is m/z 29 HCO+ with intensity amounting to about one-third of the m/z 45 peak. However, attention must be paid to several known structural isomers of formamide (Figure 2). Formamidic acid is known as the less
used to rule out the formation of nitrosomethane while the use of m/z 28 as characteristic for formaldehyde oxime is difficult here because of strong overlap with a CO desorption signal from parts of the sample holder that warm up slower than the sample itself (data not shown). 3.2. Identification of Formamide in Condensed Mixtures of CO and NH3 after Electron Exposure. TDS curves for m/z 28 and m/z 17 obtained from multilayer films of CO and NH3 (1:8) before and after electron exposure reveal the decay of the parent compounds. The formation of a new product is obvious from a new desorption signal in the m/z 45 curve with a maximum near 230 K (Figure 3a). In the second set of TDS experiments performed on multilayer films of CO and NH3 (1:1) before and after a higher electron exposure, the signals m/z 29 and m/z 30 were recorded together with m/z 45 to obtain evidence that the observed new product is in fact formamide and reveal if nitrosomethane is also produced. Higher exposure was used to ensure a higher product yield and thus a better s/n ratio. The presence of nitrosomethane can be excluded based on the lack of signal at m/z 30 (Figure 3b). Similarly compelling evidence on the absence of formamidic acid and formaldonitrone cannot be obtained here due to the lack of mass spectrometric reference data. The absence of formaldehyde oxime is tentatively concluded from a lack of a signal at m/z 28 coincident with m/z 45. Here, the proof is not as strong as for nitrosomethane due to overlap of its anticipated m/z 28 signal with residual CO (data not shown). However, based on the above review of known data for the different isomers, the coincidence and the relative intensities of the m/z 45 and m/z 29 signals clearly support that formamide has been synthesized under electron exposure. It has to be noted here that the onset of the m/z 29 peak is in fact located at a slightly lower temperature than that of the m/z 45 signal. This is caused by the side product isocyanic acid (HNCO), which desorbs at a slightly lower temperature than formamide. This was confirmed by observation of m/z 43, which is the base peak of the HNCO mass spectrum36 (data not shown). HNCO is known to form alongside formamide in UV12 as well as high-energy proton13 processing of CO/NH3 ices. To obtain additional evidence that formamide is indeed the product, we compare the desorption temperature observed in the postirradiation experiment with that observed upon depositing pure formamide (Figure 4). The desorption temperature of formamide varies with the amount of substance on the target. At very low coverages in the submonolayer regime where interaction of molecules with the metal substrate determine binding energy, a desorption temperature maximum of around 230 K is observed. This agrees closely with the desorption temperature of smaller amounts of the new product as obtained after a short electron exposure and for smaller quantities of CO (Figure 3a). Deposition of larger amounts of vapor leads to a desorption signal with a maximum near 190 K in close coincidence with the desorption temperature of the new product after longer electron exposure (Figure 3b). The position of the desorption peak remains constant upon further increase of the thickness (not shown), which is expected of multilayer coverage, where interactions between formamide molecules determine the binding energy. Here, it is important to note that formamide produced upon electron exposure of condensed mixed films of CO and NH3 desorbs at significantly higher temperature than the parent compounds and thus not from a matrix in another compound.37 Therefore, the
Figure 2. Possible structural isomers of CONH3: (a) formamide, (b) formamidic acid, (c) formaldehyde oxime, (d) nitrosomethane, and (e) formaldonitrone.
stable tautomer of formamide.32 The barrier for this tautomerization is reduced when proton transfer is assisted by water.32,33 NH3 may equally assist such proton transfer but formation of formamidic acid from CO and NH3 requires more extensive bond reorganization than formation of formamide. Mass spectrometric data for formamidic acid are unfortunately not available but in accord with its thermal decomposition that yields HCN and H2O32 a fragmentation pathway yielding a m/z 29 ion is difficult to conceive for this isomer (see Table 1). Table 1. Electron Impact Ionization Mass Spectrometric Data for Different Isomers of CONH3a m/z 28 m/z 29 formamide (H2NCHO) formamidic acid (HNCHOH) formaldehyde oxime (H2CNOH) nitrosomethane (H3CNO) formaldonitrone (H2CNHO)
a
m/z 30
m/z 45
source
6 34 0 100 no EI data available but shows thermal decay into HCN and H2O 32 2 5 100
27 32
20 4 100 70 no EI data available but collisionally activated dissociation spectra show large peak at m/z 30 and another peak at m/z 28 with about 25% intensity
27 34
31
Numbers are reported as percentages of base peak in mass spectrum.
Three further structural isomers of formamide that would require even more extensive bond reorganization if formed from CO and NH3, namely, nitrosomethane, formaldehyde oxime, and the zwitterionic formaldonitrone (Figure 2) are again tautomers of each other. Among those tautomers, formaldehyde oxime is the most stable one.33,35 The most intense signals in its mass spectrum beside the dominant M•+ peak are m/z 28 [M−OH]+ and m/z 27 [M−H2O]+31 with no m/z 29 signal at all. In contrast, the mass spectrum of nitrosomethane is known and dominated by a m/z 30 signal NO+ while m/z 29 is very weak.27 Finally, data for formaldonitrone are sparse because this tautomer has only been produced and investigated as a short-lived species.34 Consequently, it is unlikely that this product would survive long enough in the present experiments to be detected. In a collisionally activated dissociation study, the most prominent signal of the decay of formaldonitrone was m/z 30 with another signal at m/z 28 with about 25% intensity. Altogether, a TDS signal at m/z 29 is unique to formamide and was thus monitored in the present experiments. In addition, m/z 30 is 52
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Figure 3. Thermal desorption spectra of multilayer films of a 1:8 mixture of CO and NH3 on Au without electron exposure (0 μC/cm2) and after an electron exposure of 200 μC/cm2 at E0 = 8 eV (a) and a 1:1 mixture on Ta after an exposure of 1000 μC/cm2 at E0 = 8 eV (b). The films were deposited from the gas phase onto the Au substrate held at 34 K at a thickness corresponding to 12−18 monolayers (a) and onto Ta held at 30 K with a thickness corresponding to 10−14 monolayers (b). The m/z 29 and m/z 30 signals starting at 230 K are due to desorption of 13CO and C18O from parts of the sample holder that warm up slower than the sample itself.
ranges of E0. Neutral dissociation can equally occur in this regime but has a threshold behavior with yield increasing monotonically above this threshold.18 At energies well above the ionization threshold, electron impact ionization becomes the dominant process as E0 increases, as the ionization cross section for CO grows faster with energy than the ND cross section, which seems to be a general trend.38 Depending on the specific molecule, there can be an energy region close to the ionization threshold in which all three types of electron− molecule interaction processes can occur. By observing the E0 dependence of processes, it is possible to tell resonant EA processes apart from EI and ND with their typical threshold behavior. It has to be noted that in the condensed phase the energies of processes yielding ions can be lowered by up to 2 eV because of stabilization of the ion in the matrix and the width of resonances may also increase as compared to the gas phase.19,29,30 When comparing product yields for different E0, it is important to make sure that the reaction has not proceeded so far that depletion of the film plays any role. This is achieved by measuring the product yield for a given E0 but for different electron exposures (Figure 5). At low values, electron exposure and product yield show a linear correlation. This is the regime where the approximation of initial rates is valid. The amount of produced formamide begins to saturate after electron exposures of around 1000 μC/cm2. This is shown here for E0 = 8 and 16 eV (Figure 5). To compare the amounts formed at different E0, we investigate the energy dependence of formamide production both for an exposure of 200 μC/cm2 (see Figure 3a), which is clearly in the linear regime, and after 1000 μC/cm2, that is, at the onset of saturation. The data show that even though the mixing ratios of CO and NH3 as well as
Figure 4. Thermal desorption spectra of thin films of pure formamide with increasing thickness corresponding to the stated amount of deposited vapor.
desorption temperatures of the product and of the pure reference film of formamide can be directly compared. Together, we obtain here further support that formamide is in fact produced. 3.3. Dependence of Formamide Production on Electron Incident Energy. The incident electron energy E0 at which products are formed gives vital clues about the mechanism at work. It reveals whether the primary reaction step, the interaction of the electron with a molecule, produces a temporary radical anion through electron attachment (EA), a radical cation through electron impact ionization (EI), or neutral (radical) fragments by neutral dissociation (ND).18,19 Below the ionization potential(s) of the reactants, electron attachment resonant processes are observed within well-defined 53
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3.4. Reaction Mechanisms for the Formation of Formamide. The energy dependence of formamide production (Figure 6) indicates that product formation proceeds via at least two different mechanisms. The ionization threshold of NH3 in the gas phase is 10.07 eV39 and that of CO is 14.014 eV.40 The increase of the formamide yield above E0 = 12 eV can thus be explained by a mechanism initiated by ionization of the parent compounds. However, while ionization thresholds in condensed phase can be slightly lower due to stabilization of the ion,19,29,30 an onset of formamide production at around 6 eV would imply a too large shift to be explained by this effect. A resonant EA process below the ionization threshold must therefore be the initiating step of the reaction at these low energies. Neutral dissociation of NH3 at E0 > 9 eV has previously been cited to explain electron-induced reactions in condensed mixtures of NH3 and CO2,41 which is again much higher than the observed onset at around 6 eV. Additionally, the present experiment shows a resonant behavior of formamide production with maximum yield around 9 eV, which suggests that neutral dissociation is unlikely to make a significant contribution to the formamide yield at these low energies. It is known from gas phase measurements that CO has an electron attachment resonance at ∼10 eV42 and that NH3 has two resonances at 5.5 and 10.5 eV.43 The EA resonance of CO at 10 eV forms the intermediate radical anion CO•− that dissociates to form a radical anion O•− and a neutral C atom.42 This process has a gas phase cross section of 2 × 10−19 cm2.38 DEA to CO will thus lead to the formation of two very reactive species. It is conceivable that DEA to CO will ultimately lead to a stable product that we have not observed. It is, however, questionable if either of the two formed species will undergo a specific reaction rather than haphazardly react with anything in their vicinity. The intact radical anion is not expected to react with NH3 because electrostatic repulsion between the anion and the nitrogen lone-pair is strong. We thus conclude that EA to CO is not a likely initial step that leads to the observed product. It is easier to conceive that formamide production is initiated by the two DEA resonances in NH3 at 5.5 and 10.5 eV that decay into NH2• and H− or NH2− and H•. In the gas phase, the higher resonance at 10.5 eV predominantly forms NH2• and H− with a cross section of 5 × 10−19 cm2 while a cross section of about 1 × 10−19 cm2 is observed for the formation of NH2− and H•43 (Figure 7). The lower resonance, which apparently does not contribute to product formation in the present experiment (see Figure 6), will decay through both channels with roughly equal probability.44 On the basis of these general considerations, we discuss here the most likely mechanisms leading to formation of formamide
Figure 5. Dependence on electron exposure at E0 = 8 eV and E0 = 16 eV of the relative amount of formamide produced in mixed multilayer films of CO and NH3 with mixing ratio of 1:1 and thickness corresponding to 10−14 monolayers. The data points were obtained by integrating the characteristic desorption peaks in the TDS curves.
the exposure were varied, the results are remarkably similar (Figure 6).
Figure 6. Dependence on electron energy E0 of the relative amounts of formamide present in mixed multilayer films of CO and NH3 with mixing ratio of 1:8 and thickness corresponding to 12−18 monolayers after an electron exposure of 200 μC/cm2 (filled circles) and of films with mixing ratio of 1:1 and thickness corresponding to 10−14 monolayers after an electron exposure of 1000 μC/cm2 (open circles). The data points were obtained by integrating the characteristic desorption peak in the TDS curves. Error bars denote error estimate for peak area and instrument energy spread for energy. Even though the two data sets were recorded on different instruments with different mixing ratios and at different exposures, they agree very well after scaling absolute intensities to compensate for different detector gain and product amounts.
Upon increase of E0, production of formamide sets in near 6 eV and reaches a maximum around 9 eV (Figure 6). After a slight drop of the produced amount around 12 eV, formamide production increases again toward higher E0. As discussed below, we interpret this finding as being related to a reaction proceeding via a resonant dissociative electron attachment (DEA) to NH3 around 9 eV and via electron impact ionization at higher E0 with the latter being in accord with the reaction outlined in Figure 1.
Figure 7. Gas phase dissociation pathways initiated by electron attachment to NH3. The resonance is centered around 10.5 eV. Cross sections for the formation of the two anions in the gas phase have been measured by Ram and Krishnakumar.44 54
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desorption above 230 K (Figure 3). However, we note that the desorption signal in m/z 29 has an onset below that of the m/z 45 peak assigned to formamide. This may point to small contributions of the lighter product formaldehyde. However, its amount must be small because a corresponding signal in the m/ z 30 data is not apparent. Radical recombination reactions depend on two independent electron-induced molecular fragmentation events and tend to be rather unspecific. Therefore, a mixture of formamide and the products shown in Figure 8 would be the most likely outcome. However, this is not the situation encountered here. In fact, DEA leads in particular to formation of formamide. Such a more specific outcome can be rationalized by assuming that formation of the product is initiated by a single DEA event yielding H• or NH2•. These radicals then trigger a reaction sequence that only involves further intact parent molecules, CO and NH3, that are abundantly available in the vicinity of the radical. The resulting reaction mechanism, including the formation of side products, is depicted in Figure 9. The energetics of these reaction sequences has furthermore been studied by quantum chemical calculations as summarized in Figure 10. The process leading to the final product starts by the attachment of an electron to NH3. The relevant electron attachment resonance (Figure 7) is centered around 10.5 eV in the gas phase but because of the width of EA resonances in general this amounts to a possible attachment of the electron between about 9 and 12 eV in the gas phase.44 In the solid state studied here, we observe an onset of product formation at around 7 eV, implying a stabilization of the ion of about 2 eV, which is in line with typical values for the solid phase.19,29,30 On the basis of these considerations, we discuss the quantum chemical predictions on product formation via DEA based on an impinging electron having E0 = 8 eV. The NH3•− radical anion decays by rupture of a NH bond with the charge on one particle and the radical site on the other. In line with general rules on the energy distribution between two fragments,45 measurements of the kinetic energy have shown that the H• radical or H− anion carries away up to 5 eV of kinetic energy that is almost the entire excess energy.44 With a calculated threshold of 4.2 eV for the formation of NH2− and H•, this means that upon electron attachment at E0 = 8 eV, as applied in the present experiment, the H• radical will have a kinetic energy of up to 3.6 eV (shown as the striped bar in Figure 10, right). Equally, the calculated threshold of 4.0 eV for formation of NH2• and H− implies a kinetic energy of up to 0.2 eV (striped bar in Figure 10, left) for NH2•. This energy enables the fragments to move through the otherwise frozen film and to overcome potential activation barriers. As can be seen in Figure 10, the energetic H• radical contributes ample excess energy to reactions proceeding via the HCO• radical (Figure 10, right). This helps to easily overcome the activation barrier to the final product. In contrast, significantly less energy
via both DEA and electron impact ionization. This discussion is supported by quantum chemical calculations of the reaction steps following the initiating electron−molecule interaction. 3.4.1. Formation of Formamide via DEA to NH3. The fragmentation pathways initiated by DEA to NH3 (Figure 7) open up two possible reactions leading to formation of formamide. Radicals formed via DEA have an unpaired electron and are thus electrophiles that can attack the electron-rich multiple bond of CO. The intermediate state of the reaction between the NH2• radical and CO would be the H2NC•O radical (Figure 9). The hydrogen radical H• would react with CO to yield the intermediate formyl radical HC•O. In the present study, only species that desorb upon temperature increase after electron irradiation are detected. Reactive intermediates tend to react rather than desorb intact; the presence of neither intermediate radical can thus be confirmed experimentally. It should be noted, however, that HCO has been observed by in situ IR spectroscopy in a similar experiment albeit the fact that it was performed with higher E0.14 Product formation following DEA in condensed phase is often a consequence of radical recombination reactions.18,19 In the present experiment, H• and NH2• as immediate products of DEA as well as HC•O and H2NC•O resulting from their reaction with CO are the expected radicals produced in the mixed layers of NH3 and CO as studied here. The anticipated recombination products beside formamide are summarized in Figure 8. Here, formaldehyde (Figure 8a)
Figure 8. Possible side products to the formation of formamide resulting from recombination of the radicals H• and NH2• produced by DEA and the radicals HC•O and H2NC•O formed by reaction of the former with CO: (a) formaldehyde, (b) urea, (c) glyoxal, and (d) ethanediamide. Characteristic EI MS fragments of these compounds are listed in Table 2.
would result from recombination of HC•O with a second H• radical while recombination of H2NC•O with NH2• would yield urea (Figure 8b). Furthermore, dimerization of HC•O and H2NC•O should lead to production of glyoxal (Figure 8c) and ethanediamide (Figure 8d). The relevant mass spectrometric signals of these compounds are listed in Table 2. Except for formaldehyde, all of these products are expected to have a higher desorption temperature than formamide. Among those, both urea and ethanediamide have a mass spectrometric signal at m/z 17. However, a desorption signal for this mass at higher temperature is not obvious (Figure 3) even when the data are enlarged (not shown). Evidence is less clear for glyoxal as its characteristic signals at m/z 29 and 30 may be hidden in the isotope signals relating to CO
Table 2. Electron Impact Ionization Mass Spectrometric Data for Possible Side Products of the Formation of Formamide
formaldehyde (H2CO) urea ((H2N)2CO) glyoxal (OCHCHO) ethanediamide (H2NCOCONH2)
m/z 17
m/z 28
m/z 29
m/z 30
m/z 45
source
0 100 0 16
24 4 36 4
100 3 100 4
58 1 55 0
0 1 0 45
27 27 27 27
55
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is available to reactions proceeding via the H2NCO• radical due to lower kinetic energy of the NH2• radical (Figure 10, left). The calculated energetics (Figure 10) support the notion that formamide is indeed formed predominantly via the HCO• radical with possible smaller contributions from the H2NCO• route. The formation of the side product urea is energetically unfavorable due to the higher-lying transition state, whereas the formation of formaldehyde is possible but leads to a less stable final situation than formation of formamide. To our knowledge, this is the first time that a reaction leading to a larger product is proposed to be triggered by EA to NH3. In particular, the reaction between NH3 and ethylene studied earlier was found to proceed only via electron impact ionization.21 A closer examination of previous data,24 however, reveals that due to the steep slope of the onset of the reaction with E0, which is caused by rather large EI cross sections of both reaction partners, the presence of the resonance could simply have been masked. In the case of formamide presented here, the reaction sets in much more gently, because the threshold of EI to CO is located several electronvolts higher than the threshold for NH3, so the contribution of the resonant process to the formamide yield is visible here. 3.4.2. Formation of Formamide Initiated by EI. At energies above 10 eV it is assumed that in addition to the reaction proceeding via electron attachment to NH3, a second reaction pathway is available. This would proceed by electron impact ionization of NH3 and at energies above 13−14 eV also by impact ionization of CO. Both reactants can be considered nucleophiles, so a direct reaction would need to proceed via a transition state in which the free electron pair of the NH3 is attached to the electron-rich multiple bond system of CO. This is hindered by electrostatic repulsion. The ionization of one of the reaction partners turns the electron-rich nucleophile into a cation, which is an electrophile. This eliminates the energy barrier for the exothermic reaction of CO and NH3 to
Figure 9. Proposed reaction pathway for the formation of formamide and the two side products formaldehyde and urea. Reaction is triggered by the resonant attachment of an electron to NH3 at an experimentally observed E0 = 8 eV. The formed radical anion decays into either an H• or NH2• radical, which carries away a part of the excess energy and which in turn attaches to CO. The formed intermediate radicals HCO• or H2NCO• then react with NH3 to yield either formamide or one of two possible side products, formaldehyde or urea. Energetics of the reaction pathways are found in Figure 10 and indicate that the path via HCO• is energetically favorable. Radical localization on the transition states was deduced from the spin density calculated at the B3LYP/6-311++G(d,p) level of theory.
Figure 10. Energetics of the proposed reaction pathways leading to formation of formamide and possible side-products formaldehyde and urea. Attachment of an electron with arbitrarily chosen E0 = 8 eV to NH3 yields the radical anion (middle) that subsequently dissociates to yield NH2• (left) or H• (right). In line with previous gas-phase results,44 the available excess energy can partially or fully convert into kinetic energy of the radicals. The possible range of these kinetic energies is represented by the striped bars. This excess energy helps to overcome the barriers (transition states, TS) between the intermediate radicals formed by reaction of NH2• (left) or H• (right) with CO and the final products. All energies except for that of NH3•−, which was arbitrarily set to 8 eV above the ground state of NH3, were calculated at the B3LYP/6-311++G(d,p) level of theory. 56
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ACS Earth and Space Chemistry formamide (Figure 11). A similar enhancement of the reaction was observed in the case of the reaction between ethylene and NH3 to yield ethylamine21 and between ethylene and H2O leading to ethanol.25
Figure 12. Energetics of the proposed reaction pathway leading to formamide by electron impact ionization. After the initial ionization step at either NH3 or CO, the reaction proceeds downhill yielding the H2NCO• radical. For formation of neutral formamide from the radical, a proton and an electron need to be captured. The calculated energetics show that the capture of an electron by the H2NCO• radical and a subsequent acid−base type protonation is energetically more favorable than the protonation of the radical and subsequent capture of an electron. All energies were calculated at the B3LYP/6-311+ +G(d,p) level of theory.
Figure 11. Proposed reaction mechanism for the formation of formamide triggered by electron impact ionization to NH3 or carbon monoxide and subsequent reaction with the electron-rich reaction partner in a nucleophile−electrophile type reaction. The ionization of CO requires higher electron energies E0 than ionization of NH3. Fractions of e in the resonant electron structures refer to the charge distribution in the molecule as taken from density functional theory calculations performed at the B3LYP/6-311++G(d,p) level of theory. The intermediate radical cation H3NCO•+ will protonate an NH3 moiety, leading to separation of charge and radical site. The neutralization via the capture of a thermalized electron can occur either before or after proton transfer. Energetics of the reaction (Figure 12) show that capture of an electron before proton transfer is favorable.
excess energy deposited in the reaction system by ionization of one of the reactants is sufficient to drive the whole sequence of reaction steps all the way to the final product formamide thus supporting the mechanism proposed in Figure 11.
4. CONCLUSIONS Formamide was observed to form during electron exposures of mixed condensed films of CO and NH3 at cryogenic temperatures. The E0 dependence of the formation was studied and from it two different reaction mechanisms in different energy regimes were deduced. It was found that below the ionization threshold of the reactants a resonant process centered around 9 eV occurs, which we attribute to an electron attachment to NH3. The formation of the transient negative radical ion NH3•− triggers a reaction sequence at the end of which formamide is formed. At energies above the ionization threshold, we also found the production of formamide. The reaction here proceeds via the electron impact ionization of either starting molecule and a subsequent addition of the electrophilic radical cation to a nucleophilic neutral molecule. H atom migration and the capture of a thermalized electron then lead to the final product formamide. Both reaction mechanisms are examples of an atom efficient synthesis in which all atoms from the starting molecules are found in the product. The explanation of energy dependence of the reaction does not need a reaction triggered by neutral dissociation of the starting molecules, as described earlier.14 While it is possible that there is a contribution to the overall yield at higher energies, the reaction mechanism found here closely resembles that of the reaction between NH3 and olefins21,24 and between olefins and H2O.25
After the initial step of adding NH3 to CO, migration of an H atom and neutralization by capture of a thermalized electron must take place in order to finally lead to formamide. Unlike in the addition of H2O or NH3 to olefins, here the migration step is not a 1,3-migration but rather a 1,2-migration. Thus, the direct migration of an H atom within the H3NCO•+ radical cation is sterically not favorable. According to our quantum chemical calculations, a proton rather transfers to an adjacent NH3 leading to separation of charge and radical site (Figure 12). Subsequently, reprotonation of the H2NCO moiety could happen before or after capture of a thermal electron. Here, calculations show that capture of an electron and the formation of an H2NCO− anion leads to an immediate acid−base type proton transfer. Both of these reaction steps are exothermic. The reprotonation of the H2NCO• radical on the other hand is associated with an energy barrier of around 1.5 eV, rendering this reaction path unlikely. However, in any case the initial 57
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. ORCID
Jan H. Bredehöft: 0000-0002-7977-6762 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding
The work presented here was found by the Deutsche Forschungsgemeinschaft DFG under project Sw26/15-1 “Atom-efficient mechanisms of electron-induced chemical synthesis”. Notes
The authors declare no competing financial interest.
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