(OH) Radicals and Methane (CH4) - American Chemical Society

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Reactivity Between Non-Energetic Hydroxyl (OH) Radicals and Methane (CH4) Emilie-Laure Zins,†,‡,* Claire Pirim,†,‡,§ Prasad Ramesh Joshi,†,‡ and Lahouari Krim†,‡ †

UPMC Univ. Paris 06, UMR 7075, Laboratoire de Dynamique, Interactions, et Réactivité (LADIR), F-75005, Paris, France CNRS, UMR 7075, Laboratoire de Dynamique, Interactions et Réactivité (LADIR), F-75005, Paris, France § School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ‡

ABSTRACT: Reactions between dilute methane and nonenergetic hydroxyl radicals were carried out at 3.5 K. The temperature was kept low in order to characterize the stepwise reaction and prevent parasitic side reactions. The hydroxyl radicals originate from discharged H2O/ He mixtures. The reactions were monitored in situ using a Fourier transform infrared spectrometer. The formation of CH3 radicals was confirmed simultaneously with the formation of water ice. Subsequent recombination reactions lead to the formation of ethane (C2H6). Production of ethane and water ice occur preferentially to the formation of methanol.

1. INTRODUCTION Water molecules are ubiquitous in the interstellar space. Water is the primary molecular ice accreted onto dust particles within gas clouds and is also considered as the main component of comets. While icy dust grains and comets evolve within the astrophysical environment, they may encounter high and lowenergy particles as well as fast atoms or molecules with which they will likely interact. Interactions of water ice with such low or high-energetic particles may lead to a significant production of OH radicals. (Reaction 1) H 2O + Energy → OH + H

the microwave discharge, and condensed afterward at a lower temperature, such as 3 K.6 Experimental conditions employing nonenergetic OH radicals are thought to reproduce astrophysical conditions found within dense molecular clouds which are opaque to UV radiations. These coldest environments support the formation of low-energy, ground-state OH radicals. The present experimental investigation on the interaction between ground-state methane and OH radicals will give further insights about chemical reactions that may occur in these extreme astronomical environments. One of the crucial questions arising while considering the following species [CH4+OH] is whether the reaction can induce the formation of methanol. This consideration has been discussed in previous studies that are presented in section 22. The experimental setup used for the present study is depicted in section 3. Results obtained are presented and discussed in section 4.

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Within the coldest environments (e.g., dense molecular clouds) water-rich icy dust grains may be gently activated to induce the formation of nonenergetic OH radicals. In addition, water-rich icy-grains and comets are known to incorporate plenty of other simple molecules such as CH4, CO or NH3. Methane was successfully identified in molecular clouds, in both solid and gaseous state.1−3 Consequently, highly reactive OH radicals are likely to be involved in further reactions with the surrounding simple molecules. Many laboratory experiments have been carried out to investigate astronomically relevant reactions involving OH radicals. While in most of these experiments highly excited molecules or energetic particles are involved, recent studies regarding the reactivity of nonenergetic OH radicals have been published.4−7 To this end, Oba et al. dissociated pure gaseous water in a microwave induced plasma and further condensed the species obtained at temperatures as low as 10 K. In order to limit the formation of side products, a gaseous mixture containing water diluted into a rare gas8 can be subjected to © 2012 American Chemical Society

2. EARLIER THEORETICAL AND EXPERIMENTAL INVESTIGATIONS The reaction between methane and OH radicals has been theoretically investigated by different groups especially for applications at high temperatures. Herein are summarized some of these theoretical investigations. Some of the thermodynamic data discussed below come from IUPAC evaluations.9,10 Received: July 13, 2012 Revised: November 28, 2012 Published: November 28, 2012 12357

dx.doi.org/10.1021/jp306963z | J. Phys. Chem. A 2012, 116, 12357−12363

The Journal of Physical Chemistry A

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Spin projection and Moller−Plesset perturbation theory were employed by Gonzalez et al.11 to obtain the barriers and energetics associated with the reaction 2 CH4 + OH → CH3 + H 2O

high pressure and high temperature conditions. Some of these investigations are summarized in Table 2. According to the results presented in Table 2, methanol may be produced from both nonenergetic methyl and OH radicals, since this reaction is highly exothermic. Furthermore, methanol is the most stable species reported in this table. The next most stable products (namely CH2O + H2 and H2CO + H2) are almost 100 kJ/mol less stable than the methanol. Thus, in the context of the chemistry that may take place in molecular clouds, we can expect that, among the reactions listed in the Table 2, only the reaction 4 leading to the formation of methanol will take place. Thus, reactions 2, 3, and 4 mais take place in dense molecular clouds. Because of the nature of the particular cold and solid environments (icy-grains, comets), the chemical processes involving CH4 and OH radicals are susceptible to various changes, supporting investigations under very low temperatures, low pressure, and nonenergetic conditions.

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Their results demonstrated that during this reaction a transient reaction intermediate is formed: CH3--H--OH. More recently, the spin contamination was taken into account in the prediction of the geometry for the open-shell saddle point associated with this reaction,12 and rate constants were calculated by direct dynamic.13−15 Furthermore, at temperatures as low as a few kelvin, the hydrogen transfer between methane and an OH radical may occur with no activation energy. Indeed, it is well-known that, at such low temperatures, tunnelling effects may play a prominent role, thus allowing reactions that would not occur otherwise. This specific point was theoretically investigated based on the variational transition-state theory.16−20 All these investigations suggest that the reaction between methane and OH radicals is exothermic, and the activation barrier that needs to be overcome for this reaction is relatively weak. Therefore, even at very low temperatures, we may expect the hydrogen transfer between methane and hydroxyl radicals to take place, leading to the formation of CH3 radicals. Once these methyl radicals are formed, theoretical results reported in the literature suggest that two main reactions may occur under our experimental conditions: 1 Methyl radicals may recombine with each other to form ethane (reaction 3) CH3 + CH3 → C2H6

3. EXPERIMENTAL SETUP The experimental methods and setup used for the present study have been previously described.39,40 Briefly, here we summarize the approaches used to synthesize the samples studied in the solid state. For this study, OH radicals were generated from discharged H2O/He mixtures. It was not possible under our experimental conditions to determine precisely the water concentrations in the mixtures subjected to the discharge; as a consequence, all the values given are only estimates. The head of the cryostat is rotatable (Cryomech PT 405) and consists of six mirrors maintained at 3.5 K. Each mirror can be exposed to the gas coming from the microwave discharge, and then experimentally probed byFourrier Transform infrared (FT-IR) spectroscopy (Figure 1). A microwave-driven atomic source (SPECS PCR-ECS) is used to process various concentrations of H2O/He mixtures prior to their deposition onto the cryogenic metal mirror maintained at 3.5 K. The amount of water can be estimated through the nature and the intensities of peaks in the 3000− 3800 cm−1. Before each experiment, the gas ramp is saturated with the H2O/He mixture that will be used. The microwave discharge source is based on the principle of electron cyclotron resonance (ECR). The pressure in the cryostat cell is always in the 10−7 mbar range except during the gas injection, i.e., during the synthesis of the sample. During the deposition, the pressure increases up to 10−5 mbars. It is to be underlined that the deposits are reproducible. Under such pressures, impurities (e.g., H2O, CO, and CO2) may be present. Therefore, in order to prevent contamination from the background gas as efficiently as possible, a 3 L nitrogen trap is included in the device, as shown in Figure 1. When using the nitrogen trap, water and other species are only present in trace amounts (few ppm). In addition, two thermal shields complete the device. One copper shield supplements the nitrogen trap and a second brass shield is supported by the first stage of the cryostat to prevent thermal heating from the walls of the chamber. A temperature probe fixed in the copper block is used to control the temperature with an accuracy better than 0.1 K. The cryogenic mirrors may act as a pumping stage for pollutants. In order to prevent this risk, the setup was evacuated at 7 × 10−7 mbar before refrigeration of the sample holder. A series of

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This reaction is known to take place in the atmospheres of Saturn and Neptune, where methyl radicals are relatively abundant.21 Rate constants associated with this reaction were determined at 298 and 202 K21 by using a discharge-flow kinetic technique coupled to mass spectrometric detection. The values they obtained are in Table 1. Table 1. Rate Constants (in cm3 molecule−1 s−1) Associated with the CH3 + CH3 → C2H6 Reaction, at Two Different Helium Pressures, According to Cody et al.21 P = 0.6 Torr helium 298 K 202 K

(2.15 ± 0.42) 10−11 cm3 molecule−1 s−1 (5.04 ± 1.15) 10−11 cm3 molecule−1 s−1

P = 1.0 Torr helium (2.44 ± 0.52) 10−11 cm3 molecule−1 s−1 (5.25 ± 1.43) 10−11 cm3 molecule−1 s−1

Interestingly, a higher rate of reaction was obtained at the lower temperature, supporting the fact that such a recombination reaction may be considered under our experimental conditions. 2 Methyl radicals may react with OH radicals to form methanol (reaction 4) CH3 + OH → CH3OH

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This reaction may be the first step of several consecutive and/or competitive channels involving multichannel radical−radical reactions or radical-molecule processes. The former routes were experimentally22−31 and theoretically32−36 studied mainly under 12358



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Table 2. Relative Energies (in kJ/mol) Associated with Different Reaction Pathways Involving CH3 and OH Radicals CH3 + OH CH3OH CH2O + H2 H2CO + H2 trans COH + H2 HCOH + H2 cis HCOH + H2 CH2--H2O 3 CH2 + H2O 1 CH2 + H2O CH2OH + H CH3O + H

energy (298 K)37

energy (298 K)23

energy (0 K)38

energy (0 K)36

energy (0 K)33

energy (0 K)34

0.0 −385.3 −292.3

0.0 −373.8

0.0 −377.6

0.0 −378.2

0.0 −384.5

0.0 −366.5

−285.4 −69.0

−292.3 −74.3

−292.6 −75.2

−308.5 −89.5

−285.1 −68.6

−51.5

−54.9

−57.7 −35.5

−71.5 −38.5

−51.4 −20.1

2.9 75.8 42.1

2.4

0.1 −6.7 57.3

−6.7 2.1 54.3

2.1

−70.0

−33.0 2.5 17.7 54.8

57.5

65.2

Figure 1. Schematics of the experimental device used for the present study.

and 500 cm−1 using a Bruker 120 HR Fourier Transform Infrared spectrometer equipped with a KBr/Ge beam splitter and liquid N2-cooled narrow band HgCdTe photoconductor. A resolution of 0.5 cm−1 was used. Bare mirror backgrounds recorded from 4500 to 500 cm−1 prior to sample deposition were used as references in processing the sample spectra. Absorption spectra in the mid-IR region were collected on samples through a KBr window mounted on a flange separating the interferometer vacuum (10−5 mbar) from the cryostat cell. The spectra were subsequently subjected to baseline correction to compensate for infrared light scattering and interference patterns. In all the figures, IR spectra were vertically offset, but neither the scales nor the abscissa were changed. While many laboratory simulations of astronomical experiments related with the interstellar clouds are carried out either at 10 or 20 K, we performed our experiments at 3.5 K. A temperature as low as 3.5 K was chosen because it allows for reaction intermediate trapping and thus prevents side reactions. Furthermore, some studies have suggested that very low temperatures, such as at a few kelvins, may also be relevant for some astronomical environments.41−43

preliminary experiments has shown that, within the time of the experiments, the amount of water pollution (and other contaminants) are below the detection threshold of our IR spectrometer. The coinjected gas is introduced in front of the cryostat, five millimeters away from the mirror that is exposed to the microwave discharge, so that no gas phase reactions can take place. The flux of chemicals reaching the mirror directly exposed to the source was estimated at 2.0 × 10 16 mol·s−1·cm−2. This flux was estimated by taking into account the amount of gas injected, in combination with the migration of the species all around the hexagonal sampler. Helium was obtained from “Air Liquide” with purity of 99.9995% and natural demineralized water was degassed in a vacuum line. The purity of samples was confirmed spectroscopically. The expansion beam issued from the plasma source makes an angle of 10° with the source axis. The codeposited gas mixtures are introduced in front of the cryostat via separate lines. Methane/He mixtures are injected five millimeters away from the mirror so that no gas-phase reactions can take place. The angle of the IR beam is 8° with respect to the normal of the mirror. The experimental method for the IR setup is detailed herein. Infrared spectra of the samples were recorded in the transmission-reflection mode between 4500 12359



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4. RESULTS AND DISCUSSION Figure 2a shows the IR spectrum obtained when the discharged H2O/He (10/100) mixture (i.e., OH radicals) is solely injected.

(OH-stretching), characteristic of the construction of small water aggregates, the CH4/OH coinjection experiment induces the formation of the conventional pattern corresponding to water ice (wide unstructured band centered at 3275 cm−1). Furthermore, both intensities of the water absorption bands located in the 3300 cm−1 region and in the 1650 cm−1 region (HOH bending) are slightly more intense in the spectrum obtained after the coinjection experiment. This result, obtained under nonenergetic conditions, supports the formation of water and CH3 radicals via hydrogen transfers from CH4 molecules to OH radicals, as described hereafter: OH + CH4 → CH3 + H 2O

(2)

These methyl radicals subsequently recombine with each other to form C2H6,51 as follows: CH3 + CH3 → C2H6

(3)

In addition to reaction 2, in which OH radicals act as proton acceptor while participating in a hydrogen transfer from methane molecules, hydroxyl radicals may also be involved in different recombination reactions. They can recombine to form water molecules (reaction 5):40,52

Figure 2. Spectra obtained after (a) deposition of a H2O/He (10:100) mixture, (b) deposition of a CH4/He (1/100) mixture, and (c) coinjection of a discharged H2O/He (10/100) mixture and a CH4/He (1/100) mixture at 3 K.

OH + OH → H 2O + O

Under these experimental conditions, OH radicals trapped in a water ice are observed (3689 cm−1). Figure 2b shows the IR spectrum obtained after the sole deposition of the CH4/He (1/ 100) mixture. Thus, in this experiment, OH radicals are absent from the mixture. IR absorption signatures of methane clearly emerge as two intense peaks at 1300.3 (−CH2 bend) and 3212.2 cm−1 (−CH2 asymmetric stretch). In addition, a weaker peak rises at 2813.6 cm−1 (−CH2 symmetric stretch). The IR spectrum obtained when the same CH4/He (1/100) mixture is coinjected with the discharged water/He (10/100) mixture is shown in Figure 2c. As a consequence of the presence of OH radicals in this mixture, the decrease of the IR intensities of methane is observed. Concomitantly, water ice is formed, as proved by the appearance of wide bands centered at 3300 and 1650 cm−1. The IR spectrum displayed after coinjection of CH4 and OH radicals is also characterized by the appearance of two new wide bands centered at 1425 and 2850 cm−1, respectively, that correspond to the species C2H6.44−46 The structure of the water ice formed after the CH4/OH coinjection experiment (Figure 2c), is differentiated by a strongly different IR signature than the one obtained when the discharged H2O/He (10/100) mixture (i.e., OH radicals) is solely injected. Indeed, an absorption band due to the OH radical interacting with water ice (3689 cm−1) is observed in the case of the simple injection experiment. Herein, we would like to briefly comment the attribution of this absorption band. The OH−H2O complex was observed in Ar matrix at 3452.1, 3442.1, and 3428.0 cm−1.47−49 However, the OH radical interacting with a water ice was observed between 3000 and 3800 cm−1 by Djouadi et al.50 and more precisely, at very low temperature, under our experimental conditions, the OH radical interacting with a water ice was characterized by a band at 3689 cm−1.7 When OH radicals and methane are coinjected, the band due to the OH radical interacting with water ice totally disappears suggesting that all OH radicals find a reaction partner, i.e., a methane molecule. Additionally, while the solely injection of the discharged water/He (10/100) mixture induces the formation of a structured wide band centered at 3400 cm−1

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or they can encounter residual H atoms stemming from the dissociation process occurring in the atomic source and react with them to form more water molecules (reaction 6):40

OH + H → H 2O

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Reactions 5 and 6 may also be responsible of the formation of water when the discharged H2O/He mixture is solely injected. Herein we would like to emphasize that the amount of water detected in the IR spectra is indeed due to recombination reactions, and not to residual water molecules exciting the microwave discharge. Indeed, it was shown that when a small amount of water molecules dilute into Ne are introduced in the plasma source and further condensed, all the water molecules are broken down, and the fragments are isolated in a Ne matrix.53 Thus, the observation of a water ice in the present experiments is indeed due to further recombination reactions between fragments exciting the discharge. In addition to C2H6, and in much smaller concentrations, other new products are formed when OH radicals and methane are coinjected. Indeed, Figure 3, which is a zoom of the spectrum 2c, shows the presence of CH3OH and H2CO. Furthermore, the formation of CO2 is also observed (Figure 2c). These three species are due to successive oxidation of the methyl radical. Indeed, and in competition with reactions 2, 5 and 6, OH radicals can react with methyl radicals to form methanol through a thermodynamically favorable reaction, according to theoretical studies mentioned previously (reaction 4). CH3 + OH → CH3OH

(4)

OH radicals can further oxidize the methanol formed to generate H2CO (reactions 7 and 8).54,55 CH3OH + OH → H3CO + H 2O

(7)

H3CO + OH → H 2CO + H 2O

(8)

From this latter product, a two-step reaction, involving the formation of the HCO radical as an intermediate (reactions 9 12360



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5. ASTRONOMICAL IMPLICATIONS AND CONCLUSIONS In dense interstellar clouds where nonenergetic OH radicals stemming from water ices may be present in close vicinity to methane molecules, the production of highly reactive methyl radicals via hydrogen transfers between OH radicals and CH4 molecules may occur. These transient methyl radicals are likely to recombine to form ethane and water ice as byproducts. Thus, OH radicals can play a fundamental role by creating carbon−carbon bonds and thus participating in the formation of alkane chains. Production of ethane and water ice occur preferentially to the formation of methanol. Our experimental results suggest that under nonenergetic conditions in addition to recombination reactions, a stepwise oxidation of methyl radicals to the formation of carbon dioxide takes place (Figure 4). These reactions may take place without any additional energy, thanks to tunnelling effects. Indeed, it is well-known that, at such low temperatures, tunnelling effects may play a prominent role, thus allowing reactions that would not occur otherwise. Furthermore, OH radicals can react with ethane via a hydrogen transfer to form C2H5.59 Therefore, further reaction routes may be considered: OH + CH4 → CH3 + H 2O (2)

Figure 3. Formation of H2CO and CH3OH observed after coinjection of a discharged water/He (10/100) mixture and a CH4/He (1/100) mixture at 3 K.

and 10), results in the generation of carbon dioxide (reaction 11). H 2CO + OH → HCO + H 2O

(9)

HCO + OH → CO + H 2O

(10)

CO + OH → CO2 + H

(11)

CH3 + CH3 → C2H6

As a highly reactive species, the HCO radical is not observed under our experimental conditions. As a consequence, OH radicals are involved as reagents in 9 different processes (reactions 2, 4−11). Two types of experiments may have helped us to further gain some insight onto the reaction pathways: 1 First, by varying the relative amounts of OH radicals and methane injected, it might have been possible to favor (with an excess of radicals) or disfavor (with an excess of methane) the formation of methanol over the formation of ethane. On the other hand, under our experimental conditions, namely OH/CH4: 10/1, the whole amount of hydroxyl radical initially deposited reacts when methane is coinjected, as proved by the disappearance of the band at 3689 cm−1. This total consumption of OH radicals, due to recombination reactions 5 and 6, hinders a concentration effect study. 2 Second, time-dependent experiments that are useful to ascertain a tunneling effect cannot be carried out, since such investigations require an excess of reactants.56−58

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OH + C2H6 → C2H5 + H 2O

(12)

C2H5 + CH3 → C3H8

(13)

Propane thus formed (reaction 13) may further react with OH radicals to form the intermediate species C3H7 and possibly form a new C−C bond with a surrounding methyl radical, as follows: OH + C3H8 → C3H 7 + H 2O

(14)

C3H 7 + CH3 → C3H8

(15)

Many experimental and theoretical studies are reported in the literature about hydrogen transfer between OH radicals and alkanes. As an example, reactions 12 and 14 were experimentally studied using the flash photolysis−resonance fluorescence technique under high temperatures and high pressure conditions.59 Theoretical investigations also showed that such processes are likely to take place.60 Under our experimental conditions, the occurrences of the carbon−carbon growth processes are likely to be hindered by

Figure 4. Reaction pathways tentatively proposed from our experimental data. 12361



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competitive reactions as well as by low-energy conditions. On the other hand, while side reaction byproducts may be formed in low concentrations, their IR signatures may be masked by the bands due to C2H6. Since such reaction routes may be involved in reactions leading to the formation of biologically relevant species, it might be interesting to consider such reaction pathways in the case of dense interstellar clouds containing methane.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the SMART Federation from the Université Pierre et Marie Curie, Paris VI and the PCMI (Physique et Chimie des Milieux Interstellaires) grant. P.R.J. thanks the Erasmus Mundus External cooperation Window Lot 15 (EMECW15) for providing scholarship.

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