Rate Constants and H-Atom Product Yields for the Reactions of O(1D

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A: Kinetics and Dynamics

Rate Constants and H-Atom Product Yields for the Reactions of O(D) Atoms with Ethane and Acetylene from 50 to 296 K 1

Dianailys Nunez-Reyes, and Kevin M Hickson J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b02267 • Publication Date (Web): 01 May 2018 Downloaded from http://pubs.acs.org on May 2, 2018

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The Journal of Physical Chemistry

Rate Constants and H-atom Product Yields for the Reactions of O(1D) Atoms with Ethane and Acetylene from 50 to 296 K Dianailys Nuñez-Reyesa,b and Kevin M. Hickson,a,b,* a

Université de Bordeaux, Institut des Sciences Moléculaires, F-33400 Talence, France b

CNRS, Institut des Sciences Moléculaires, F-33400 Talence, France

Abstract The gas phase reactions of atomic oxygen in its first excited state with ethane and acetylene have been investigated in a continuous supersonic flow reactor over the temperature range 50 K to 296 K. O(1D) atoms were produced by the pulsed laser photolysis of ozone at 266 nm. Two different types of experiments, kinetics measurements and H-atom product yield determinations, were performed by detecting O(1D) atoms and H(2S) atoms respectively by vacuum ultraviolet laser induced fluorescence. The measured rate constants are in agreement with previous work at room temperature and little or no temperature dependence was observed as the temperature is decreased to 50 K. H-atoms yields were found to be independent of temperature for the reaction of O(1D) with ethane. These product yields are discussed in the context of earlier dynamics measurements at higher temperature. Due to the influence of secondary reactions, no H-atom yields could be obtained for the reaction of O(1D) with acetylene.

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1. Introduction The reactions of atomic oxygen in its first excited state 1D, with hydrocarbons have been extensively studied given their importance in atmospheric and combustion chemistry

1–17

.

Likewise, recent studies highlight the relevance of these reactions as a possible approach for the formation of complex organic molecules on dust grains in star forming regions.18,19 Among all the hydrocarbons, significant attention has been paid to the dynamics and kinetics of the CH4 reaction with O(1D) due to its importance in the formation of OH radicals, which participate in the chemistry of the Earth’s ozone layer.15 In contrast, the reactions of larger saturated and unsaturated hydrocarbons such as ethane (C2H6) and acetylene (C2H2) with O(1D) are less well characterized. In the case of the O(1D) + C2H6 reaction, even though the reaction has numerous possible exit pathways,8,20 only four have been identified experimentally by crossed molecular beam measurements.7,8 O(1D) + C2H6 → CH3CHOH/C2H5O + H

(1)

→ CH2CHOH/CH3CHO + H2

(2)

→ C2H5 + OH

(3)

→ CH2OH + CH3

(4)

Several experimental and theoretical dynamics studies

4,8,16,21,22

suggest that the reaction

mechanism is dominated by oxygen insertion into the C-H bond forming a strongly bound ethanol intermediate complex that falls apart through several reaction pathways. Oxygen insertion into the C-C bond is thought not to occur as the lowest five singlet potential energy surfaces are all seen to be repulsive in the entrance region.16 Furthermore, a direct H-atom 2 ACS Paragon Plus Environment

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abstraction mechanism has also been proposed for the channel leading to OH formation.

6–8,20

Although most studies reported in the literature agree on the mechanism of the O(1D) + C2H6 reaction, the situation regarding the relative branching ratios is different. While Park and Wiesenfeld21 reported an absolute yield of 3% for channel (3) leading to OH formation and Matsumi et al.23 predicted a relative yield of 10% for channel (1) leading to H-atom formation, more recently Shu and coworkers7,8 reported the following relative branching ratios of 70% , 25%, 3%, and 2% for the channels (4), (3), (1), (2) respectively using the crossed molecular beam technique coupled with electron impact ionization at a collision energy of 33.5 kJ/mol. On the theoretical side, Sun and coworkers20 reported branching ratios of 60%, 8%, 4% and 1% for the channels (4), (3), (1) and (2) respectively and predicted a 27% yield for an additional channel leading to H2O formation. In contrast, the measurement of absolute rate constants for the O(1D) + C2H6 reaction has received less attention, with only a few determinations,3,23,24 and only one over a range of temperatures (220-300 K),23 with the rate constant values differing between them by a factor of 2 at room temperature. In the case of the O(1D) + C2H2 reaction, there are four exothermic exit pathways, O(1D) + C2H2 → 1CH2 + CO

(5)

→ 3CH2 + CO

(6)

→ HCCO + H

(7)

→ C 2 O + H2

(8)

with two of them (channels (5) and (7)) leading to spin-allowed products. There are very few previous studies of this reaction in the literature. Girard and Chaquin25 performed a theoretical investigation of this process providing basic information on the nature and stability of the likely 3 ACS Paragon Plus Environment


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primary products. As the focus of this article was the generation of potential energy surfaces to simulate the various H2C2O addition products of the O(1D) + C2H2 reaction, no further calculations were performed to determine the unimolecular dissociation products, which are expected in the gas-phase at low pressure. To the best of our knowledge, there are no other experimental or theoretical studies of the dynamics of this reaction. However, conclusions can be drawn from other related work26,27 treating some parts of the potential energy surface involved in this reaction. Furthermore, studies of the reaction of ground state atomic oxygen, O(3P), with C2H2 can offer valuable insight.28,29 There has only been one previous measurement of the rate constant for the O(1D) + C2H2 reaction at room temperature. 30 In the present paper, rate constants for both the O(1D) + C2H6 and O(1D) + C2H2 reactions were measured over the temperature range 50-296 K using a continuous supersonic flow reactor. Pulsed Laser Photolysis (PLP) and Vacuum Ultraviolet Laser Induced Fluorescence (VUV-LIF) were employed as O(1D) production and detection methods respectively. In addition, temperature dependent product branching ratio measurements were performed by following H-atom formation by VUV-LIF. Numerical simulations were performed to validate the branching ratio determination method. The experimental methods are described in Section 2. The results are presented in Section 3 and discussed in Section 4. Concluding remarks are given in Section 5. 2. Experimental methods The description of the technique,31,32 the experimental setup33–35 and the characteristics of the axisymmetric Laval nozzles36 used during this experiment can be found in previous work. Briefly, three argon based Laval nozzles were used in a continuous supersonic flow reactor to generate flows with characteristic temperatures of 50, 75 and 127 K. Experiments at room temperature were performed without a nozzle and by lowering the flow velocity. Although a 4 ACS Paragon Plus Environment

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range of nozzles is available in our laboratory, this work has been restricted to temperatures that can be attained with argon based Laval nozzles to avoid the fast removal of O(1D) atoms by collisions with carrier gases other than argon.34 Although it would have been advantageous to use helium as the carrier gas in these experiments given the much slower quenching rate of the O(1D) + He reaction,24 we do not currently possess any helium based Laval nozzles. O(1D) was produced in situ within the supersonic flow by the PLP of O3 at 266 nm using a frequency quadrupled Nd:YAG laser with pulse energies around 23 mJ for a beam with a 6mm diameter and a repetition rate of 10 Hz. To synthesize O3 molecules, a flow of O2 was introduced into a quartz cell continuously irradiated by a high-pressure mercury lamp producing O(3P) atoms from O2 photolysis. O3 formation occurred through the termolecular O(3P) + O2 + M → O3 + M reaction; a process which was enhanced by the use of higher pressures (700 Torr). The output of the cell was connected to the Laval nozzle reservoir. Residual O(3P) atoms formed in this way and those produced by PLP of O3 (with a quantum yield of approximately 10% around 266 nm)37 are unreactive with C2H6 and C2H2 at room temperature and below. 38 Two different types of experiments were performed in this study. Firstly, by following reagent O(1D) atoms it was possible to determine temperature dependent rate constants for the O(1D) + C2H6 and O(1D) + C2H2 reactions. Secondly, by following H(2S) atom formation it was possible to determine temperature dependent branching ratios for H-atom production. Both O(1D) and Hatoms were followed by the pulsed VUV-LIF method on resonance at 115.215 nm and 121.567 nm for O(1D) and H(2S) respectively. These wavelengths were produced using the second harmonic (532 nm) of a Nd:YAG laser to pump a dye laser. The resulting narrowband tunable beam was directed into a BBO (Beta Barium Borate) crystal to produce UV light that was focused into a cell containing a mixture of noble gases for frequency tripling (Xe for O(1D) 5 ACS Paragon Plus Environment


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detection34,39 and Kr for H(2S) detection,36 in both cases Ar was added for phase matching). The resulting VUV light was collimated at the output of the cell by a magnesium fluoride (MgF2) lens and directed into the reaction chamber perpendicular to the supersonic flow and the detector. The fluorescence emission of the sample was collected as a function of delay time between pump and probe lasers using a solar-blind photomultiplier tube (PMT) that was isolated from the flow reactor by a LiF window. The zone between the PMT and the window was maintained under vacuum to prevent supplementary losses by atmospheric absorption; moreover, a LiF lens was used to focus the fluorescence onto the PMT. Finally, the output of the PMT was connected to a boxcar integrator and a PC for signal acquisition and processing. The temporal profile of the fluorescence signal consisted of at least 70 points with 30 laser shots recorded at each delay time. In addition, several points were recorded with the probe laser firing before the pump laser to establish the baseline level. The gases (C2H6 99.5%, C2H2 99.6%, Ar 99.999%, Kr 99.99%, Xe 99.998%, O2 99.999%, H2 99.9999%) used in this study were flowed directly from cylinders through digital mass flow controllers into the reservoir. The flow controllers were calibrated by measuring the pressure rise in a known volume generated by each of the gases used in the experiment. 3. Results 3.1 Rate constants The hydrocarbons C2H6 and C2H2 were maintained in large excess with respect to O(1D) so that the pseudo-first-order approximation could be applied. Under these conditions, the decays of O(1D) atoms can be described by a functional fit of the form =

(9) 6

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Where is the O(1D) VUV fluorescence signal at a given time t, is the initial O(1D) VUV fluorescence signal, and is the pseudo-first-order rate constant for the loss of O(1D) atoms. = , where is the second-order rate constant, is the concentration of hydrocarbon and represents mainly the first-order loss of O(1D) through deactivation with argon. Argon concentrations were in the range (1.26 - 2.59) 1017 cm-3 and the rate constant for O(1D) quenching by Ar has been measured with a value around 6 10-13 cm3 s-1 over the 50-300 K temperature range.34 Other losses such as deactivation with residual O2 in the reactor and diffusional loss from the probe volume were expected to be negligible in comparison. Typical O(1D) pseudo-first-order kinetic decays are shown in Figure 1.

Figure 1 O(1D) VUV LIF signal as a function of time at 75 K. (Blue open squares) without C2H2;



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(red open circles) with [C2H2] = 4.6 1014 cm-3. Solid blue and red lines represent functional fits to the data using expression (9). The second-order rate constants were obtained from a weighted linear least-squares fit to the data when is plotted against the excess reagent concentration. The weighting used was based on the statistical uncertainties generated during the fitting procedure to the pseudo-first-order kinetic decays. At each temperature, at least 39 pseudo-first-order decays were recorded with a minimum of 12 different concentrations. Examples of these plots obtained for the O(1D) + C2H6 and C2H2 reactions are shown in Figure 2.


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Figure 2 Measured pseudo-first-order rate constants as a function of hydrocarbon concentration. Upper panel O(1D) + C2H6 reaction at 50 K (blue open diamonds) and at 296 K (red solid circles). Lower panel O(1D) + C2H2 reaction at 50 K (magenta open squares) and at 75 K (dark yellow solid triangles). Solid lines represent weighted fits to the individual data with statistical uncertainties (1σ) derived from fits to the pseudo-first-order kinetic decays.



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The y-axis intercept values of these plots essentially represent the pseudo-first-order loss rate for O(1D) atoms through collisions with the carrier gas Ar. The rate constants obtained for the O(1D) + C2H6 and O(1D) + C2H2 reactions are displayed as a function of temperature in Figure 3 and are summarized in Table 1. The error bars are a combination of the statistical and systematic errors. Systematic errors could arise from several sources including uncertainties in the calibration of the mass-flow controllers and the subsequent determinations of the concentrations of the reactants and the supersonic flow density as well as possible errors in pressure gauges and other instruments. These systematic errors are estimated to be around 10%.


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Figure 3 Measured rate constants as a function of temperature. Upper panel, the reaction of O(1D) + C2H6; (dark red triangle) Matsumi et al.23 ; (dark yellow diamond) Dillon et al.3; (black square) Schofield 24 ; (blue circles) this work. Lower panel, the reaction of O(1D) + C2H2; (green open square) Carl30; (red triangle) this work. Error bars represent the combined statistical and systematic uncertainties.



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Table 1 Measured rate constants for the O(1D) + C2H6 and O(1D) + C2H2 reactions.

T/K

N

b

[C2H6] / 1014 cm-

kO(1D) + C2H2

kO(1D)+C2H6 N

3

/ 10

-10

b

14

[C2H2] / 10 cm

3 -1

-3

/ 10-10 cm3 s-1

cm s

42

0-10.7

(3.5 ± 0.3)c

42

0 -12.8

(3.5 ± 0.3)

127±2a 63

0-7.8

(2.7 ± 0.3)

42

0-9.4

(3.2 ± 0.3)

75±2

41

0-3.8

(3.1 ± 0.3)

41

0-4.7

(3.1± 0.3)

50±1

40

0-5.3

(3.2 ± 0.3)

39

0-1.4

(2.6 ± 0.4)

296

a

Uncertainties on the calculated temperatures represent the statistical (1σ) errors obtained from

Pitot tube measurements of the impact pressure; c

b

Number of individual measurements;

Uncertainties on the measured rate constants represent the combined statistical and systematic

errors. Product Branching ratios In a second type of experiment, relative branching ratios for H-atom formation channels were determined by following the formation of product H-atoms. By comparing the VUV LIF signal intensities of atomic hydrogen produced by the target reaction with the ones produced by a reference reaction, namely the O(1D) + H2 reaction with a known H-atom yield of 100%,35 it was possible to extract absolute branching ratios. Some typical H-atom formation curves are shown in Figure 4. These profiles can be described by a functional fit of the form

= −

(10)

Where A is the signal amplitude, is the loss of H by secondary reactions and diffusion from the reactor and = , where is the second order rate constant for the O(1D) + X 12 ACS Paragon Plus Environment

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reaction and includes the contribution of each of the different exit channels = ⋯, is the hydrocarbon concentration and represent the first-order loss of O(1D) through deactivation with argon as explained previously. These curves are characterized by an initially increasing part to the maximum intensity (!"# ) followed by a slower decay. It can be shown that !"# can be described by the following expression !"# =

$% $& $⋯$' (

)*+

(11)

Where the maximum time, ,!"# , is given by ,!"# =

ln

(12)



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Figure 4. H(2S) VUV LIF intensity as a function of the time recorded at 127 K. (Blue solid circles) [C2H6] = 7.75 x 1014 cm-3; (red solid squares) [C2H6]= 8.32 x 1013 cm-3. Black arrows indicate the maximum intensity and maximum time in each case. In previous work,13 we used fits of the type given by expression (10) to obtain A values for the target O(1D) + CH4 and reference O(1D) + H2 reactions, allowing temperature dependent branching ratios to be extracted from the data. In this earlier paper, it was necessary to ensure that the values of ,!"# for each profile were identical to accurately determine these branching ratios due to the non-negligible quenching contribution of O(1D) atoms by argon. In the present experiments, an alternative method was implemented to obtain product branching ratios based on measurements of the peak intensity recorded at different ,!"# values for different concentrations of coreagent for the target and reference reactions. Prior to the experiments, ,!"# was calculated using expression (12) for a range of coreagent concentrations using values of k′ that were already determined in this study through kinetic experiments following O(1D) loss. Values of were obtained from previous H-atom detection experiments performed under similar conditions (where secondary reactions are unimportant). The calculated ,!"# values varied between 12 and 33 µs as a function of the coreagent concentration at all the temperatures, with values in the range 2000-5000 s-1. During the experiments, the delay time between photolysis and probe lasers was set to ,!"# , and !"# was recorded for a given coreagent concentration. The procedure was then repeated for different coreagent concentrations (with different ,!"# values). The exponential part of expression (11) can be approximated by a Taylor series expansion ( # ≈ 1 − 2 as the product ,!"# is small, so that plots of !"# against ,!"# for the target and reference reaction yield


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straight lines as shown in Figure 5. Finally, the branching ratios can be extracted from the ratio of the slopes of the two plots.

Figure 5 Maximum intensity, !"# , as a function of the maximum time, ,!"# , at 50 K; (red squares) reference O(1D) + H2 reaction; (blue circles) target O(1D) + C2H6 reaction. Solid lines represent weighted fits to the individual data based on the standard error derived from an average of the number of laser shots recorded at each ,!"# . (see text for details) The plots of !"# against ,!"# consist at least of 6 different concentration points with each point itself the average of at least 200 laser shots. The slopes of the curves were obtained from a weighted linear least-squares fit to the data with the weight derived from the standard error



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obtained from the averaging procedure. Due to the presence of residual gases in the chamber such as C2H6, !"# was corrected to account for absorption losses of the VUV excitation laser and fluorescence intensities. This correction was estimated to be less than 25% for C2H6 over the entire temperature range. The absolute branching ratios as a function of temperature for the O(1D) + C2H6 reaction are reported in Table 2. Table 2 H-atom yields for the O(1D) + C2H6 reaction as a function of temperature

a

H-atom yield

T/K

Na

296

4

0.18 ± 0.007b

127±2c

6

0.18 ± 0.02

75±2

4

0.18 ± 0.03

50±1

4

0.18 ± 0.01

O(1D) + C2H6

Number of branching ratios determinations. bThe error bars reflect the statistical uncertainties at

the 95% confidence level. cUncertainties on the calculated temperatures represent the statistical (1σ) errors obtained from Pitot tube measurements of the impact pressure. Unfortunately, no branching ratios could be determined for the O(1D) + C2H2 reaction due to the influence of secondary reactions leading to the production of H-atoms. In this case, it is likely that one of the major reaction products, 1CH2 (+ CO), reacts rapidly with C2H2 to form H-atoms (and C3H3) as products, with a yield that increases from 0.28 at 195 K to 0.88 a 298 K. 40,41 To validate this new methodology, branching ratios were determined at 127 K using the previous methodology implemented in our laboratory where the C2H6 and H2 concentrations used in the experiments are adjusted to obtain H-atom formation curves with similar time constants (k′). These curves are fitted using expression (10), in which the signal amplitude A represents the


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theoretical H-atom yield in absence of competing losses. In these experiments, three pairs of Hatom formation profiles were recorded and the signal amplitude was corrected for absorption losses of the VUV light by residual C2H6 in the chamber. A H-atom yield of 0.19 ± 0.03 was obtained with this methodology, in excellent agreement with the branching ratios reported in Table 2. As a secondary check of the new method for the determination of the branching ratios, numerical simulations were performed using the differential integrator FACSIMILE. Here, the branching ratios obtained for the O(1D) + C2H6 reaction during the experiments were used as input parameters in order to generate H-atom formation profiles at different concentrations of C2H6. Similarly, H-atom formation profiles were generated for the O(1D) + H2 reaction. The simulations included all the different processes that could occur in the reactor such as the deactivation of O(1D) through collisions with the carrier gas as well the diffusional losses of both O(1D) and H(2S) from the detection region. The branching ratios determined by plotting the simulated values of !"# against ,!"# were identical to the input values, indicating the negligible influence of secondary processes in the branching ratio measurements. 4. Discussion In the case of the O(1D) + C2H6 reaction, Matsumi et al.23 reported a rate constant of (6.3 ± 0.3) 10-10 cm3 s-1 at room temperature following the time evolution of the H product signal by VUVLIF. More recently, Dillon et al.3 extended the available kinetics data over the temperature range 220-300 K by detecting the OH product of the O(1D) + C2H6 reaction by LIF, finding a temperature independent rate constant of (3.4 ± 0.7) 10-10 cm3 s-1. The latter result is in good agreement with the recommended value of 3.2 10-10 cm3 s-1 from the critical evaluation of relative rate constants made by Schofield.24 As can be seen in Figure 3, the rate constants for the O(1D) + C2H6 reaction measured in the present work are in excellent agreement with the previous 17 ACS Paragon Plus Environment


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experimental result of Dillon et al.3 and the evaluation of Schofield24 while the experimental value reported by Matsumi et al.23 is two times larger at room temperature. This difference has been attributed3 to possible errors in the concentration determination in the work of Matsumi et al.23 No temperature dependence is observed in the rate constant as the temperature is decreased to 50 K, in agreement with the previous results from Dillon et al.3 at higher temperature. The general consensus is that the mechanism of the O(1D) + C2H6 reaction is dominated by an O(1D) insertion into the C-H bond forming an energized ethanol intermediate complex that falls apart to give several exit channels. As the lifetime of this intermediate is calculated to be only a few picoseconds due to rapid unimolecular dissociation,20 complex stabilization will not occur under the present experimental conditions. The channel leading to OH production has attracted the most attention, due to the possible contribution of direct H-atom abstraction6–8,20 alongside the insertion mechanism. On the experimental side, there are several previous determinations of the product branching ratios for the reaction of O(1D) with C2H6. Shu et al. 7,8 studied the reaction of

18

O(1D) + C2H6 using a crossed molecular beam technique coupled with a mass-selective,

electron impact ionization detector at collisional energies of 33.5 kJ/mol. The authors reported the following relative product branching ratios: CH2OH + CH3 (4) with a 70% yield; C2H5 + OH (3) with a 25% yield; CH3CHOH + H (1) with a 3% yield and CH2CHOH/CH3CHO + H2 (2) with a 2% yield. On the theoretical side, Sun et al.20 reported branching ratios at 33.5 kJ/mol in parallel with the experimental results of Shu et al. 7,8 using the combined quantum chemistry calculation and Rice–Ramsperger–Kassel–Marcus (RRKM) approach. They found contributions of 60%, 8%, 4% and 1% for the channels 4, 3, 1 and 2 respectively and predicted a 27% yield for an additional channel leading to H2O, which was not detected in the crossed molecular beam experiment. Sun et al.20 underestimated the experimental branching ratio for OH formation


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obtained by Shu et al.7,8; a result that was attributed to the non-consideration of the abstraction channel in those calculations. In addition, Gonzalez et al.6 performed an experimental characterization of the OH state distributions produced by the O(1D) + C2H6 reaction at an average collision energy of 50 kJ/mol. Although their experimental results were not fully conclusive regarding the influence of the direct abstraction mechanism, their complementary analysis using quasiclassical trajectory calculations predicted an occurrence of 1-2% of abstraction like reactive trajectories. These authors analyzed the influence of the collision energy on the dynamics of the reaction, comparing with the experimental work of Park and Wiesenfeld21 performed at 26 kJ/mol, concluding that insertion followed by unimolecular dissociation should be the most important reaction mechanism at low and intermediate temperature.6 In addition to these studies of the O(1D) + C2H6 reaction at high collision energies, an earlier study by Park and Wiesenfeld21 based on the energetic characterization of nascent OH products by LIF, predicted an absolute yield of 3% for the channel leading to OH formation (3) at thermal collision energies (300 K equivalent to 3.7 kJ/mol). Furthermore, Matsumi et al.23 determined Hatom formation yields at room temperature in a similar manner to the present experiments by following atomic hydrogen production by VUV LIF at 121.567 nm. They determined the H-atom yield to be around 10% by comparing the H-atom and D-atoms yields of the O(1D) + C2H6 and O(1D) + D2 reactions respectively. These authors stated that at this temperature, the H-atom product is mostly formed from rupture of the O-H bond (C2H5O + H channel) in contrast with the findings at high collision energies7,8. The H-atom product yields measured in this work, summarized in Table 2, are independent of temperature in the 50-296 K range. The present results are in reasonable agreement with the value reported by Matsumi et al. at room temperature but they are somewhat higher than the experimental and theoretical H-atom branching ratios of



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Shu et al.7,8 and Sun et al.20 described above. This finding is not unexpected as the most thermodynamically favorable channel (CH3CHOH + H) is thought to dominate at these higher collision energies (equivalent to temperatures around 2700 K)7,8 whereas the kinetically favorable products (C2H5O + H) are likely to dominate at room temperature and below.23 In the case of the O(1D) + C2H2 reaction, there is only one previous determination30 of the rate constant using a time resolved chemiluminescence detection method. Carl30 reported a rate constant of (3.1 ± 0.2) 10-10 cm3 s-1 at 295 K, which is in excellent agreement with the rate constant determined in this work at 296 K. Furthermore, as can be seen in Figure 3 there is little or no variation of the reaction rate as the temperature is lowered, with the differences well within the combined error bars. There is very little information regarding the dynamics of the O(1D) + C2H2 reaction. Girard and Chaquin25 theoretically studied this process, calculating the potential energy surface using unrestricted Density Functional Theory (UDFT). Several minima were found on the potential energy surface, first due to the formation of a C=O bond as a result of the O(1D) addition to the C≡C bond leading to formylcarbene (HCCHO) and/or oxirene (C2H2O) structures and secondly due to the insertion of O(1D) into a C-H bond, resulting in ethynol (CHCOH) as the most likely product. From the stability calculations, these authors suggested that at low temperatures both oxirene and formylcarbene structures could exist, with formylcarbene the more stable species by approximately 4.18 kJ/mol, while at room temperature a further step is expected involving the interconversion of formylcarbene/oxirene into ketene. As the focus of this article was the generation of potential energy surfaces for the simulation of the addition products of the O(1D) + C2H2 reaction in matrix type experiments, no further calculations were performed to determine the unimolecular dissociation products that are expected in the gas-phase. 20 ACS Paragon Plus Environment

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Further information regarding the potential energy surface for the O(1D) + C2H2 reaction can be derived from theoretical studies of the reaction between O(3P) and C2H2, where intersystem crossing (ISC) to the singlet state could play an important role.28,29 These potential energy surfaces, calculated at the unrestricted UB3LYP /6-31G (d,p) level, were used to perform nonadiabatic dynamics studies for O(3P) + C2H2 at three different reagent collisions energies (34.3; 39.7; 54.8 kJ/mol) to investigate their effect on the extent of ISC. Branching ratios for the primary product channels in the O(3P) + C2H2 reaction were determined to be around 80% for the channel leading to H+HCCO and 20% for the production of

1,3

CH2 + CO at all collisional

energies. However, the product yield of 1CH2 was calculated to decrease from 11% to 5% with increasing collision energy, with a corresponding increase in the branching ratio for 3CH2 formation due to a lower probability for ISC. These authors present the potential energy surface for the O(1D) + C2H2 system predicting an adiabatic pathway leading to 1CH2 + CO. Interestingly, the potential energy surface for the O(1D) + C2H2 system involves the formation of ketene (H2C=C=O) as an intermediate. The dynamics of ketene photodissociation has been extensively studied both experimentally and theoretically. The CASPT2 study of Xiao and coworkers26 predicts that once ketene is excited to its first excited singlet S1 state, it can then evolve to either the ground triplet T1 or ground singlet S0 state (this latter state corresponding to the potential energy surface involved in the O(1D) + C2H2 reaction). Once in the S0 state, the unimolecular dissociation of ketene can yield H + HCCO and 1CH2 + CO as adiabatic products and 3CH2 + CO as non-adiabatic products through ISC. The relative branching ratios between the respective products channels will depend somewhat on the photon energy. If the energy is much larger that the corresponding dissociation barrier, adiabatic product formation should be favored so that 1CH2 + CO and H + HCCO will be the major products with 3CH2 + CO as minor products.



Page 22 of 29

This hypothesis is corroborated by the experiments of Fockenberg27 who studied the product distribution of ketene photolysis at 300 K, measuring the following product yields 1CH2 + CO (66 ± 8)%, HCCO + H (17 ± 7)% and 3CH2 + CO (6 ± 9)%. Unfortunately, in this work, we were not able to determine H-atom branching ratios to clarify the reaction mechanism. However, the O(1D) + C2H2 reagents are 266.9 kJ/mol above the O(3P) + C2H2 reagents. As a result, the intermediate 1C2H2O complex is highly energetic and is therefore expected to have a short life-time reducing considerably the probability of ISC to the triplet surface. Consequently, although there are four exothermically accessible exit channels for this reaction, only two of them (reactions 5 and 7) lead to spin allowed products, which should therefore represent the major exit channels. Indeed, our observation of significant quantities of secondary H-atoms clearly indicates that 1CH2 + CO products represent one of the major channels of the O(1D) + C2H2 reaction. 5. Conclusions In this study, rate constants for the reaction of excited state oxygen atoms, O(1D), with C2H6 and C2H2 have been determined for the first time over the 50-296 K range using a continuous supersonic flow reactor. O(1D) was produced by the pulsed laser photolysis of O3 molecules and vacuum ultraviolet laser induced fluorescence was used as a detection method. The measured rate constants for these reactions are in good agreement with the previously reported literature values. Moreover, considering the experimental errors, little or no temperature dependence was observed for both reactions over the entire temperature range. In addition to kinetic measurements, a new methodology has been applied for the determination of branching ratios for the H-atom production channels of the O(1D) + C2H6 reaction. This method has been shown to be valid through comparison with the results of test experiments and through numerical simulations. 22 ACS Paragon Plus Environment

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Branching ratios for the H-atom formation channels of the O(1D) + C2H2 reaction could not be determined due to atomic hydrogen formation by secondary reactions in this system. AUTHOR INFORMATION

Corresponding Author * Correspondence to: [email protected]. Tel: +33 (0)5 40 00 63 42 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Acknowledgement K.M.H. and D.N.R acknowledge support from the French program ‘‘Physique et Chimie du Milieu Interstellaire” (PCMI) of the CNRS/INSU with the INC/INP co-funded by the CEA and CNES. References 1.

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Rate Constants and H-Atom Product Yields for the Reactions of O(1D

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