Electron-Induced Formation of Ethyl Methyl Ether in Condensed

Article Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX

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Electron-Induced Formation of Ethyl Methyl Ether in Condensed Mixtures of Methanol and Ethylene Fabian Schmidt, Petra Swiderek, and Jan H. Bredehöft* University of Bremen, Institute of Applied and Physical Chemistry, Fachbereich 2 (Chemie/Biologie), Leobener Straße/NW 2, Postfach 330440, D-28334 Bremen, Germany

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S Supporting Information *

ABSTRACT: Electron-induced reactions in condensed mixtures of ethylene (C2H4) and methanol (CH3OH) lead to the formation of ethyl methyl ether (EME, C2H5OCH3), as shown by post-irradiation thermal desorption spectrometry (TDS). In contrast to the electron-induced reaction between water (H2O) and C2H4, product formation as a consequence of proton transfer following electron attachment (EA) to C2H4 is not observed in the analogous reaction between CH3OH and C2H4. However, a resonant process centered around 5.5 eV and a threshold-type increase of product yield starting at 9 eV is observed. On the basis of the presence and absence of particular side products after irradiation of the mixture as well as of the pure parent compounds, reaction mechanisms related to the two energy regimes are proposed. Below the ionization threshold of the reactants, dissociative electron attachment (DEA) to CH3OH triggers the reaction sequence by producing reactive methoxy radicals, which attack neighboring C2H4 molecules. The resulting adduct then abstracts a hydrogen atom to yield EME. Above but near the ionization threshold, electron impact ionization (EI) produces primarily intact molecular cations, which drive the reaction by converting the repulsive Coulomb force between the high electron densities at the reactive sites of the two neutral parent species into an attractive force. This again induces the formation of an adduct between the two reactants that rearranges to the product EME. Fragmentation of the molecular CH3OH+• cation into CH3O+, however, may provide an additional reaction pathway toward EME. In this scenario, CH3O+ attacks a neighboring C2H4 molecule. The resulting adduct is then neutralized by a thermalized electron and abstracts a hydrogen atom from a nearby CH3OH molecule to yield EME.

1. INTRODUCTION Electron irradiation can induce chemical transformations, which include radiation damage in DNA strands,1,2 the degradation of organometallic precursors in focused electron beam induced deposition (FEBID),3 and chemical reactions occurring inside interstellar ices.4,5 Although electron-induced processes are usually associated with their destructive nature, electrons can also induce the formation of more complex molecules from smaller entities, which we refer to as electron-induced synthesis.6 A profound understanding of such electron− molecule interactions is desired in technical processes like FEBID where electron irradiation leads not only to the deposition of chemically well-defined nanostructures but also to the formation of undesired contaminants.3,7 Moreover, the role of low-energy secondary electrons is now thoroughly discussed in the field of astrochemistry for initiating chemical transformations on interstellar dust particles.4,5,8 The reason for this is that copious numbers of secondary electrons are produced when electromagnetic or particle radiation interacts with condensed matter.9 Electron-induced synthesis can be triggered by different processes, namely, electron attachment (EA), electronic excitation of a reactant, and electron impact ionization (EI).6 When an electron is attached to a molecule, a transient negative ion (TNI) is formed. The TNI often decays by producing anions © XXXX American Chemical Society

and neutral radicals in a process called dissociative electron attachment (DEA), which can be driven by the antibonding character of the molecular orbital housing the additional electron and/or a positive electron affinity of one of the fragments. However, in the condensed phase, the TNI may be stabilized by the matrix and thus survive.6,9 Such EA and DEA processes are particularly prominent in the energy regime below the ionization potential(s) of the reactant(s) and are observed in well-defined and narrow energy ranges called resonances. Following electronic excitation of the parent compound into a repulsive state, neutral dissociation (ND) produces neutral radicals. ND occurs above the electronic excitation energy of the parent compounds and shows a threshold behavior. Usually, the role of ND often remains unclear because it is difficult to monitor this process.10 However, Zlatar et al.11 showed, using the compound Pt(PF3)4, that electron-induced fragmentation via ND may be more relevant than DEA. Finally, above but near the ionization threshold(s) of the reactant(s), EI produces radical cations and is usually more important than DEA. Similar to ND, EI shows a threshold behavior and the total ion yield thus increases steadily with electron energy.6,9,12 Received: October 19, 2018 Revised: December 5, 2018 Published: December 11, 2018 A

DOI: 10.1021/acs.jpca.8b10209 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

mixtures of methanol (CH3OH, Merck, ≥99.9%) and ethylene (C2H4, Air Liquide, 99.95%) was investigated by TDS. All experiments were performed in an ultrahigh vacuum chamber evacuated to a base pressure of 10−10 mbar by a turbomolecular pump. The condensed mixtures were prepared by leaking a defined amount of the premixed gases from a gas handling manifold into the vacuum chamber where the gases condense on a polycrystalline Ta substrate held at ∼30 K. The amounts of gases leaked into the chamber were determined by monitoring the pressure drop in the gas handling manifold with a MKS Baratron type 622B capacitance manometer. A film thickness of the C2H4/CH3OH mixtures corresponding to 7−13 monolayers was prepared, which was estimated by observing at which pressure drop the transitions from monolayer to multilayer desorption signals occur. The stoichiometric ratios of the reactants were controlled by the composition of the gas mixture in the gas handling manifold. This was achieved by leaking defined amounts of each compound from a reservoir into the gas handling manifold. This procedure, however, assumes equal sticking coefficients of the two compounds. Therefore, the stoichiometric ratio of the reactants was further reviewed by comparing the mass spectrometric peak areas of the reactants in the TDS, taking into account the partial cross sections of the monitored fragments for electron impact ionization.26 The results revealed that the true ratio of the compounds in the cryogenic mixture differs from that in the gas handling manifold in favor of C2H4. A true 1:1 ratio in the cryogenic mixture was obtained when both reactants were premixed as a C2H4/ CH3OH (1:1.4) mixture in the gas handling manifold. Exposures were carried out with a commercial STAIB NEK150-1 electron source with an energy resolution of 0.5 eV. Neutral species that desorbed during irradiation were monitored using a quadrupole mass spectrometer (QMS) residual gas analyzer (Stanford Research System RGA 200). The QMS has an electron-impact ion source operating with electrons of energy E0 = 70 eV. After irradiation, the remaining film was desorbed upon resistive heating of the substrate with a heating rate of 1 K/ s while monitoring four mass-over-charge ratios simultaneously. After each experiment, the substrate temperature was held at 450 K for 2 min in order to desorb remaining species on the sample holder, which warms up more slowly than the substrate. We never observed that the peak areas deviate by more than 10% when the same experiment was repeated. Therefore, we estimate the error of the product yield to be 10% with respect to the absolute value. 2.2. Chemical Synthesis. EME was synthesized in a Williamson ether synthesis using methyl iodide (CH3I, Acros Organics, 99%, Cu stabilized) and sodium ethoxide prepared by dissolving sodium (Na, Merck) in absolute ethanol (C2H5OH, VWR Chemicals, 99.94%). The gaseous product was isolated by condensing it in a cold trap immersed into ethanol that was cooled down to −35 °C by liquid nitrogen. Mass spectra of the headspace (Supporting Information, Figure S1) of the colorless liquid give evidence that it is indeed EME and that it could be used for our purposes without further purification. 2.3. Calculations. DFT calculations of the energetics of neutral and radical species were performed at the B3LYP/6311++G(d,p) level of theory using the Gaussian 09 package27 with the RB3LYP and UB3LYP methods for closed shell and open shell species, respectively. The Berny algorithm was used for geometry optimizations, and optimized geometries were verified by performing a frequency analysis. In order to compare mass spectrometric signals of different compounds quantita-

In recent case studies, we investigated the reactions between water (H2O) and ethylene (C2H4) as well as those between ammonia (NH3) and C2H4 upon electron irradiation of condensed mixtures of these compounds.13,14 At electron energies above the ionization thresholds, we observed the formation of ethanol and ethylamine, respectively. To explain the formation of these products, we proposed a reaction sequence where one of the reactants is ionized, thus lowering the activation barrier for the reaction. In C2H4/H2O mixtures (1:1), we further found another reaction pathway leading to formation of ethanol at lower energies that is triggered by nondissociative EA to C2H4 followed by a proton transfer from H2O to the TNI C2H4•−. These reactions may serve as proof of principle, providing simple but powerful reactivity concepts capable of explaining the pathway from small ubiquitous molecules in space toward chemical complexity. Moreover, they offer a way of controlling reaction routes and the outcome of a chemical reaction by simply tuning the electron energy. However, the question is, whether these reaction concepts can be transferred to systems with larger molecules. To this end, we already investigated the reactions between C2H4 and ethylamine, C2H4 and diethylamine, and propene and NH3.15 We could show that the same reaction occurs in the case of these more complex derivatives, which emphasizes the general reaction concept. The aim of the present study is to reveal whether the same electron-induced reactions that occur between H2O and C2H4 are also efficient in the case of alcohols and C2H4. Therefore, the formation of ethyl methyl ether (EME) in condensed mixtures of C2H4 and CH3OH was studied by post-irradiation thermal desorption spectrometry (TDS). To ensure that no significant amounts of products are desorbing during irradiation, we further recorded electron-stimulated desorption (ESD) spectra. However, we never observed that EME desorbs in any of the samples during irradiation. In an ESD experiment, excited neutral species that are desorbing during electron irradiation of the sample are monitored with a mass spectrometer. These kinds of experiments have frequently been applied to monitor the formation of negative or positive ion fragments in the condensed phase as a function of electron energy to discriminate among DEA, EI, and other electron−molecule interactions.16−20 ESD experiments can also be performed at constant electron energies, which provide additional information about reaction kinetics.21 Unfortunately, ESD is limited to comparatively small and volatile molecules due to the lower desorption efficiencies of larger molecules. Thus, TDS experiments after irradiation usually provide deeper insights into the chemical transformations. In a TDS experiment, the sample is heated with a well-defined rate of temperature increase while continuously monitoring desorbing species with a mass spectrometer. The desorption behavior is described in detail by the Polanyi− Wigner equation and depends, e.g., on the activation energy for desorption, the heating rate, and the surface coverage.22 When going from pure substances to mixtures, desorption behavior may also depend on the molecular environment.23,24 TDS thus allows one to distinguish desorbing species not only by their mass-over-charge ratios but also by their desorption temperature. In combination, ESD and TDS experiments have been shown to be a suitable method for studying electron-induced chemical reactions.25

2. EXPERIMENTAL SECTION 2.1. Post-Irradiation Thermal Desorption Spectrometry. The electron-induced formation of EME in condensed B


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The Journal of Physical Chemistry A tively, we usually use electron impact ionization cross sections that are published in the NIST database.28 However, no such values are available for CH3OH. Therefore, we calculated cross sections for electron impact ionization. These values were obtained via the binary-encounter-Bethe (BEB) model,29,30 utilizing the GAMESS software package31,32 for the calculation of orbital properties. For electron impact energies of 70 eV, we obtained total ionization cross sections of 5.26 Å2 for CH3OH and 5.03 Å2 for C2H4. Thus, the BEB cross section for C2H4 from the NIST database (5.115 Å2) differs slightly from the one we calculated. We ascribe this small difference to the different basis sets from which these two values were obtained.30

3. RESULTS AND DISCUSSION 3.1. Identification of Ethyl Methyl Ether. Motivated by the identification of ethanol after irradiating mixtures of C2H4 and H2O with low-energy electrons,13 we replaced H2O by CH3OH to check whether the same reaction principles can be transferred to larger molecules. As a proof of concept, we prepared condensed mixtures containing equal amounts of C2H4 and CH3OH and irradiated these with low-energy electrons (E0 = 16 eV). After irradiation, depletion of the reactants and the formation of new products can be observed by TDS (Figure 1). Depletion of C2H4, which desorbs at 60 K, was studied by monitoring the m/z = 27 [M − H]+• fragment instead of the m/z = 28 molecular ion M+• to avoid overlaps with signals arising from the m/z = 28 fragment of CH3OH and the possible formation of CO. CH3OH, which desorbs at 125 K, was monitored by observing the m/z = 31 fragment, which is the dominant signal in its mass spectrum (Supporting Information, Table S1).33 To give evidence for the formation of the anticipated product EME, we monitored mass-over-charge ratios m/z = 45 and 60 during TDS, because these are the characteristic ions in the mass spectrum of EME (Supporting Information, Figure S2). After irradiation, we observed two new signals, the first of which is located at a desorption temperature of ∼100 K, whereas the second signal is located at ∼160 K (Figure 1). The first signal, located at 100 K, is assigned to EME. The assignment was verified by mixing small amounts of pure EME with CH3OH. Both the peak area ratios of the m/z = 45 and m/z = 60 signals as well as the desorption temperature of the product seen in Figure 1 are in accord with TDS data of the mixed films of EME and CH3OH (Figure 2) and thus corroborate the assignment. There is a small desorption feature at ∼100 K in the m/z = 46 curve (Figure 1), a signal that is high in intensity in the mass spectrum of dimethyl ether33 but almost absent in the mass spectrum of pure EME (Supporting Information, Table S2). While the ratio between the m/z = 46 and m/z = 45 signals is about 0.025 in pure EME, the same ratio is about 0.15 in the sample that was irradiated at 16 eV. Therefore, we suppose that, besides EME, small amounts of dimethyl ether have formed as a side product during irradiation. Ethanol, which equally shows a m/z = 46 signal (Supporting Information, Table S2), is excluded because it desorbs at a significantly higher temperature.13 Because their desorption peaks overlap, dimethyl ether also contributes to the overall signal at m/z = 45 to some extent. Although the signal at around 160 K is present in the irradiated sample as well as in the reference mixture (Figure 2), we exclude that this second desorption signal in the m/z = 45 curve originates from EME. The similarity, however, is only visible in the m/z = 45 signal but not in any other fragment of EME. Furthermore, this second desorption signal is only

Figure 1. Thermal desorption spectrum before and after electron exposure of a multilayer mixture of CH3OH and C2H4 (1:1) with 250 μC/cm2 at an energy of E0 = 16 eV. Depletion of reactants is deduced from the signal decrease for CH3OH (m/z = 31) at 125 K and C2H4 (m/z = 27) at 60 K, which can be seen more clearly at higher exposures (Supporting Information, Figure S1). Signals at m/z = 45, 46, and 60 after electron irradiation that are absent in the nonirradiated sample give evidence that new products have formed.

present, when EME is formed at electron energies above the ionization energies (Figure 2, bottom spectrum) of the parent compounds, whereas it is absent when EME is formed at lower energies (Figure S3). This makes it unlikely that this second desorption signal originates from monolayer desorption or matrix effects. Our tentative conclusion is that the signal at ∼160 K in the reference mixture stems from ethanol as an impurity, which is the solvent in the ether synthesis. Thus, it is likely that small amounts of ethanol vapors were co-condensed during isolation of the gaseous ethyl methyl ether. In the case of the irradiated sample, we assign the TDS signal at 160 K to a side product of the reaction. However, we decided to not assign a specific side product to this signal. This is because we also observed signals at m/z = 41, 42, 43, 54, 56, and 57 in this temperature range (not shown) which are associated with several side products like butadiene and hexane formed during irradiation of pure C2H4.15 Therefore, we cannot exclude that the m/z = 45 and 60 signals may also result from two or more different species desorbing in this temperature range which makes an unambiguous assignment of the 160 K signal a difficult task. 3.2. Energy Dependence of Ethyl Methyl Ether Production. The energy dependence of the product yield for EME was investigated. In order to be able to compare product C


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are no longer negligible, which in turn makes it difficult to compare product yields obtained for different electron energies. With increasing electron energy, the reaction rate usually increases and product yields at electron energies much higher than 15 eV may not behave linearly at exposures of 300 μC/cm2. However, we assume that deviations from this linear behavior are negligible in the energy range up to 18 eV so we can compare product yields obtained after exposures of 250 μC/cm2 while achieving high product yields. The energy dependence of the product yield obtained after this exposure shows a threshold for the formation of EME located at around 9−10 eV (Figure 3b). With increasing electron energy, the product yield then increases steadily, indicating a process triggered by EI. The ionization thresholds for the parent compounds in the gas phase are IECH3OH = 10.8 eV and IEC2H4 = 10.5 eV,33 but in condensed phase, these are usually lowered by 1−2 eV, due to stabilization of the ion by polarization of the surrounding medium.9 In contrast to the electron-induced hydration of C2H4,13 no product formation was observed in the energy region below 9 eV. In the case of C2H4 and H2O, however, irradiations were carried out with higher exposures of 600 μC/cm2. Therefore, EME might not be detected with exposures of only 250 μC/cm2 if it is formed in minor amounts. In a second set of experiments, electron irradiation was repeated in the energy range from 2.5 to 10 eV but with higher exposures of 8000 μC/cm2 (Figure 4). Now, at these higher exposures, formation of EME in the lower energy range can be observed. The product yield in the energy range from 2.5 to 10 eV shows a resonant-like behavior with maxima at 5.5 and 8 eV, thus indicating that EA is involved in the formation process (Figure 4b). At electron energies corresponding to the higher resonance, EI may also start to contribute to the overall product yield. Thus, for investigations concerning the reaction mechanism below the ionization threshold, we focused on the lower energy resonance at 5.5 eV to be able to rule out any processes triggered by EI. We suggest that, below the ionization threshold, formation of EME at exposures of 250 μC/cm2 was not observed due to the low product yield. This low product yield can be illustrated by plotting the product yields for

Figure 2. Thermal desorption spectrum after electron exposure of a multilayer mixture of CH3OH and C2H4 (1:1) with 250 μC/cm2 at an energy of E0 = 16 eV. Two new signals at m/z = 45 are present after irradiation (bottom spectrum), among which the signal at lower temperatures was assigned to EME. The assignment was verified by mixing small amounts of pure EME with CH3OH (upper spectra). The small desorption feature at 160 K in the m/z = 45 mass trace of the EME (upper spectra) is an impurity in the synthesized ether, whereas the signal at 160 K in the irradiated film (bottom spectrum) could not be assigned yet.

yields, it is important to ensure that the approximation of initial rates is valid, i.e., that product formation is not limited by depletion of the starting materials or product degradation. To this end, the dependence of yields on electron exposure was measured. Experiments performed at electron energies of 15 eV show a linear increase of product yield with increasing exposure in the range from 0 to 300 μC/cm2 (Figure 3a). At higher exposures, the product yield shows a saturation behavior indicating that film depletion, product degradation, or both

Figure 3. Peak areas for the m/z = 45 EME signal obtained by TDS before and after electron irradiation of mixed multilayer films of CH3OH and C2H4 (1:1). Error bars denote the estimated error for peak area and energy spread. (a) Dependence of product yield on exposure at E0 = 15 eV and (b) dependence of product yield on incident electron energy E0 after irradiation with 250 μC/cm2. The y-axis has the same scale as that in Figure 4; thus, product yields can be compared directly. D


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Figure 4. Peak areas for the m/z = 45 EME signal were obtained by TDS before and after electron irradiation of mixed multilayer films of CH3OH and C2H4 (1:1). Error bars denote the estimated error for peak area and energy spread. (a) Dependence of product yield on exposure at E0 = 5 eV with linear fit. The intersection point of the dashed vertical and horizontal lines represents the product yield expected after an exposure of 250 μC/cm2, as derived from the fit parameters. (b) Dependence of product yield on incident electron energy E0 after irradiation with 8000 μC/cm2. The y-axis has the same scale as that in Figure 3; thus, product yields can be compared directly.

different exposures at energies of 5 eV (Figure 4a). The intersection point of the dashed vertical and horizontal lines in Figure 4a marks the product yield expected after exposure with 250 μC/cm2 that is clearly below the noise level and thus the detection limit of our setup. Experiments performed with different exposures at 5 eV show a linear behavior of the product yield, even at values as high as 16 000 μC/cm2, suggesting that film depletion and product degradation are negligible during irradiation. 3.3. Reaction Mechanisms below the Ionization Threshold. The electron-induced formation of ethanol13 from C2H4 and H2O can be triggered by nondissociative EA to C2H4, which has a low-lying resonance at 1.5 eV in the gas phase.34 Subsequently, a proton is transferred from H2O to C2H4− to form C2H5•, which drives the reaction. By replacing H2O with CH3OH, the analogous reaction would end up with EME, which was identified by TDS after electron exposure at 5 eV. In contrast to the formation of ethanol from C2H4 and H2O,13 experiments performed at different E0 values (Figure 3b and Figure 4b) revealed that the product yield of EME does not increase when decreasing E0 in the range from 4 to 2 eV. This indicates that the analogous nondissociative EA to C2H4 and subsequent capture of a proton from an adjacent CH3OH molecule do not play a role in the formation of EME. At energies above 6 eV, dissociative EA to C2H4 might occur, which is known to produce H−, CH−, CH2−, and C2H− fragments in the gas phase35,36 as well as H−, CH−, and CH2− fragments in the condensed phase17 (Table 1). However, there is no obvious formation route to EME starting from any of these fragments. We thus conclude that a process involving EA to C2H4 is not a likely initial step for the observed formation of EME. A much more simple and more intuitive starting point for the formation of EME is dissociative EA to CH3OH, which produces CH3O• and H−. Our assignment of the observed resonance at 5.5 eV to this DEA channel yielding H− is in accord with crossed beam experiments performed in the gas phase37 and ESD experiments in the condensed phase38 (Table 1) when considering that polarization forces in the condensed phase can shift peak positions by about 1−2 eV to lower energies.9

Table 1. Anions and Peak Positions in the Gas and Condensed Phase Observed after DEA to CH3OH and C2H4, Respectively peak positions (eV) fragment methanol (CH3OH)

ethylene (C2H4)

H−

O−/OH− CH3O− CH− CH2− CH3− H− CH− CH2− C2H−

gas phase

condensed phase

6.4a 7.9a 10.2a 10.3a 3.0d,e

6.0b,c 7.8c 8.0d 11.0c,f 7.5c,f 7.5c,f

7.6g 7.9h 8.8h 8.5h

10.0i 10.6i

a

Reference 37. bShoulder. cReference 38. dReference 42. eReference 43. fWas assigned to reactive scattering of O−. gReference 35. h Reference 36. iReference 17.

In the gas phase, experiments with deuterated methanol species39,40 suggest that H− in the case of this resonance stems solely from the −OH group and not from the −CH3 group. In the condensed phase, however, ESD data on deuterated species by Parenteau et al.41 suggest that the H− stems from the −OH group as well as from the −CH3 group. At higher energies, other resonances with different products, which result from the reactive scattering of O−, are known (Table 1).38 It is, however, not obvious how EME can be formed by the reaction of O− with C2H4. On the other side, a resonance that yields CH3O− is located at considerably lower energies than the 5.5 eV observed here (Table 1)42,43 and thus obviously does not play a role in the observed formation of EME. Our hypothesis is that the reaction proceeds via the addition of CH3O• to the electron-rich double bond of C2H4 to form the 2-methoxyethyl radical CH3OC2H4• as an intermediate that can abstract a hydrogen atom from an adjacent molecule to form the E


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(short-lived) intermediates, which are not directly accessible by post-irradiation analysis. We want to emphasize that the proposed reaction mechanism in Scheme 1 is similar to that of a free radical polymerization in which CH3OH acts as an initiator by producing CH3O• radicals. On the other side, C2H4 acts as a monomer unit. Inspired by this analogy, we searched for side products that result from radical recombination, chain propagation, and radical transfer reactions, which are all typical reactions in free radical polymerizations.46 In doing so, we need to keep in mind that HOCH2• radicals might also be present in the film and participate in these types of reactions. HOCH2• radicals are produced directly by dissociation of the TNI41 and possibly by intra- or intermolecular conversion of CH3O• into HOCH2•. This conversion was observed experimentally at 77 K by Toriyama and Iwasaki47,48 and was proposed to occur even at temperatures as low as 6 K.49 The formation of side products at incident electron energies of 5.5 eV was investigated with post-irradiation TDS. Upon heating of the sample, different mass-over-charge ratios were monitored, which were chosen on the basis of published mass spectra.33,50 Whenever possible, we monitored mass-overcharge ratios that are not present in the mass spectra of the parent compounds but characteristic and dominant in the spectra of the anticipated products (Supporting Information, Table S1 and Table S2) to avoid overlap with the much more intense signals of the parent compounds. In the present study, we did not find any evidence for the formation of ethylene glycol (HOC2H4OH), methoxymethanol (HOCH2OCH3), or dimethyl peroxide (CH3OOCH3) (data not shown) at electron energies of 5.5 eV that would form upon recombination of HOCH2• and CH3O• radicals. We neither find any evidence for the formation of dimethoxy ethane (CH3O− C2H4−OCH3) that would form upon recombination of the CH3OC2H4• intermediate with a CH3O• radical. We ascribe the absence of any of these radical recombination products to the low radical densities in the film compared with the higher densities of CH3OH and C2H4 that favor radical−molecule interactions.

Scheme 1. Proposed Reaction Pathway for the Formation of EME during Electron Irradiation at 5.5 eV in Condensed Multilayer Mixtures of C2H4 and CH3OH (1:1)a

After EA to CH3OH, the TNI dissociates into CH3O• that can add to C2H4 to form an intermediate CH3OC2H4• radical. This intermediate can react with CH3OH to form EME. a

observed product (Scheme 1). Our DFT calculations show that the reaction between a CH3OC2H4• radical and C2H4 is endothermic by +0.36 eV. In contrast, the reaction of CH3OC2H4• with CH3OH to form EME and HOCH2• or CH3O• is exothermic by −0.31 and −0.03 eV, respectively. For a true prediction of whether the reaction proceeds spontaneously, the change in Gibbs free energy ΔG would have to be known, rather than the change in internal energy ΔU, which we calculated. They are related by ΔG = ΔU + pΔV − TΔS; the pΔV term is negligible, since no pressure−volume work is done by the system. The TΔS is likewise extremely small, since differences in entropies S0 between small molecules and their respective radicals are small.44 In addition, the reaction proceeds at relative low temperatures; consequently, the Gibbs free energy is mainly affected by the changes in internal energy of the reaction, which makes the reaction between CH3OC2H4 and CH3OH the more plausible scenario. Any proposed reaction mechanisms are usually elaborated by careful study of side products of the reaction.45 The structural motifs present in these side products give valuable hints at

Figure 5. (a) Thermal desorption spectra of a multilayer mixture of C2H4 and CH3OH (1:1) before and after an electron exposure of 8000 μC/cm2 at 5.5 eV, upon which two new signals at m/z = 29 and m/z = 30 can be observed that were assigned to C2H6 and CH2O. (b) Assignment of the signal at 88 K to CH2O was verified by TDS of a reference sample prepared by mixing small amounts of pure CH2O with CH3OH (4:100). The signal of CH2O in the reference sample is located at the same temperature as the signal in the irradiated sample, which confirms this assignment. F


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characteristic m/z = 56 (Supporting Information, Table S2) curve can be seen in the TDS (data not shown). On the basis of the intensity ratios in Figure 6, however, we cannot exclude the formation of iso-butane because the mass spectra of iso-butane and n-butane are very similar for the m/z = 41, 42, and 43 signals (Supporting Information, Table S2). We notice that neither C2H6 nor n-butane is formed when irradiating pure C2H4 at 5.5 eV, which can be deduced from the absence of signals at m/z = 30 and 43 (Figure 7a) which are characteristic of C2H6 and n-butane (Supporting Information, Table S2). This implies that the additional hydrogen atoms must come from CH3OH. Support for this scenario is given by the fact that CH2O is not formed when pure CH3OH is irradiated at E0 = 5.5 eV (Figure 7b), suggesting that the formation of these products is linked to each other. We propose that the formation of CH2O, C2H6, and n-butane is triggered by CH3O• radicals according to Scheme 2. These reactions are thus directly linked to the formation of EME and give additional support to the proposed formation mechanism in Scheme 1. In a first reaction step, CH3O• radicals are produced by DEA to CH3OH. In a subsequent step, the CH3O• radical transfers one of its hydrogen atoms to a nearby C2H4 molecule to form CH2O and ethyl radicals C2H5•. C2H6 is formed when the C2H5• radicals react with CH3OH, whereas n-butane is formed by reaction of C2H5• radicals with C2H4 and subsequent capture of a hydrogen atom from CH3OH. On the basis of the reactions shown in Scheme 2, we conclude that no iso-butane is formed. This is because [1,2]-migration of a hydrogen atom and migration of a methyl group would be necessary to form isobutane from n-butane. [1,2]-Migration of a hydrogen atom in nbutyl radicals, however, is inhibited because this isomerization proceeds via a four-membered cyclic transition state that has high ring strains. Therefore, beta scission of n-butyl radicals is the dominant process, which again yields C2H5• radicals and C2H4. Thus, no new products are formed by beta scission.51 As mentioned before, the proposed formation mechanisms are similar to those of free radical polymerization reactions. The absence of higher oligomers like n-hexane, however, indicates that radical transfer reactions between any radical intermediates and CH3OH are more dominant than chain propagation.46 This might be due to the low mobility of molecules in the cryogenic mixture, which limits the amount of C2H4 monomers that is available for the propagating oligomer chain. Moreover, beta scission of n-butyl radicals, which again yields C2H4 and C2H5• radicals, is a competitive process that lowers the efficiency of chain propagation. Taking into account the energy dependence for the formation of EME and the formation pathways of the side products, we conclude that the observed formation of EME is triggered by DEA to CH3OH, which yields CH3O• radicals and H−. These CH3O• radicals attack the double bond of C2H4 to form an intermediate CH3OC2H4• radical, which finally abstracts a hydrogen atom from CH3OH to yield EME. 3.4. Reaction Mechanisms above the Ionization Threshold. At electron energies where electron impact ionization of the parent compounds occurs, we observe two signals at ∼100 and ∼160 K in the m/z = 45 curve, the first of which is EME. Above a threshold of 9 eV, the product yield of EME increases steadily with increasing electron energy (Figure 3b). This behavior is typical of a reaction triggered by EI of one of the reactants. Similar results were obtained for the reactions of C2H4 with NH3 and H2O to form ethylamine14 and ethanol,13 respectively. We adopted the proposed reaction mechanism to explain the formation of EME. The reaction between the neutral

After irradiation, however, two new signals appear around 65 and 88 K with mass-over-charge ratios of m/z = 29 and 30 (Figure 5a). The signals at m/z = 30 are clearly absent in the case of the nonexposed sample, while the corresponding signals at m/ z = 29 partially overlap with the multi- and monolayer desorption peaks of the C2H4 isotopologue 13C2H4 (verified by comparing the peak areas of the m/z = 28 and 29 desorption signals, not shown), as can be seen in the spectra of the nonexposed sample (Figure 5a, lower spectrum). The new signals at 65 and 88 K were assigned to ethane C2H6 and formaldehyde CH2O. The assignment of C2H6 is based on the results of Böhler et al.15 who observed similar desorption temperatures of C2H6 after irradiating mixed films consisting of C2H4 and NH3. To verify the assignment of CH2O, small amounts of pure CH2O (obtained by heating paraformaldehyde) were mixed with CH3OH. TDS of the so prepared films show desorption features identical to the signal in the irradiated films, confirming our assignment (Figure 5b). In addition, more complex hydrocarbons are formed, as can be seen by a desorption feature near 98 K with fragments at m/z = 41, 42, and 43 (Figure 6) that are prominent in several

Figure 6. Thermal desorption spectra of a multilayer mixture of C2H4 and CH3OH (1:1) before and after an electron exposure of 8000 μC/ cm2 at 5.5 eV, upon which new signals at m/z = 41, 42, and 43 can be observed. The signals were assigned to n-butane.

saturated hydrocarbons (see the Supporting Information, Table S2).33 On the basis of the intensity ratios of these signals, we ascribe this desorption feature to n-butane, which is in line with previous experiments, where we observed a desorption temperature of ∼115 K for n-butane.15 Significant contributions of 1-butene, 2-butene, and n-hexane to the signal at 98 K can be excluded because no signal in the G


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Figure 7. (a) Thermal desorption spectra of a multilayer mixture of C2H4 and CH3OH (1:1) and of pure C2H4 after an electron exposure of 8000 μC/ cm2 at 5.5 eV, respectively. The left signal at 65 K in the m/z = 30 curve and the signal at 98 K in the m/z = 43 curve correspond to C2H6 and n-butane, respectively. Formation of C2H6 and n-butane only occurs when CH3OH is present. (b) Thermal desorption spectra of a multilayer mixture of C2H4 and CH3OH (1:1) and of pure CH3OH after an electron exposure of 8000 μC/cm2 at 5.5 eV, respectively. The signals at 88 K in the m/z = 29 and m/z = 30 curves correspond to CH2O. Formation of CH2O only occurs when C2H4 is present, pointing toward a key role of C2H4.

Scheme 2. Proposed Reaction Mechanism for the ElectronInduced Formation of Formaldehyde (CH2O), Ethane (C2H6), and n-Butane (C4H10) in Condensed Mixtures of C2H4 and CH3OH (1:1) as a Concurrent Reaction to the Formation of EMEa

Scheme 3. Proposed Reaction Mechanism Proceeding via the Intact CH3OH•+ Radical Cation and C2H4•+ Radical Cation, Respectivelya

a

Methoxy radicals produced by DEA to CH3OH can transfer a hydrogen atom to C2H4 to form C2H5• radicals and CH2O. C2H5• radicals can abstract a hydrogen atom from CH3OH to form C2H6 or attack another C2H4 molecule to form an n-butyl radical. The n-butylradical, in turn, can react with CH3OH to form n-butane.

parent compounds is hindered because of the repulsive Coulomb force between the electron-rich double bond of C2H4 and the lone pairs located at the oxygen atom of CH3OH. By ionization of either C2H4 or CH3OH, the repulsive force is converted into an attractive force that lowers the activation barrier for the reaction. Intramolecular [1,3]-hydrogen migration in the resulting adduct and subsequent neutralization with a thermalized electron then leads to the formation of EME (Scheme 3). The formation of EME shows that the reaction mechanism known from smaller molecules like H2O or NH3 can, in principle, also be applied to larger molecules like alcohols.

a

Ionization of one of the reactants lowers the activation barrier for the reaction. Intramolecular [1,3]-hydrogen migration in the formed adduct between the molecular cation and the other reactant and subsequent neutralization with a thermalized electron lead to the formation of EME.

H


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The Journal of Physical Chemistry A The premise of the proposed reaction mechanism in Scheme 3 is that EI of one of the reactants only forms intact molecular ions, which we refer to as soft ionization, and thus drives product formation by lowering the activation barrier. However, according to the NIST database,33 the appearance energies (AEs) for fragments of CH3OH are considerably lower than those of NH3 and H2O (Figure 8). Bowen and Maccoll52

Scheme 4. Alternative Reaction Mechanism for the Formation of EMEa

a

In contrast to NH3 and H2O, CH3OH readily undergoes fragmentation even slightly above the ionization threshold. CH3O+ cations, which are produced by fragmentation of CH3OH+, attack C2H4 to form an intermediate CH3OC2H4+ cation. This intermediate cation can be neutralized by a thermalized electron and capture a hydrogen atom from CH3OH to form EME.

two energy resonances to rule out any contributions of EI processes. For the formation of EME, we propose a mechanism triggered by DEA to CH3OH which results in the formation of CH3O•. The CH3O• radical then attacks the electron-rich double bond of C2H4. Subsequently, the resulting adduct captures a hydrogen atom from a nearby CH3OH to form EME. In contrast to the reaction between H2O and C2H4,13 the reaction between CH3OH and C2H4 cannot be triggered by EA to C2H4. The formation mechanism of EME at 5.5 eV is similar to that of a free radical polymerization where CH3O• radicals act as a reaction initiator and C2H4 as a monomer. As a side process, CH3O• can initiate the formation of hydrocarbon oligomers by transfer of one of its hydrogen atoms to a C2H4 monomer. However, in contrast to a typical polymerization, radical transfer reactions between intermediate alkyl radicals and CH3OH molecules dominate the reaction. Thus, the formation of oligomers that are larger than n-butane is suppressed. At energies above but near the ionization threshold, we also observed the production of EME. We can explain its formation in analogy to the formation of ethanol and ethylamine starting from C2H4 and H2O13 or NH3,14 respectively. For these compounds, one of the reactants is ionized by the impinging electron, which yields the intact molecular cation. This ionization converts the repulsive Coulomb force between the high electron densities at the reactive sites of the two neutral parent compounds into an attractive force, which drives the reaction by lowering the activation barrier. However, unlike in the cases of H2O and NH3, fragmentation of CH3OH is nonnegligible. Therefore, reactive fragments of CH3OH may provide an additional reaction pathway to EME, which would be similar to a cationic polymerization reaction. We could show that the same reaction mechanism that accounts for the reaction between C2H4 and H2O can be applied to reactions between C2H4 and alcohols. Thus, the results of the present study contribute to previous attempts15 to obtain a more generalized picture of these electron-induced reactions.

Figure 8. Ionization energies and appearance energies for different fragments of CH3OH, NH3, and H2O in the gas phase.33

reported an AE value of 11.67 eV for the fragment CH3O+ and provided mass spectra of CH3OH recorded at electron energies of approximately 12 eV, showing a relatively high intensity for the CH3O+ fragment (17% of the intensity of the [M]+• ion). This shows that fragmentation of CH3OH is non-negligible. We suppose that fragmentation of CH3OH accounts for a large number of side products, which we observed above the ionization threshold (not shown), in line with a previous study on electron-induced reactions in condensed methanol.53 Therefore, reactive fragments of CH3OH may provide an alternative reaction pathway to EME that supplements the one depicted in Scheme 3. The most likely alternative is the addition of CH3O+ to C2H4 to form an intermediate CH3OC2H4+ cation. This cation can be neutralized by a thermalized electron to yield CH3OC2H4• and subsequently capture a hydrogen atom from a nearby CH3OH molecule to form EME. In analogy to the free radical polymerization like reaction that is proposed for the formation of EME below the ionization threshold, this alternative mechanism would correspond to a cationic polymerization (Scheme 4).54 However, we do not find any specific side products that would result from such a reaction between CH3O+ and C2H4 that are not also formed upon irradiation of pure CH3OH53 or pure C2H415 above their ionization energies and thus can give direct support for this hypothesis.

4. CONCLUSION We have observed the formation of EME during electron irradiation of condensed multilayer mixtures of C2H4 and CH3OH (1:1). At higher exposures, we could identify two resonance-like structures at 5.5 and 8.0 eV in the product yield curve below the ionization threshold. We focused on investigating the mechanism corresponding to the lower of the I


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.8b10209. Figure S1, peak areas for the m/z = 27 and m/z = 31 signals for different exposures at 15 eV showing depletion of the parent compounds C2H4 and CH3OH; Figure S2, comparison of our mass spectrum of ethyl methyl ether with that of the NIST database; Table S1, relative ion intensities in the mass spectra of CH3OH and C2H4 according to the NIST database; Table S2, relative ion intensities in the mass spectra of relevant molecules according to the NIST database (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +49 421 218 63201. ORCID

Fabian Schmidt: 0000-0002-2944-791X Jan H. Bredehöft: 0000-0002-7977-6762 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work presented here was funded by the Deutsche Forschungsgemeinschaft DFG under project number SW26/ 15-2.

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Electron-Induced Formation of Ethyl Methyl Ether in Condensed

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