Low-Energy Electron-Induced Hydroamination Reactions between

Article pubs.acs.org/JPCC

Low-Energy Electron-Induced Hydroamination Reactions between Different Amines and Olefins E. Böhler, J. H. Bredehöft, and P. Swiderek* University of Bremen, Institute of Applied and Physical Chemistry, Fachbereich 2 (Chemie/Biologie), Leobener Straße/NW2, Postfach 330440, D-28334 Bremen, Germany ABSTRACT: Thermal desorption spectrometry (TDS) was applied to investigate reactions initiated by electron exposure of multilayer condensed films containing both an olefin and an amine or ammonia. In all cases, electron-induced hydroamination reactions leading to addition of ammonia or an amine to the CC double bond of the olefin were detected. The dependence of the hydroamination product yield on the electron energy was studied for mixtures of ethene with ammonia, ethylamine, and diethylamine and points, in all three cases, to an ionization-driven process. Concurrent reactions include the formation of larger hydrocarbons as well as fragmentation of the reactants and, in particular, of the amines so that higher substituted analogues are obtained even without the presence of an olefin. The mechanisms of these reactions are reviewed and an ionization-driven olefin dimerization is proposed that closely resembles the previously described hydroamination. The results underline the general concept of electron-induced synthesis driven by ionization but also demonstrate how the complexity of the reactions increases as the reactants become larger.

1. INTRODUCTION Chemical reactions induced by electron−molecule interactions contribute to a variety of processes in nature and technology. Low-energy electrons are particularly important here because they are released abundantly as secondary electrons when ionizing radiation interacts with matter.1 These low-energy species can prepare molecules in different reactive states, including short-lived anion states, excited neutrals, and cations, which typically have a tendency toward dissociation.2−5 This can, for example, initiate radiation damage of DNA6−8 or contribute to the degradation of materials under exposure to Xrays or high-energy electron beams used in surface analysis.9−11 Secondary electrons released from an underlying solid under a focused keV electron beam fragment adsorbed organometallic precursor molecules in electron beam induced deposition (EBID) to form solid deposits.12,13 Similarly, low-energy electron−molecule interactions contribute to the formation of neutral or charged reactive molecules and fragments in plasmas.3 While these examples point to the destructive nature of electron−molecule interactions, the resulting reactive particles may also interact with a reaction partner to yield larger products.5 Simple examples include the formation of D2O2 upon electron exposure to D2O ice14 and the production of ozone (O3) in condensed O2.15 However, more complex species may also be formed as exemplified by a variety of products such as ethanol (CH3CH2OH), dimethylether (CH3OCH3), and ethylene glycol (HOCH2CH2OH) identified after electron irradiation of condensed methanol.2 From a synthetic point of view, reactions that can couple an entire molecule to another structure are particularly interesting. Reactions of this type are, for example, useful to attach molecules to surfaces, leading to surface functionalization. As an © 2014 American Chemical Society

example, a C−H bond of an acetonitrile molecule can be dissociated by interaction with an impinging electron having an appropriate initial kinetic energy (E0). The thus released atomic hydrogen can activate a hydrogenated diamond surface and the remaining CH2CN fragment can recombine with the resulting radical site at the surface.16 Similar reactions may also lead to coupling of two molecules, a process that may then be termed as an atom-efficient synthesis.5 Recently, we have provided evidence of an electroninduced reaction that couples two molecules to a product incorporating all atoms of the two reactants. In fact, irradiation of a condensed mixture of ethene (C2H4) and ammonia (NH3) with electrons having E0 just above the ionization threshold of the reactants induces the formation of ethylamine (C2H5NH2).17 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 activation barrier of the reaction between the two molecules so that the adduct formation can take place (Figure 1). The neutral product is then formed by recapture of a thermalized electron present within the molecular layer while electron exposure continues. This electron-induced hydroamination reaction was recently also used to functionalize a selfassembled monolayer (SAM) terminated by CC double bonds.18 The aim of the present study is to reveal whether the same reaction induced by electron irradiation at E0 just above the ionization threshold is also efficient in the case of other larger reactants thus leading to an atom-efficient synthesis of more Received: February 3, 2014 Revised: March 12, 2014 Published: March 12, 2014 6922

dx.doi.org/10.1021/jp501192v | J. Phys. Chem. C 2014, 118, 6922−6933

The Journal of Physical Chemistry C

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99.95%), propene (H2CCHCH3, Air Liquide, 99.95%), ethylamine (C2H5NH2, Sigma Aldrich, 97%), and diethylamine ((C2H5)2NH, Sigma Aldrich, ≥99.5%). For most electron exposure experiments, sample films were prepared from gas mixtures containing equal partial pressures of the two reactants. However, experiments on pure condensed films of one specific reactant were also performed for comparison. Additional experiments aiming at reference data on desorption temperatures were performed with use of 1-propylamine (C3H7NH2, Sigma Aldrich, 98%), n-hexane (n-C6H14, VWR, 96%), cyclohexane (Merck, 99.9%), 2-methylpentane (Merck, 98%), cis-hex-3-ene (Alfa Aesar, 97%), and triethylamine ((C2H5)3N, Fluka, ≥99.5%). 2.3. Product Identification and Quantification. Products of electron-induced reactions were identified by monitoring characteristic mass over charge ratios as known either from mass spectra of the pure substances acquired upon leaking the vapor into the UHV chamber or from literature mass spectra.20 Furthermore, characteristic trends in the desorption temperatures were monitored to support this assignment.21 Here, it is important to note that only multilayer desorption temperatures of pure one-component layers are characteristic of a compound while the desorption temperatures at monolayer coverage and in mixed films depend on the specific interactions with the underlying surface and the adjacent molecules.21 In the present experiments, the desorption temperature of the amines turned out to vary over a wide range but decreased with increasing coverage within the monolayer regime and was also influenced by codeposits. Therefore, the observed variation of desorption temperatures at submonolayer coverage and the possible presence of byproducts must be taken into account when drawing conclusions from the desorption temperatures. Relative product quantities are derived here from the integrated desorption peaks measured for characteristic fragment ions as produced in the QMS by using the partial ionization cross sections for their formation. As described previously,18 these latter quantities are derived by taking into account the contribution of the specific mass fragments to the total intensity within the mass spectrum and using either tabulated22 or estimated total electron impact ionization cross sections at an electron energy of 70 eV, i.e., the energy applied in the QMS to produce the cation fragments of the respective molecules. This allows us to outline trends in production efficiency for specific compounds without explicit use of reference samples as has been described earlier.23

Figure 1. Proposed mechanism of an electron-induced hydroamination reaction between NH3 and ethene.17

complex amines. However, as the molecular complexity of the reactants increases, more attention must be paid to concurrent reactions. Therefore, we first investigate side reactions of ethene and NH3 that lead to products other than ethylamine and determine how the yield of the desired amine can be optimized. Product formation is then investigated in mixtures where either ethene is replaced by a larger olefin, namely propene (H2CCHCH3), or NH3 is replaced by different amines, namely ethylamine and diethylamine ((C2H5)2NH). The propene experiments aim in particular at a possible steric effect of the methyl group while the use of amines as reactant reveals if more than one hydrocarbon side chain can be introduced at a nitrogen atom by an electron-induced hydroamination.

2. EXPERIMENTAL SECTION 2.1. Experimental Setup. Electron-induced reactions in condensed multilayer molecular films of the used reactants were investigated by postirradiation thermal desorption spectrometry (TDS) or electron-stimulated desorption (ESD). In the experiments that were performed in an ultrahigh vacuum chamber evacuated by a turbomolecular pump to a base pressure of 10−10 Torr,19 product formation was monitored after (TDS) or during (ESD) electron exposure of the condensed films. The films were deposited at ∼35 K on a polycrystalline Au foil by leaking defined amounts of the gases via a gas handling manifold onto the Au substrate. During leaking the pressure in the main chamber increased by one order of magnitude. The film thickness was estimated by observing the transition from monolayer to multilayer desorption signals with increasing coverage.19 Alternatively, the monolayer coverage was estimated by comparing the density of a given compound with that of a substance for which the amount of vapor required for monolayer formation is known. The condensed films were exposed to low-energy electrons from a commercial flood gun (SPECS FG15/40) having an energy resolution of 0.5−1 eV. Desorbing neutral molecules were analyzed by 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 either during irradiation (ESD experiment) or after irradiation upon heating the Au foil with a rate of 1 K/s by resistive heating with Ta ribbons spot-welded to the gold substrate (TDS experiment). Four different masses were monitored simultaneously during a typical experiment. 2.2. Chemicals and Film Preparation. The following chemicals were used as reactants in the experiments: ammonia (NH3, Air Liquide, 99.98%), ethene (C2H4, Air Liquide,

3. RESULTS AND DISCUSSION 3.1. Yield of Main Product and Side Products in the Reaction between NH 3 and Ethene. As previously described, the electron-induced loss of the reactants and the production of ethylamine in mixed condensed films of ethene and NH3 can be monitored by TDS recorded at the relevant masses.5,17 In Figure 2, formation of ethylamine is obvious from desorption signals with their maximum around 170 K and characteristic intensity ratio17,20 in the TDS curves recorded at 30 and 45 amu. In addition, significant amounts of the byproducts N2 and ethane (C2H6) have been observed with desorption maxima at 37 and 70 K, respectively, upon electron exposure at E0 = 15 eV.17,18 Furthermore, the dependence of the characteristic ethylamine signals on E0 suggests that the reaction is initiated by electron impact ionization.17 The ionization-driven fragmentation of ethene and NH3 in the gas phase only sets in at approximately 13 and 15 eV, i.e., about 2.5 6923



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Figure 4. Dependence on electron energy E0 of the relative amounts of N2, ethane, and ethylamine formed after an exposure of 800 μC/ cm2 in multilayer films of 1:1 mixtures of ethene and NH3 with thickness corresponding to 20−30 monolayers. The data points were obtained by integrating the characteristic desorption peaks in the TDS curves and correcting for the respective partial ionization cross sections as described in the text.

Figure 2. Thermal desorption spectra of multilayer films of a 1:1 mixture of ethene and NH3 without electron exposure (0 μC/cm2) and after an electron exposure of 800 μC/cm2 at E0 = 15 eV. The films were deposited from the gas phase at a thickness corresponding to 20−30 monolayers and exposed at 32 K.

Figure 5. Relative amounts of N2, ethane, and ethylamine formed after an electron exposure of 800 μC/cm2 at E0 = 15 eV in multilayer films of mixtures of ethene and NH3 with varying composition. The film thickness corresponds to 20−30 monolayers. The data points were obtained by integrating the characteristic desorption peaks in the TDS curves and correcting for the respective partial ionization cross sections as described in the text. The lines only serve as a guide to the eye.

comparable to the gas phase. However, this has not been verified so far as the dependence of the concurrent formation of N2 and ethane on E0 was not investigated. Also, the experiments were restricted to samples produced from equal amounts of vapor of the two reactants.17 Therefore, the dependence of the formation of the three products ethylamine, ethane, and N2 on E0 and on the mixing ratio of ethene and NH3 is now studied in more detail to reveal the most favorable conditions for the formation of ethylamine. The relative amounts of the products ethylamine, ethane, and N2 are deduced from the TDS data recorded for their molecular cations as produced in the QMS. This evaluation uses total electron impact ionization cross sections at 70 eV of 2.805 Å2 for N2 and 6.422 Å2 for ethane.22 The total ionization cross section of ethylamine is not known and has thus been estimated

Figure 3. Dependence on electron exposure at E0 = 9 eV and E0 = 13 eV of the relative amounts of N2, ethane, and ethylamine formed in multilayer films of a 1:1 mixture of ethene and NH3 with thickness corresponding to 20−30 monolayers. The data points were obtained by integrating the characteristic desorption peaks in the TDS curves and correcting for the respective partial ionization cross sections as described in the text.

and 5 eV above the gas phase ionization threshold.24 Coupling of the intact reactants leading to ethylamine should thus dominate at E0 near the threshold under the assumption that the fragmentation behavior in the condensed phase is 6924



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Figure 6. Electron-stimulated desorption from a 10−15 monolayer NH3 film recorded at 17 and 28 amu during electron exposure of 6200 μC/cm2 at E0 = 15 eV. At the beginning of the experiment, the target is negatively biased and the electron gun is set at a low energy where ESD is not observed. At time (A) the gun was set to E0 = 15 eV but the target remained negatively biased. At time (B) the negative bias was removed and the target grounded. At time (C) the negative bias was applied again to stop electrons from reaching the target thus finishing ESD, and at time (D) the gun energy was lowered again.

Figure 8. Thermal desorption spectra of 20−30 monolayer mixed films of ethene and NH3 (1:1) and pure 10−15 monolayer films of ethene recorded at 30 amu prior to and after electron exposure of 800 μC/cm2 at E0 = 15 eV.

Table 1. Fragmentation Pattern of C4 Hydrocarbons Observed in Mass Spectra Recorded by Using Electron Impact Ionization at 70 eV (ref 20)a relative ion intensities at m/z molecule C4H10 isomers n-butane isobutane C4H8 isomers 1-butene 2-butene (Z) 2-butene (E) isobutene C4H6 butadiene

58

56

12 3

1

39 57 48 56

54

53

43

41

39

1 1

100 100

29 38

14 17

100 100 100 100

34 31 36

2 4 4 2

5 8 8 5

95

71

100

a

The listed masses are used for identification of products formed upon electron exposure of condensed films of ethene.

may be confused with an impurity signal of CO that may stem from the filament of the electron gun. However, the intensities of ethylamine and ethane generally approach saturation after an electron exposure of 800 μC/cm2 with no evidence of significant loss due to further reaction. The TDS signals reached after this exposure are therefore used to evaluate the E0 dependence of product formation (Figure 4). Figure 4 shows that significant amounts of ethane are already formed below the gas phase ionization threshold while the amount of N2 again scatters too much to be considered reliable. The ratio of the amount of ethane to ethylamine increases from 1.4:1 at 8 eV to 2.7:1 at 15 eV suggesting that the byproduct is less favored at E0 near the ionization threshold. However, a significant preference for ethylamine formation at E0 just above the ionization threshold cannot be deduced. It must be noted that the threshold for formation of these two products is roughly 8 eV, which is about 2 eV below the gas phase ionization threshold. However, this is a typical value of stabilization energy for ions in the condensed phase.5 Nonetheless, the immediate appearance of ethane indicates

Figure 7. Thermal desorption spectra of 20−30 monolayer films NH3 recorded at 32 amu prior to and after electron exposure of 200 μC/ cm2 at E0 = 15 and 25 eV. The curves reveal formation of hydrazine (N2H4).

as 6.2 Å2 by assuming that it roughly corresponds to the average of the values of 5.9 Å2 for methylamine25 and 6.54 Å2 for npropylamine.26 When comparing the yields of ethylamine, N2, and ethane, it is also important to make sure that the data represent an electron exposure that has not yet led to a significant consumption of these products by further electron-induced reactions. Figure 3 therefore shows the evolution of the relative product amounts observed in TDS after increasing electron exposure. The amounts of N2 show a significant scatter. Due to its very low desorption temperature, slight differences in the target temperature may lead to strong variations in the trapping efficiency of N2 in the condensed film. Also, the signal of N2 6925



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Figure 9. Thermal desorption spectra of 20−30 monolayer films of C2H4 recorded at the indicated masses (a) prior to and after electron exposure of 200 μC/cm2 and (b) after electron exposure of 800 μC/cm2 at E0 = 15 eV.

Table 2. Fragmentation Pattern of C3 Amines As Well As C3 and C6 Hydrocarbons Observed in Mass Spectra Recorded by Using Electron Impact Ionization at 70 eV (ref 20)a relative ion intensities at m/z molecule propene propane 1-aminopropane 2-aminopropane C6H14 isomers n-hexane 2-methylpentane dimethylbutane C6H12 isomers 1-n-hexene 2-n-hexene (Z) 2-n-hexene (E) 3-n-hexene (Z) 3-n-hexene (E) 2-methyl-pent-1-ene 4-methyl-pent-2-ene 4-methyl-pent-2-ene (Z) 4-methyl-pent-2-ene (E) 2,3-dimethyl-but-2-ene C6H10 isomers 1,5-hexadiene 1,4-hexadiene 2,4-hexadiene (Z,Z) 2,4-hexadiene (E,Z) 2,4-hexadiene (E,E) 4-methyl-pent-1,3-diene 2,3-dimethyl-but-1,3-ene a

69

59

9 3

58

56

55

44

43

42

41

30

27 1 100

2 23 2 7

70 6 3 17

100 13 5 11

2 100 2

2 8

1 1

4 1

45 4 2

7 5 12

3 3 4

81 100 100

41 54 83

70 29 45

24 27 20 30 40 40 17 89 91 90

100 25 23 27 28 100 50 7 7 7

64 100 100 100 100 47 9 10 11 8

2

59 14 12 14 17 10 100 6 7 7

72 57 45 61 69 32 31 8 9 9

95 50 37 66 81 89 68 100 100 100

1 2

6 12 6 7 6 5 6

1 1

6 6 5 6 6 8 11

100 51 32 35 36 42 58

1

1 1 2 2

1 1 4 3

1 1 3 5

1

1 1 1 1

The listed masses are used for identification of products formed upon electron exposure of condensed mixed films of propene and NH3.

clusters yields NH4+ and NH2 radicals already at energies between 9 and 10 eV.27 The formation of NH2 radicals very likely initiates further reactions that can also lead to reduction

that an efficient reaction channel acting as the source of atomic hydrogen must be open already near the ionization threshold. In fact, it has been reported that photoionization of small NH3 6926



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Figure 12. Thermal desorption spectra of 20−30 monolayer mixed films of propene and NH3 (1:1) recorded at several characteristic masses of C6 hydrocarbons after electron exposure of 800 μC/cm2 at E0 = 15 eV.

Figure 10. Thermal desorption spectra of 20−30 monolayer mixed films of C3H6 and NH3 (1:1) and 10−15 monolayer pure films of C3H6 recorded prior to and after electron exposure of 800 μC/cm2 at E0 = 15 eV.

resulting ions remain trapped in the film.30 This process can thus deliver hydrogen atoms required for reduction of ethene to ethane while the remaining fragments may not as efficiently induce the formation of ethylamine. In addition, Figure 5 shows the dependence of the amount of the three main products ethylamine, ethane, and N2 on the mixing ratio of the reactants ethene and NH3. While the amount of produced N2 increases proportional to the quantity of NH3 in the sample and the amount of ethane decreases

of ethene to ethane (Section 3.2). In addition, we note that while the rate of formation is generally small at 5 eV, the ratio of the amount of ethane to ethylamine increases again to 2.5:1 at this E0. This may be traced back to fragmentation of NH3 by dissociative electron attachment as known from earlier gas phase measurements.28 While the same process was not detected in ESD experiments on condensed films of NH3,29 it is possible that dissociation nonetheless occurs but the

Figure 11. (a) Thermal desorption spectra of thin films of 1-aminopropane with increasing thickness corresponding to the stated number of monolayers (ML). The stated thickness has an estimated margin of error of 15%. (b) Thermal desorption spectra of multilayer films of different C6 hydrocarbons. 6927



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Figure 13. (a) Thermal desorption spectra of thin films of diethylamine with increasing thickness corresponding to the stated number of monolayers (ML). The stated thickness has an estimated margin of error of 15%. (b) Thermal desorption spectra of 20−30 monolayer mixed films of ethene and ethylamine (1:1) and (c) thermal desorption spectra of pure films of ethylamine, both recorded after an electron exposure of 800 μC/cm2 at E0 = 15 eV.

Figure 15. Thermal desorption spectra of 25−35 monolayer mixed films of ethene and diethylamine (1:1) and of 35−45 monolayer pure films of diethylamine, both recorded after an electron exposure of 800 μC/cm2 at E0 = 15 eV.

Figure 14. Thermal desorption spectra prior to and after electron exposure of 800 μC/cm2 at E0 = 15 eV of 35−45 monolayer films of diethylamine.

dissociation steps as well as formation of N−N bonds between fragments from two NH3 molecules suggesting that intermediate species are involved. Results on ESD of N2 from a condensed multilayer film of NH3 (Figure 6) support this assumption. In this experiment, gas was initially deposited on a Au substrate and at time (A) (see Figure 6) the electron gun was set to E0 = 15 eV. A negative bias remained applied to the target to prevent electrons from reaching the sample. However, the electron beam did impinge on a set of clamps that serve to hold the sample in place and always remain grounded. Desorption between times (A) and (B) thus originates from the clamps. After 300 s (B), the negative bias to the sample was

accordingly, the detected amount of ethylamine remains relatively constant although less ethene was present. This indicates that the yield of ethylamine, i.e., its quantity as compared to the initial quantity of the reactant ethene, increases with increasing percentage of NH3. An excess of NH3 is thus favorable for the production of ethylamine. 3.2. Reactions Leading to Formation of Ethane and N2. ESD and TDS experiments were performed to obtain more detailed insight into the reactions leading to formation of ethane and N2 and to identify possible further side products. The formation of N2 from NH3 requires multiple bond 6928



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Figure 17. Summary of electron-induced hydroamination reactions studied in this work. (a) Formation of primary, secondary, and tertiary amines. (b) Isomers produced in the case of different substitution on the two unsaturated carbon atoms.

reaction of adsorbed NH2 on Pt(111) was suggested to produce atomic nitrogen and further hydrogen atoms according to32

Figure 16. Dependence on electron energy E0 of the relative amounts of (a) diethylamine formed after an exposure of 800 μC/cm2 in multilayer films of 1:1 mixtures of ethene and ethylamine with thickness corresponding to 20−30 monolayers and (b) triethylamine formed after an exposure of 800 μC/cm2 in multilayer films of 1:1 mixtures of ethene and diethylamine with thickness corresponding to 25−35 monolayers. The data points were obtained by integrating the characteristic desorption peaks in the TDS curves measured at 58 (diethylamine) and 86 amu (triethylamine). Note that the error bars are larger than in Figures 3−5 due to smaller intensities of the signals.

2NH 2(a) → NH3(g) + N(a) + H(a)

In contrast to this, the present result shows that low-energy electrons can induce the formation of N2H4 in multilayer films of NH3 where trapping of NH2 radical intermediates by a metal surface is unlikely. N2H4 has also been observed previously, besides other NxHy species, in electron exposure experiments on multilayer NH3 films performed at E0 = 5 keV.33 The slow increase of N2 ESD (Figure 6) can thus be traced to the gradual accumulation of these species that are further decomposed to N2. The formation of the byproduct ethane requires a source of hydrogen. While it is conceivable that H atoms released upon dissociation of NH3 react with ethene yielding ethane, this product can also be formed, to a lesser extent, from the hydrocarbon alone (Figure 8). In fact, a 30 amu desorption signal appears in TDS recorded after electron exposure of pure films of ethene. Although this signal is considerably larger in the case of the mixed film giving evidence that NH3 in fact acts as a reducing agent, this result shows that H atoms may also be supplied in pure films of C2H4. This requires reactions leading to formation of hydrocarbons with a lower H:C ratio than C2H4. As shown below, different C4 hydrocarbons can be identified in thin films of ethene following electron exposure. They can result from coupling of two ethene molecules. Their hydrogen content is analyzed here. C4 hydrocarbons differing in H content can be distinguished because of their distinct mass spectrometric fragmentation pattern (Table 1). A low exposure of 200 μC/cm2 was applied first in the search for such products to minimize the contribution of subsequent reactions (Figure 9a). Table 1

removed and ground potential applied to the substrate to induce ESD from the sample. As a result and in accord with previous data,18 desorption of NH3 increased sharply but then dropped continuously due to depletion of the NH3 amount present on the substrate. In contrast, desorption of N2 increased slowly after the start of electron exposure giving clear evidence that N2 is released through a reaction of an intermediate product. After reaching the exposure of 6200 μC/ cm2, the target was again negatively biased to interrupt ESD. At this point, N2 production stopped abruptly suggesting again that it results from the electron-induced decomposition of an intermediate product. TDS results show that a signal appears in the 32 amu curve following electron exposure giving evidence of hydrazine (N 2H 4) formation (Figure 7). Earlier experiments on electron-induced dissociation of NH3 adsorbed at 100 K on a Pt(111) surface using E0 = 50 eV have provided evidence of the formation of chemisorbed NH2 and NH species.31 These species decomposed to release H which desorbed as H2 upon warming the surface while the remaining N atoms finally recombined and desorbed at 700 K. A disproportionation 6929



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in the TDS curve recorded at 39 amu points to formation of different species. The signal intensities at 120 K in the 54, 53, and 39 amu curves are comparable. This behavior is similar to the mass spectrum of butadiene (Table 1). On the other hand, the dominant 110 K peak at 41 amu together with the less intense signals at 56 and 39 amu reveal again the formation of butane isomers (Table 1). Finally, also the formation of butane is now more obvious from a signal at 110 K in the 43 amu TDS curve and a second desorption peak appears at 130 K in the 56 amu curve pointing to the beginning formation of even larger products (see Section 3.3). The results suggest that electron exposure of condensed layers of ethene in fact leads to a mixture of larger hydrocarbons. Similar cross-linking reactions have been described before.35 A rough estimate based again on the mass spectra and total ionization cross sections reveals that production of butene is about three times larger than that of butadiene. This is reasonable because formation of 1-butene requires the least amount of bond reorganization (see Section 3.5). However, formation of butane does not lead to release of atomic hydrogen, a process that is required for the formation of ethane in the absence of NH3. An additional source of hydrogen which is probably more important was identified by preliminary ESD experiments that give evidence of desorption of acetylene (C2H2) from condensed layers of ethene.36 In conclusion, while NH3 clearly acts as a reducing agent under electron exposure, electron-induced dissociation of C−H bonds in ethene can also release sufficient amounts of atomic hydrogen to produce a noticeable although smaller amount of ethane. 3.3. Reaction between Propene and NH3: Steric Effects. The addition of NH3 to an unsaturated hydrocarbon may be subject to steric effects, i.e., a carbon atom carrying substituents may be a less favorable site for addition of the amino group. Therefore, propene (C3H6) was used as reactant instead of ethene and formation of either 1-aminopropane (CH3CH2CH2NH2) or 2-aminopropane ((H3C)2CHNH2) was anticipated. The mass spectra of these two products are distinctly different. While 59 amu is characteristic beside the dominant 30 amu signal in 1-aminopropane, 44 amu is the most intense fragment in 2-aminopropane and 58 amu is more intense than the molecular cation at 59 amu (Table 2). In fact, desorption signals with maxima between 140 and 150 K appear in the TDS curves recorded at these masses after electron exposure at E0 = 15 eV of condensed films produced from a mixture of equal amounts of NH3 and propene (Figure 10). This desorption temperature lies within the range of those observed for pure films of 1-aminopropane at coverages above the monolayer as shown in Figure 11a. However, desorption is very likely modified by intermolecular interactions with other molecules in a mixed layer so that the desorption temperature of the product and the pure reference film can differ.21 For reference, the same experiment was also performed by using pure films of propene (Figure 10). In fact, the characteristic signals of 1-aminopropane and 2-aminopropane are absent after exposure of pure propene giving further evidence that the observed products result from coupling of propene and NH3. Figure 10 also confirms the production of propane as is obvious from the similar intensities of two desorption peaks around 85 K in the 44 and 43 amu TDS curves (compare Table 2). As in the case of ethene (Section 3.2), the reduction of propene also proceeds in its pure film but is again more efficient in the presence of NH3 (Figure 10). However, a second desorption peak near 130 K appears in both

Figure 18. Comparison of reaction mechanisms: (a) proposed electron-induced dimerization of ethane, (b) electron-induced hydroamination reaction between NH3 and ethene, (c) acid-catalyzed dimerization of ethene, and (d) electron-induced proton transfer in between two molecules of NH3.

reveals that 41 and 56 amu are characteristic fragment masses of butene isomers (C4H8). The TDS curves recorded at these masses after 200 μC/cm2 at E0 = 15 eV in fact show signals with maximum near 120 K and relative intensities reminiscent of the mass spectrum of a butene isomer. In addition, a desorption peak at slightly higher temperature in the 54 amu curve gives evidence of formation of butadiene (C4H6) and thus reveals a possible source of hydrogen. On the other hand and consistent with the absence of a signal in the 58 amu curve, only small amounts of a butane isomer (C4H10) have been detected at 43 amu with desorption maximum near 115 K (not shown). The latter is presumably n-butane as the formation of isobutane would require reorganization of C−C bonds. The desorption temperature is higher than reported values for butane at multilayer coverage34 and thus points to an amount of product within the submonolayer regime. To accumulate larger amounts of product and thus facilitate their identification, the experiment was repeated for longer electron exposure of 800 μC/cm2 (Figure 9b). In this case, a double peak with components centered around 110 and 120 K 6930



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roughly 4:1 and thus give evidence of formation of diethylamine (Figure 13b). A number of differently structured isomers of diethylamine with the same elementary composition and also dimerization products of ethylamine such as 1,2-diethylhydrazine were further considered as possible products. However, they can all be ruled out as dominant products as none of these compounds reproduces the observed relative intensities at 58 and 73 amu.20 Figure 13a shows that, as for 1-aminopropane (Figure 11a), the desorption temperature of pure diethylamine shifts strongly depending on the coverage. The maximum is located at 285 K for the lowest investigated coverages and shifts down to 140 K in the multilayer regime (Figure.13a). However, a comparison of the integrated desorption peaks suggests that the amount of diethylamine produced after an electron exposure of 800 μC/cm2 at E0 = 15 eV (Figure 13b) is similar to that of a reference layer with coverage corresponding to 0.2 monolayers. This is traced back to the presence of other molecules21 including remaining amounts of ethylamine in the exposure experiment and shows again that, in the case of amines, the desorption temperature cannot be used to identify the product. For reference, electron exposure was also performed on pure films of ethylamine. TDS curves recorded during this experiment show that diethylamine is also produced from ethylamine alone (Figure 13c), suggesting that electroninduced cleavage of a C−N bond of ethylamine is already efficient at E0 = 15 eV and that the resulting C2 fragment reacts with intact ethylamine. This is in line with reported appearance energies of 13 and 13.5 eV for C2H5+ and NH3+ in electron impact ionization of ethylamine in the gas phase.22 Also, the mass spectrum recorded by using electron impact ionization at 70 eV shows that ethylamine undergoes loss of NH3 yielding C2H4+ and thus an intermediate of the electron-induced hydroamination reaction (Figure 1) as the second most important fragmentation channel (32%).20 Previous results on electron-induced desorption from adsorbed diethylamine performed at E0 = 600 eV have also pointed toward release of ethene38 and thus support the relevance of such a process in the formation of higher substituted amines. Here, the formation of ethene in pure layers of diethylamine following electron exposure at E0 = 15 eV was observed in TDS curves at 16, 28, and 30 amu (Figure 14). Production of ethane is deduced from the small peak at 78 K in the 30 amu curve. However, the desorption peak at the same temperature in the 28 amu curve is more intense than expected for ethane based on the characteristic intensity ratio of 27:100 for its 30 and 28 amu fragments. In line with the discussion above, this gives evidence that ethene is also produced. In addition, formation of CH4 is observed at 52 K in the 16 amu curve, in line with α-cleavage yielding CH2NH2+ with a threshold of around 10 eV being the most efficient gas phase fragmentation channel.20 This efficient release of CH3 radicals explains the production of CH4 as observed in the present TDS experiments. In analogy, these results indicate that diethylamine cannot only be formed via an electron-induced hydroamination but also as a result of electron-induced fragmentation of the reactant, in this case ethylamine. Next, the formation of triethylamine in condensed mixed layers of ethene and diethylamine was also investigated. Here, in search for triethylamine, the characteristic dominant fragment at 86 amu as well as the molecular ion at 101 amu were monitored following electron exposure. This was done for both mixtures of ethene and diethylamine as well as for pure

cases in the 43 amu curve giving evidence for the formation of larger hydrocarbons. In analogy to the formation of C4 hydrocarbons in the case of electron exposure of ethene, C6 hydrocarbons are expected in the case of propene. In fact, Figure 11b shows that typical desorption temperatures of such compounds are similar to the value observed here. To substantiate this assignment and thus rule out contributions of this product to the characteristic signals of the amines, further characteristic masses of C6 hydrocarbons were monitored and included in Figure 12. Peaks at 130 K appear in several TDS curves, namely 41, 42, 43, 55, 56, and 69 amu. Comparison of the relative intensities of these signals to those of diverse hexane, hexene, and hexadiene isomers (Table 2) reveals that several C6 products may contribute. While the relative intensities of signals at 41, 42, and 43 amu are reminiscent of the mass spectrum of n-hexane, a 69 amu fragment can only stem from hexene isomers. Formation of this latter product is analogous to that of butene following electron exposure of ethene (Section 3.2). In conclusion, electron exposure of films containing propene most likely yields a mixture of different C6 hydrocarbons. However, the mass spectra of these compounds generally have a very low intensity at 30, 44, 58, and 59 amu, supporting that the 140 K peak in these curves should not be ascribed to a hydrocarbon. As all evidence points toward production of both 1aminopropane and 2-aminopropane, the relative quantities of these products can now be estimated. This is based on the assumption that the total electron impact ionization cross sections at 70 eV of these two compounds are comparable. We thus deduce from the mass spectra obtained at the same ionization energy that the dominant fragments with 30 and 44 amu make up for 72% and 57% of this cross section, respectively. We can then compare the integral intensities of the 140 K peaks for these two masses and estimate that the ratio of the yields of 1-aminopropane and 2-aminopropane is roughly 3:2. In consequence, the discrimination between these two products by steric effects is relatively weak. 3.4. Reaction between Ethene and Ethylamine or Diethylamine: Synthesis of Secondary and Tertiary Amines. Hydroamination reactions in organic chemistry are not only applied to add NH3 to unsaturated hydrocarbons but also to amines still carrying at least one hydrogen atom.37 Electron exposure experiments were thus performed on condensed layers containing ethene and either ethylamine or diethylamine to verify if electron exposure in fact leads to an electron-induced synthesis of secondary and tertiary substituted amines. In analogy to the mechanism depicted in Figure 1, this should yield diethylamine or triethylamine ((C2H5)3N), respectively. The most intense signals in the mass spectrum of diethylamine relate to fragments with masses 30 (85%) and 58 amu (100%) (Figure 13a). Also, 30 amu is the strongest signal of ethylamine20 and desorption of this compound proceeds over a wide temperature range with maximum of the multilayer desorption near 125 K and, similar to 1-aminopropane (Figure 11a), with a long tail extending up to room temperature. By using 30 amu, it is thus very difficult to detect minor amounts of products that desorb at higher temperature. Therefore, the search for diethylamine rather relies on its molecular ion at 73 amu with relative intensity of 22% in mass spectra recorded by using electron impact ionization at 70 eV.20 In fact, desorption signals around 200 K in the TDS curves recorded at 58 and 73 amu appear with an intensity ratio of 6931



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bond. Hydrogen migration with the same stereochemistry and neutralization by a further electron in close analogy to the electron-induced hydroamination reaction (Figure 18b) yields 1-butene. This is a very likely scenario considering the close similarity of well-known acid-catalyzed cation reactions (Figure 17c). In the latter, protonation of an olefin such as ethene produces the cationic species and loss of the catalytic proton from the adduct yields the dimerization product. Formation of N2H4 in NH3, however, must follow a different mechanism as ionization in NH3 clusters is known to lead to proton transfer (Figure 17d).27 Here, recombination of the resulting NH2 radicals is a likely reaction yielding the product.

diethylamine (Figure 15). Desorption peaks with the expected intensity ratio20 of 4:1 are detected in the TDS curves of the mixture after an exposure of 800 μC/cm2. However, the intensity of these signals is too small in the case of the pure diethylamine layers thus excluding an unequivocal identification. This may be related to a decrease in sensitivity of the mass spectrometer at higher masses. As a surprising finding, a desorption temperature of 150 K is observed for triethylamine. This is considerably lower than for submonolayer amounts of diethylamine as formed in a multilayer film of ethylamine (Figure 13). This striking difference may stem from the relation between the desorption temperatures of the product and the major component of the irradiated layers. The multilayer film of ethylamine desorbs near 125 K and thus at lower temperature than diethylamine at multilayer coverage (140 K). Diethylamine thus probably still remains at the surface after the bulk of the reaction matrix has evaporated so that a small quantity is detected with a characteristic submonolayer desorption signal. Triethylamine multilayers, on the other hand, desorb at 140 K and thus at the same temperature as multilayers of diethylamine. This result is also supported by relatively similar multilayer desorption temperatures of the two compounds reported previously.39,40 Also, the monolayer desorption signal (not shown) remains as low as 200 K even for similarly small quantities as shown for diethylamine in Figure 13. This suggests that triethylamine interacts less strongly with a surface probably due to steric hindrance by the three ethyl side groups. Therefore it may desorb together with the reaction matrix consisting mainly of diethylamine and thus before actually forming a somewhat more strongly bound submonolayer film. Finally, the energy dependence of the formation of diethylamine in mixtures of ethene and ethylamine and of triethylamine in mixtures of ethene and diethylamine was investigated. This was again done by integrating the characteristic desorption signals of diethylamine near 200 K in the 58 amu TDS curves (Figure 16a) and of triethylamine near 150 K in the 86 amu TDS curves (Figure 16b). Similar to the formation of ethylamine in mixtures of ethene and NH3, production starts to increase above a threshold within the range of typical ionization energies, supporting the assumption that the same ionization-driven hydroaminantion reaction mechanism is in effect. 3.5. Summary of Molecular Synthesis Induced by Electron-Impact Ionization. The preceding sections have revealed that electron-induced hydroaminations as identified previously in mixed condensed layers of ethene and NH3 can also lead to synthesis of larger amines when different olefins or alkylamines are used as reactants. In analogy to the reaction mechanism shown in Figure 1, the concept of an electroninduced hydroamination can thus be summarized in Figure 17. In fact, secondary and tertiary amines can be obtained according to Figure 17a. However, the steric effect of a methyl group on the olefin in this reaction is weak so that a mixture of products is obtained (Figure 17b). We also found evidence for synthesis of larger hydrocarbons from ethene and propene as well as N2H4 from NH3. Therefore, we propose here the most probable mechanisms leading to these products based on a comparison with the already described electron-induced hydroamination but also taking into account known mechanisms of similar reactions. Removal of an electron from the highest occupied π* orbital of ethene yields a radical cation (Figure 18a). The cationic site of the mesomeric structure may add to a further unsaturated

4. CONCLUSION The aim of this study was to verify the general validity of an electron-induced hydroamination reaction that was previously described to occur in condensed mixed layers of ethene and NH3 at E0 above the ionization threshold of the two reactants.17 In fact, the same reaction also takes place when ethene is replaced by propene yielding the two different isomers of aminopropane. Also, ethylamine and diethylamine react with ethene to yield secondary and tertiary amines, again in an ionization-driven reaction. However, side reactions have also been monitored. In particular, electron exposure also leads to production of larger hydrocarbons. This process can be explained by an ionization-driven reaction analogous to the hydroamination mechanism. However, the reactant ethene can also be delivered by the fragmentation of ethylamines so that higher substituted amines are produced even in pure amine layers. While the results underline the general concept of an electron-induced synthesis driven by ionization, they also demonstrate the complexity of the reactions as the reactants become larger. These data provide new evidence that concepts from organic chemistry can be applied to understand the outcome of electron-induced reactions.5

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +49 421 218 63200. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support provided by the DFG and the Cost Action CM0601 “Electron Controlled Chemical Lithography” (ECCL) is gratefully acknowledged.

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REFERENCES

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