Branching Ratios in the Hydroxyl Reaction with Propene - The Journal

ARTICLE pubs.acs.org/JPCA

Branching Ratios in the Hydroxyl Reaction with Propene Lev N. Krasnoperov,*,† Nadezhda Butkovskaya,‡ and Georges Le Bras‡ †

Department of Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, United States ‡ Institut de Combustion, Aerothermique, Reactivite et Environnement (ICARE), CNRS-INSIS, 45071 Orleans, France ABSTRACT: The branching ratios for the reactions of attachment of hydroxyl radical to propene and hydrogen-atom abstraction were measured at 298 K over the buffer gas pressure range 60-400 Torr (N2) using a subatmospheric pressure turbulent flow reactor coupled with a chemical ionization quadrupole mass spectrometer. Isotopically enriched water H218O was used to produce 18Olabeled hydroxyl radicals in reaction with fluorine atoms. The β-hydroxypropyl radicals formed in the attachment reactions 1a and 1b, OH þ C3H6 f CH2(OH)C•HCH3 (eq 1a) and OH þ C3H6 f C•H2CH(OH)CH3 (eq 1b), were converted to formaldehyde and acetaldehyde in a sequence of secondary reactions in O2- and NO-containing environment. The 18O-labeling propagates to the final products, allowing determination of the branching ratio for the attachment channels of reaction 1. The measured branching ratio for attachment is β1b = k1b/(k1a þ k1b) = 0.51 ( 0.03, independent of pressure over the 60-400 Torr pressure range. An upper limit on the hydrogen-abstraction channel, OH þ C3H6 f H2O þ C3H5 (eq 1c), was determined by measuring the water yield in reactions of OH and OD radicals (produced via H(D) þ NO2 f OH(OD) þ NO reactions) with C3H6 as k1c/(k1a þ k1b þ k1c) < 0.05 (at 298 K, 200 Torr N2).

’ INTRODUCTION Oxidation of unsaturated hydrocarbons is of general interest for both atmospheric and combustion chemistry. Propene is of special interest being an intermediate in combustion mechanisms, an atmospheric pollutant, and a model compound as the smallest asymmetric alkene. Oxidation of propene is mainly driven by reactions with hydroxyl radical in both combustion and atmospheric environments. Under atmospheric conditions, abstraction is a minor channel and the major route is attachment of hydroxyl to the double bond.1 It is generally assumed that attachment proceeds mainly to the peripheral carbon atom of the double bond, forming a 1-hydroxy-2-propyl radical (reaction 1a) OH þ C3 H6 f CH2 ðOHÞC 3 HCH3

ð1aÞ

f C 3 H2 CHðOHÞCH3

ð1bÞ

The notion of the domination of channel 1a (ca. 65%) in the attachment is quite common (e.g., refs 2 and 3) and mainly based on the unpublished work of Cvetanovic.4 An estimate based on the site-specific reactivity increments (structure-activity relationship, SAR5,6) yields a branching ratio of ca. 90% for channel 1a. However, there are some indications that channel 1b could be of comparable importance. In an EPR study of the radicals formed in the liquid phase of propene/H2O2 mixtures under UV light, the lines assigned to the two isomers produced in channels 1a and 1b were of comparable intensity.7 Moreover, theoretical calculations suggest that channel 1b should be a major channel, as it is more energetically favorable.8 Comparable r 2011 American Chemical Society

importance of the two channels is also indicated in a recent theoretical paper.9 Abstraction channel 1c is important at high temperatures, such as in combustion of propene10 OH þ C3 H6 f H2 O þ C3 H5

ð1cÞ

This channel dominates at temperatures above 700 K, where positive temperature dependence is observed.10-12 The addition pathways 1a and 1b dominate at low temperatures, where the overall rate constant of reaction 1 exhibits a negative temperature dependence.12-14 At ambient temperatures and below, the abstraction channel, eq 1c, is of minor importance. The branching ratio for this channel at ambient temperature does not exceed 5%.15 Recent theoretical calculations resulted in ca. 1% for the abstraction channel.9 This study is an experimental determination of the branching ratio for attachment of hydroxyl radical to propene. The branching ratios in reaction 1 were measured at 298 K over the buffer gas pressure range 60-400 Torr (N2) using a subatmospheric pressure turbulent flow reactor coupled with a chemical ionization (CI) quadrupole mass spectrometer. In the determination of the branching ratio for the addition channels, 18O-labeled hydroxyl radicals were used to trace the attachment position via isotopic analysis of the final products in reaction mixtures containing O2 and NO. The hydroxypropyl radicals formed in Received: July 30, 2010 Revised: January 4, 2011 Published: March 02, 2011 2498

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The Journal of Physical Chemistry A

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the attachment reactions 1a and 1b were converted to formaldehyde and acetaldehyde in a sequence of secondary reactions. The 18 O-labeling propagates to the final products allowing determination of the branching ratios in reaction 1. The branching ratio for the abstraction channel 1c was estimated based on the H2O and HOD yields in reactions of OH and OD radicals with propene.

’ EXPERIMENTAL SECTION Approach. The approach is based on the following sequence of secondary reactions following hydroxypropyl radical formation in reactions 1a and 1b with hydroxyl radicals isotopically labeled by 18O. The reaction sequence is well established.2 The sequence consists of attachment of the oxygen molecule, converting hydroxyalkyl radicals into peroxy radicals (the star symbol denotes 18O atom) CH2 ðOHÞC 3 HCH3 þ O2 f CH2 ðOHÞCðOO 3 ÞHCH3 ð2aÞ

C 3 H2 CHðOHÞCH3 þ O2 f CðOO 3 ÞH2 CHðOHÞCH3

ð2bÞ

Peroxy radicals are converted into alkoxy radicals in reactions with NO CH2 ðOHÞCðOO 3 ÞHCH3 þ NO ð3aÞ f CH2 ðOHÞCðO 3 ÞHCH3 þ NO2 CðOO 3 ÞH2 CHðOHÞCH3 þ NO f CðO 3 ÞH2 CHðOHÞCH3 þ NO2

ð3bÞ

It is generally accepted that β-hydroxypropoxy radicals produced in reactions 3a and 3b undergo fast unimolecular dissociation (>1 106 s-1, at 298 K and 1 bar)2,16-18. Decomposition of 1-hydroxy-2-propoxy radical from eq 3a produces acetaldehyde and isotopically labeled CH2(*OH) radical, which forms isotopically labeled formaldehyde in reaction with molecular oxygen (or NO) CH2 ðOHÞCðO 3 ÞHCH3 f CH2 ðOHÞ þ CH3 CHO ð4Þ CH2 ðOHÞ þ O2 ðor NOÞ f HO2 ðor HNOÞ þ H2 CO ð5Þ In a similar manner, decomposition of 2-hydroxy-1-propoxy radical formed in reaction 3b produces formaldehyde and isotopically labeled CH(*OH)CH3 radical and, subsequently, isotopically labeled acetaldehyde CðO 3 ÞH2 CHðOHÞCH3 f CH2 O þ CHðOHÞCH3 ð6Þ CHðOHÞCH3 þ O2 ðor NOÞ f HO2 ðor HNOÞ þ CH3 CHO

ð7Þ

Fast abstraction reactions of the weakly bound hydrogen atom from the hydroxyl group of the radical lead to the same final products CðO 3 ÞH2 CHðOHÞCH3 þ O2 ðor NOÞ f HO2 ðor HNOÞ þ CH2 O þ CH3 CHO ð8Þ In the present work, the occurrence of the fast decomposition

of β-hydroxypropoxy radicals was confirmed experimentally by measuring NO2 and CH3CHO concentrations in the OH/C3H6/ O2/NO system. The concentrations, obtained at 50 and 200 Torr, were equal within the experimental uncertainty of ca. 5%. Consequently, attachment of isotopically labeled OH to the peripheral carbon atom leads to isotopically labeled formaldehyde, and attachment to the central atom leads to isotopically labeled acetaldehyde, irrespective of the fine details of the transformation mechanism of the two isomers of hydroxypropoxy radical. Therefore, measuring the relative isotopic composition of formaldehyde and acetaldehyde (the first approach) or just the isotopic composition of acetaldehyde (the second approach) when 18O isotopically enriched hydroxyl radical is used allows determination of the branching ratios in the attachment reaction 1. The analysis shows that in the first approach neither information on the initial enrichment of hydroxyl radicals nor the detailed reaction mechanism is required. However, the second approach relies upon the initial isotopic enrichment, a detailed reaction model, and the reaction time. In this study, we pursued both approaches. Chemical ionization with negative-ion mass spectrometry allows detection of only acetaldehyde out of the two final products, although with excellent sensitivity. Proton transfer ionization with positive-ion mass spectrometry allows detection of both formaldehyde and acetaldehyde, although with a somewhat lesser sensitivity. Determination of the branching ratio of the abstraction channel 1c was based on the yield of water, H2O, and HDO in the reaction of OH and OD radicals with propene in comparison with the water yield when propene was replaced with cyclohexane, where abstraction is the only channel of the reaction. Experimental Setup. The experimental setup is described in detail elsewhere;19 only a brief description is given here. The experiments were performed using a subatmospheric pressure turbulent flow reactor (i.d. = 2.4 cm) coupled with an ionmolecule reactor and a quadrupole mass spectrometer. The pressure of the reacting mixture (N2 as the bath gas) was varied over the 60-400 Torr range. All experiments were performed at ambient temperature, 295 ( 2 K. Typical flow velocity was ca. 17-18 m/s; the Reynolds numbers were in the range 30007100. The flow at these Reynolds numbers is turbulent, which provides fast radial mixing of the reactants. 18O-Labeled hydroxyl radicals were produced in a movable concentric injector (i.d. = 1.1 cm) using reaction of fluorine atoms with H218O F þ H2 18 O f HF þ 18 OH

ð9Þ

Fluorine atoms were produced in microwave discharge in F2/He mixtures in a discharge tube of the movable injector (see Figure 1 of ref 19). The maximum distance between the movable inlet and the sampling orifice was 50 cm, which corresponds to the maximum reaction time of ca. 30 ms. In the abstraction channel evaluation, hydroxyl and hydroxylD radicals were produced in the reactions of H and D atoms with nitrogen dioxide in the movable injector H þ NO2 f OH þ NO

ð10Þ

D þ NO2 f OD þ NO

ð10DÞ

The branching ratio for the H-atom abstraction by OH radical was determined at 200 Torr and 298 K by measuring the yield of 2499

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water in the OH þ propene and OD þ propene chemical systems OH þ C3 H6 f H2 O þ C3 H5

ð1cÞ

OD þ C3 H6 f HDO þ C3 H5

ð1cDÞ

The amount of H2O or HOD formed was compared with that from the analogous reactions with cyclohexane proceeding via a sole water forming channel OH þ C6 H12 f H2 O þ C6 H11

ð11Þ

OD þ C6 H12 f HDO þ C6 H11

ð11DÞ

A very small secondary kinetic isotope effect is expected for this type of abstraction reaction,20-22 implying that the branching ratios are similar for reactions 1c and 1cD. Propene, oxygen, nitric oxide, and cyclohexane were introduced directly to the turbulent flow reactor, where they were mixed with the carrier gas flow (N2) upstream of the tip of the movable inlet. Typical concentrations of the reactants were [OH] = (1.5-6.0) 1011, [O2] = (0.1-2.0) 1016, [NO] = (0.3-1.5) 1014, [C3H6] = (1.7-70) 1013, and [C6H12] = (1.6-2.0) 1013 molecules cm-3. Gases and Reactants. Nitrogen, used as the bath gas in all experiments, was obtained by evaporation of liquid nitrogen (Air Liquide). The purity of all other gases and reagents were as follows: AlphaGaz 2 argon (>99.9999%); AlphaGaz 2 helium (>99.9999%); AlphaGaz 2 oxygen (>99.9995%); nitrogen dioxide, NO2, Air Liquide (0.50 ( 0.01%) in N2; propene, C3H6, Air Liquide N25 (>99.5%); cyclohexane, C6H12, Riedel-deHaen:: > 99.5%; water-18O, Aldrich, 97% 18O atom percent. Nitric oxide purchased from AlphaGaz (>99%) was additionally purified in a series of freeze-thaw cycles from melting ethanol. The residual level of impurities was estimated as 3 10-4. Ion-Molecule Reactor and Chemical Ionization. After sampling through an orifice in a conical Teflon skimmer from the turbulent flow reactor, the species of interest were ionized in the ion-molecule flow reactor (stainless steel tubing, i.d. = 4.0 cm, 20 cm long, Ar carrier gas, ca. 44 m/s linear velocity, ca. 1 Torr pressure). Electrons were produced by thermoemission from a heated filament and accelerated using 20 V potential to produce primary Arþ ions by electron impact ionization. Chemical ionization agents (SF6, NF3, H2O) were introduced downstream the filament. Primary negative ions SF6- and F- were produced by electron attachment to SF6 and dissociative attachment to NF3, respectively. Positive ions (H3Oþ(H2O)n) used in the proton transfer reactions (PTR) were produced by addition of water vapor together with SF6 downstream the filament. In the negative-ion mode, only acetaldehyde out of the two important final products could be reliably detected using F- negative ions.23 In the positive-ion mode, both formaldehyde and acetaldehyde were detected. To measure the concentration and isotopic composition of acetaldehyde, ions C 2 H 3 O- (m/z = 43) and C 2 H3 18 O- (m/z = 45) were used. In the positive-ion mode, detection of formaldehyde was performed via protonated formaldehyde CH2O 3 Hþ (m/z = 31) and CH218O 3 Hþ (m/z = 33) and its water cluster CH2 O 3 Hþ(H 2 O) (m/z = 49) and CH 2 18 O 3 Hþ(H 2 O) (m/z = 51). Detection of acetaldehyde was always performed via protonated acetaldehyde with one water ligand: CH 3 CHO 3 H þ(H 2 O) (m/z = 63) and CH3CH18O 3 Hþ(H2O) (m/z = 65). Although other clusters

for both formaldehyde and acetaldehyde with different numbers of water molecules were observed, the choice of these two was governed by the minimal interference from the background peaks. In the calibrations and some additional kinetic experiments, NO2 was detected in the negative mode at m/z = 46 using charge exchange with SF6-. Isotopic composition of hydroxyl was measured via conversion of OH to HNO3 in the reaction OH þ NO2 f HNO3 with detection of nitric acid at m/z = 82 (HNO3 3 F-) and m/z = 84 (HN18OO2 3 F-) using F- transfer from SF6- and via conversion to HONO in the reaction OH þ NO f HONO with detection of nitrous acid at m/z = 46 and 48 (NO2- and N18OO-, respectively) using reaction with F-.

’ RESULTS Attachment Branching, Positive-Ion Mode. The most reliable and accurate results were obtained in the positive-ion mode using PTR, which allowed simultaneous observation of major reaction products, formaldehyde and acetaldehyde. In the experiments, reactive mixtures containing molecular oxygen, nitric oxide, propene, and the products from the movable injector were probed at variable distance of the movable injector tip from the sampling orifice. Isotopically labeled hydroxyl radicals were generated inside the injector in reaction of atomic fluorine with 18 O-enriched water (97%). Some unlabeled hydroxyl radical were also formed in the reaction of fluorine atoms with residual water present in the carrier gas. It cannot be ruled out that hydrogen atoms, formed in the discharge through dissociation of molecular hydrogen, water, and hydrocarbons, present in helium as impurities, were also formed. However, kinetic analysis shows that under the experimental conditions used hydrogen atoms are quickly converted to HO2 radicals in reaction with molecular oxygen. Subsequently, HO2 radicals are converted to hydroxyl in reaction with nitric oxide. Both processes are happening on a very short time scale so that the net impact of hydrogen-atom generation from impurities was dilution of hydroxyl radicals with the major natural isotope, 16O. In the best cases, the initial isotopic composition of OH was ca. 40% of 18O. In typical experiments, the initial compositions in the main reactor were about 15-20% of 18OH. Figures 1-3 show the progression from raw data to branching ratio. Figure 1 shows the profiles of the isotopomers of acetaldehyde and formaldehyde, both 16O- and 18O-containing, in a F/H2O//C3H6/O2/NO reaction system. Solid lines in Figure 1 represent fits by the reaction mechanism given in the Appendix. The profiles extrapolate to negative distances due to the combined physical shift of the injector relative to the sampling orifice and back diffusion of the reaction products caused by fast turbulent diffusion. Fast radial mixing in a turbulent flow is achieved via fast turbulent (eddy) diffusion. However, fast diffusion simultaneously leads to distortion of the axial profiles.24 For reaction products, when concentration increases with distance, back diffusion leads, to a first approximation, to a shift of the profile to shorter distances, which appears as a negative “zero shift”. Zero shifts similar to that observed in the current study were also observed in previous kinetic studies using the same experimental setup under similar experimental conditions (pressures, flow velocities, and Reynolds numbers) (e.g., ref 25, Figure 2.) The zero shift was obtained in a separate experiment by observation of the HNO3 build-up profile in a simple reaction mechanism, H þ NO2 f OH þ NO, OH þ NO2 f HNO3, 2500



Figure 1. Axial profiles of the isotopomers of acetaldehyde and formaldehyde obtained in the positive-ion mode using PTR. The intensities of the 18O-isotopomer peaks increased 10 times: (solid lines: fit by the reaction mechanism (see text); [C3H6] = 1.69 1013, [NO] = 3.30 1013, [O2] = 1.24 1016 molecules cm-3. Pressure 200 Torr (carrier gas N2). The ratios I31/I33 and I63/I65 were used in the calculations of k1b/k1a using eq E5.

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Figure 3. Branching ratio for attachment channel 1b calculated according to eqs E5 and E6 as a function of the injector distance (experiment as in Figure 1, 298 K, 200 Torr, N2).

and formaldehyde reach plateaus, the concentrations of the unlabeled acetaldehyde and formaldehyde continuously increase with the reaction time. This is caused by the chain nature of the reaction in the OH/C3H6/O2/NO system. HO2 radicals with the natural abundance of the oxygen isotopes, formed in reactions 5 and 7, are converted back to hydroxyl radicals with the natural isotopic composition in the reaction with nitric oxide HO2 þ NO f OH þ NO2

ð12Þ

Therefore, the content of the isotopically labeled acetaldehyde and formaldehyde decreases with the progress of the reaction, as shown in Figure 2. As follows from the reaction mechanism, in the steady-state regime, the rates of production of labeled acetaldehyde and formaldehyde can be written as (Aa = CH3CHO, Fa = CH2O, asterisk (*) denotes 18O-labeled compounds)

Figure 2. Axial profiles of the 18O mole fraction in acetaldehyde and formaldehyde for the experiment shown in Figure 1: (O) acetaldehyde, (0) formaldehyde. The solid lines are constructed from the solid curves shown in Figure 1 (fit by the reaction mechanism). Note the comparable production of labeled acetaldehyde and formaldehyde as well as dilution with the 16O isotope caused by the chain reaction.

under conditions where the first reaction is ca. 20 times faster than the second one. In the fit the sensitivity coefficient for acetaldehyde as well as the initial isotopic composition of OH were used as fitting parameters. The relative sensitivity of acetaldehyde and formaldehyde (4.84) was obtained from the ratio (I63 þ I65)/(I31 þ I33). The branching ratio was fixed at the value obtained using eqs E5 and E6 (0.484 for this experiment). The fitted initial isotopic composition is 0.156. The signal amplitudes for acetaldehyde and formaldehyde are different because of the different sensitivity toward these species. While 18O-labeled acetaldehyde

d½Aa=dt ¼ k1b ½18 OH½C3 H6

ðE1aÞ

d½Fa=dt ¼ k1a ½18 OH½C3 H6

ðE1bÞ

Dividing eq E1a by eq E1b and integrating with the initial condition Aa* = Fa* = 0 at t = 0 results in ½Aa=½Fa ¼ k1b =k1a ðE2Þ Therefore, just the ratio of the concentrations of the labeled acetaldehyde and formaldehyde directly provides the ratio of the rate constants for the two attachment channels of reaction 1. To obtain the ratio from the experimental data, the ratio of the sensitivities for acetaldehyde and formaldehyde, SAa/SFa, is required. The generally accepted skeleton mechanism of the reaction (reactions 1a-8) implies that the products, acetaldehyde and formaldehyde, are formed in equal quantities. Therefore, the sensitivity ratio can be determined from the experimental data, as shown in Figure 1 ðE3Þ ½Aa = ½Fa ¼ ðI Aa = I Fa Þ=ðSAa = SFa Þ where IAa* and IFa* are the intensities of the corresponding peaks of 18O-labeled acetaldehyde and formaldehyde (m/z = 65 and 33 2501



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in these measurements) and SAa/SFa is the ratio of the sensitivities toward acetaldehyde and formaldehyde. Taking into account the equality of the amounts of acetaldehyde and formaldehyde produced in the reaction, this ratio is expressed as SAa =SFa ¼ ðI Aa þ I Aa Þ=ðI Fa þ I Fa Þ

ðE4Þ

Combining eqs E2, E3, and E4 leads to k1b =k1a ¼ ð1 þ I Fa = I Fa Þ=ð1 þ I Aa = I Aa Þ β1b ¼ k1b =ðk1b þ k1a Þ ¼ ðk1b = k1a Þ=ð1 þ k1b =k1a Þ

ðE5Þ ðE6Þ

Equations E5 and E6 show that the branching ratio for attachment channels 1a and 1b can be obtained simply from the ratios of the isotopic peaks of formaldehyde and acetaldehyde. It should be stressed that in the derivation of eq E5 only the equality of the concentrations of acetaldehyde and formaldehyde was used. No other additional information (such as the initial enrichment of hydroxyl radicals, more detailed mechanism of the reaction, exact accounting for axial and radial mixing, production of other active species in the discharge, such as H atoms, O atoms, FO radicals, etc.) is required for or can influence the outcome of such determination. It is known that the axial mass transfer does distort the axial profiles. However, for turbulent mixing the distortion is expected to be independent of the molecular mass by the virtue of eddy diffusion. Figure 3 shows the branching ratio β1b calculated from the profiles according to eqs E5 and E6. The result shows remarkable stability with respect to the progress of the reaction; the value of β1b is stable within a few percent. Similar measurements were performed at different pressures over the 60-400 Torr range. No pressure dependence of the branching ratio has been found within the experimental error. It should be stressed that determination of the attachment branching ratio using the above approach is based on the equality of the concentrations of formaldehyde and acetaldehyde produced in the reaction under the conditions of the study. In the photolysis of HONO-NO-propene-air mixtures at about atmospheric pressure at ca. 50 ppm concentration level, Niki et al.26 observed quantitative conversion of propene to formaldehyde and acetaldehyde in equal amounts within the stated accuracy of the concentration determination of (10%. Vereecken et al.16 studied OH-initiated oxidation of propene in the presence of nitrogen oxides under conditions relevant to the atmosphere in an environmental chamber. The major products observed were CH2O and CH3CHO with the equal yields of 1.05 ( 0.10 at room temperature. There could be several potential reasons for unequal formation of CH3CHO and CH2O. First, decomposition of oxy radicals produced in reactions 3a and 3b could be nonquantitative. Theoretical calculations (Vereecken et al.16) indicate that the major fraction of the chemically activated radicals (>80% at p < 1 atm) undergoes prompt dissociation. However, even the minor residual thermalized fraction thermally dissociates on a time scale shorter than 160 ns, providing quantitative dissociation of the oxy radicals under the current experimental conditions. The second reason could be nonquantitative conversion of CH2OH and/or CH3CHOH to HCHO and CH3CHO, respectively. The major route for RCHOH radicals is reaction with O2 via addition to form a HOC(R)OO peroxy which then quickly decomposes to RCHO þ HO2. In the presence of NO, however, for more complex HOC(R)OO radicals with a larger R group reaction with NO that ultimately

Figure 4. Profiles of the two isotopomers of acetaldehyde recorded in the negative-ion mode. [C3H6] = 6.13 1014, [NO] = 3.28 1013, [O2] = 1.23 1016 molecules cm-3; 298 K, 200 Torr, N2. (Solid lines) Fit by the model, R18O = 0.243.

leads to HC(O)OH þ R might play a role.27 However, it is not the case for small radicals such as CH2OH, where chemically activated HOCH2OO produced in the CH2OH þ O2 reaction dissociates to CH2O þ HO2 within 50 ps.28 Similarly, this minor route seems unlikely to be of any importance for the CH3CHOH þ O2. Finally, some inequality in the formation of acetaldehyde and formaldehyde could arise from the nonequal yields of the hydroxynitrates CH3CH(OH)CH2ONO2 and CH3CH(ONO2)CH2OH from CH3CH(OH)CH2OO þ NO and CH3CH(OO)CH2OH þ NO reactions. Nitrate formation from the CH3CH(OH)CH2OO þ NO and CH3CH(OO)CH2OH þ NO reactions is fairly small (1.5% at room temperature and atmospheric pressure29 and less at lower pressures). Therefore, even if the nitrate yields from the two peroxy radicals differ slightly,29 the net effect on the HCHO/CH3CHO formation ratio is less than 1%. Numerical simulations using an extended reaction mechanism (see Appendix) showed that the acetaldehyde/formaldehyde ratio deviates from unity less than 1% after 1 ms of the reaction time. Therefore, the assumption of equal yields of CH3CHO and HCHO is consistent with the existing experimental studies16,26 and with the mechanistic expectations. Model calculations show that consumption of acetaldehyde and formaldehyde does not exceed 0.3%, and this could not affect the acetaldehyde/formaldehyde ratio. Attachment Branching, Negative-Ion Mode. Since formaldehyde is undetectable in the negative-ion mode, only temporal profiles of the two isotopomers of acetaldehyde could be measured. Figure 4 shows sample profiles of the signal intensities for acetaldehyde and 18O-acetaldehyde. In this specific experiment the labeled acetaldehyde quickly reaches its maximum and stays almost constant; the concentration of the unlabeled acetaldehyde continuously increases with the reaction time due to the chain reaction. As mentioned earlier, the isotopic composition of the acetaldehyde is time dependent and cannot be compared directly with the initial isotopic composition of hydroxyl radical. To extract the branching ratio from these experiments, additional input information is necessary. First, the initial isotopic composition of hydroxyl radicals must be known. Second, because of the 2502



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Table 1. Summary of the Measurements of the Branching Ratios in the Attachment Reactions 1a and 1b [C3H6] /1 1014

[NO] /1 1014

[O2] /1 1016

Torr

MS mode

molecule cm-3

molecule cm-3

molecule cm-3

200

negative

6.4

1.04

0.19

pressure/

200 200 100

60

400

negative positive positive

positive

positive

6.27 0.17 0.18

0.17

0.24

1.02

0.13

0.33 0.33

0.31

distanceb/cm

β1bc

28

8

0.52

28

29

0.57

28

29

0.48

43

8

0.43

45

8

0.47

1.24

9.5

0.48

1.01

50 50

0.49 0.55

25

0.54

8

0.51

0.95

0.34

injector R18Oa/%

1.11

50

0.53

25

0.53

8

0.57

50

0.52

25 8

0.53 0.52

8

0.52

8

0.53

50

0.51

Isotopic composition of hydroxyl radicals, R18O = [18OH]/([ 18OH] þ [OH]). b Sample data. In several cases detailed profiles were taken. c The branching ratio for the attachment, β1b = k1b/(k1a þ k1b). a

dilution of the products with the natural isotope, which depends on the chain length, a reliable comprehensive reaction mechanism is required. To extract the branching ratio of reaction 1 from the isotopic acetaldehyde profiles, the reaction was modeled by a mechanism of elementary reactions. The simulations and fits by numerical solutions of the corresponding system of ordinary differential equations were performed using Scientist software.30 The mechanism was constructed as follows. First, an extended mechanism of 47 relevant elementary reactions was built. Then, the reaction rates under typical experimental conditions (200 Torr N2, 298 K, [NO] = 3.36 1013, [O2] = 1.24 1016, [C3H6] = 1.63 1013 molecules cm-3) were calculated. The initial concentration of hydroxyl radicals was deliberately set to the highest value ([OH]ini = 1 1012 molecules cm-3) to maximize the contribution of the radical-radical reactions. Then, all reactions, whose maximum rate on the time interval 0-30 ms did not exceed 0.1% of the largest rate, were discarded. After this, the total impact of the discarded reactions on the production of acetaldehyde was assessed not to exceed 1%. The reduced model that incorporates 18O-labeled species is in the Appendix. In the model, the kinetic isotope substitution (16O f 18O) effect is neglected. For the reactions, when literature data are not available, the rate constants were accepted as for similar reactions. The model consists of 34 elementary reactions (without the isotope-substituted species). The mechanism skeleton, the main reactions comprising the chain, is highlighted in bold. This mechanism was used to process the data on the isotopic composition of acetaldehyde alone as well as to verify the main conclusions that follow from the skeleton mechanism, i.e., how accurate is the conclusion about the equality of the concentrations of acetaldehyde and formaldehyde and the validity of eq E2, derived under the steady-state assumption. Numerical simulations using the mechanism showed that the acetaldehyde/formaldehyde

Figure 5. Branching ratio for the attachment channels as a function of pressure: (b) positive mode, both acetaldehyde and formaldehyde detection (PTR); (O) negative-ion mode, only isotopic composition of acetaldehyde is measured.

ratio deviates from unity less than 0.8% after 1 ms and less than 0.4% after 3 ms. Equation E2 is fulfilled with an accuracy better than 0.4% after 1 ms and 0.2% after 3 ms for the conditions of the experiment shown in Figure 4. The results of the measurements together with the measurements in the positive-ion mode are summarized in Table 1. The dependence of the branching ratio on pressure is shown in Figure 5. No pressure dependence is observed within the pressure range 60-400 Torr. The average value of the branching ratio for attachment is (error limit is one 2503



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Table 2. Determination of the Abstraction Branching Ratio in the OH (OD) þ Propene Reaction experiment 1

reactant

[C6H12]

concentration

1.61 10

1.43 10

7467 ( 128

4254 ( 61

3337 ( 174

124 ( 133

ΔI18

a

net H2O signal 2

[H2] b

no reactant 13

0.037 ( 0.040 [C6H12]

concentration

1.97 10

1.46 10

ΔI19 net HOD signal

16 877 ( 131 2883 ( 216

14 124 ( 193 130 ( 259

13 994 ( 172

[C3H6]

no reactant

[C3H6]

no reactant

branching ratio

0.045 ( 0.089

reactant

[C6H12]

concentration

1.70 10

1.46 10

ΔI19

13

13

10 464 ( 51

7497 ( 51

2994 ( 101

-22 ( 101

branching ratio

0 ( 0.034

6.5 1011

[D2] b

[OD] c

5.9 10

4.7 1011

[D2] b

[OD] c

12

3.4 10

13

net HOD signal

a

5.9 10 4130 ( 105

branching ratio

13

[OH] c 12

reactant a

3

[C3H6] 13

12

4.8 1011

7520 ( 87

ΔI18 and ΔI19 are the changes in the signal intensity of H2O and HOD, respectively, when switching the discharge on (in counts per second). b Concentration in the reactor with the discharge off. c Initial concentration in the reactor. Concentrations are in molecule cm-3. a

standard deviation) β1b ¼ k1b =ðk1a þ k1b Þ ¼ 0:51 ( 0:03 ð298 K, 60 - 400 Torr, N2 Þ

ðE7Þ

Due to the much better accuracy, only the data obtained in the positive-ion mode using PTR, which allowed detection of both formaldehyde and acetaldehyde, were used in the final eq E7. It should be stressed again that this branching ratio is based on the equal molar formation of acetaldehyde and formaldehyde. Abstraction Channel. The branching ratio for the H-atom abstraction channel (reaction 1c) was determined at 200 Torr and 298 K by measuring the yield of water in the OH þ propene and OD þ propene chemical systems OH þ C3 H6 f H2 O þ C3 H5

ð1cÞ

OD þ C3 H6 f HOD þ C3 H5

ð1DcÞ

The amounts of H2O and HOD formed were compared with the amount of water formed in the reaction of hydroxyl with cyclohexane, where abstraction is the only channel, eqs 11 and 11D. A very small secondary kinetic isotope substitution effect is expected for this type of abstraction reaction,20-22 implying that the branching ratios are similar for reactions 1c and 1Dc. The reactions were carried in the main reactor at reaction times of about 27 ms with propene and cyclohexane concentrations ensuring complete consumption of hydroxyl radicals. Radicals OH and OD were produced in the reactions of H or D atoms with NO2 in the movable injector. The H and D atoms were produced by dissociation of H2 or D2 in microwave discharge. Preprepared 10% H2/He and D2/He mixtures were flowed into the He carrier flow passing through the discharge. The water isotopes were detected in positive mode as H2Oþ (m/z = 18) and HODþ (m/z = 19) ions from the charge exchange reaction of H2O and HOD with Arþ. The sensitivity to H2O was determined by introducing metered flows of isotopic H218O water, which was detected at m/z = 20. This allowed evaluation of the initial concentrations of OH and OD radicals (6.5 1011 and 4.7 1011 molecules cm-3, respectively).

The experimental conditions and results of these measurements are summarized in Table 2. The accuracy of the measurements was largely reduced by the high background signal level for both isotopes and by formation of water in the injector. The H2O background signal at m/z = 18 (ca. 60 000 cps) is due to the water impurity in Ar, He, and N2 carrier gases; the background signal at m/z = 19 (ca. 20 000 cps) originates mainly from the H3Oþ ions formed in the proton transfer reaction H2Oþ þ H2O inside the ion-molecule reactor. There are additional processes forming water when switching on the discharge H þ NO2 f OH þ NO

ð13Þ

OH þ OH f H2 O2

ð14aÞ

f H2 O þ O

ð14bÞ

OH þ H2 O2 f H2 O þ HO2

ð15Þ

OH þ HO2 f H2 O þ O2

ð16Þ

In the case of D atoms, D2O is produced in the D analogues of reactions 13-16. In addition, in the deuterated system some amounts of H atoms and OH radicals are formed due to the H2O trace impurities in helium passing the discharge. This leads to OH þ OD and other cross-reactions producing HOD. The last column in Table 2 shows the increase of the H2O and HOD signal intensities when switching on the discharge in the absence of the reactants in the reactor caused by the secondary processes in the injector. In experiment 3, the D2 flow was decreased compared to experiment 2, which significantly reduced formation of water in the injector. The experimental errors for the water signal intensity, ΔIM, were calculated as the standard deviation of the mean value for 8-10 measurements. Then, the propagated errors were calculated for the “net signal due to the reaction”, which is the difference between the signals with and without the reactant and for the abstraction branching ratio, which is the ratio of the signals using propene and cyclohexane as the reactants. As can be 2504



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seen from Table 2, the water signal that originates from the hydroxyl reaction with C3H6 is less than its experimental error. The average result of the measurements for the water forming branching ratio is 0.027 ( 0.031. This value can be compared with the extrapolation of Tsang’s recommendation10 down to 300 K, k1c/(k1a þ k1b þ k1c) = 0.03. However, in all experiments the increment of the water signal upon addition of propene never exceeded the measurement error. Therefore, the measurements only set an upper limit on the branching ratio for the abstraction channel k1c = ðk1a þ k1b þ k1c Þ < 0:05 ð298 K, 200 Torr, N2 Þ

ðE8Þ

’ DISCUSSION The observed branching ratio for addition of hydroxyl radical to propene, 0.51 ( 0.03, is close to the 50/50 ratio, which is suggestive of the potential role of other factors that might lead to equalization of the oxygen isotopic composition of acetaldehyde and formaldehyde. Such factors could be fast isotope exchange between acetaldehyde and formaldehyde in homogeneous and heterogeneous processes as well as fast migration of hydroxyl between two sites in β-hydroxypropyl radicals formed in reactions 1, oxygen atom in chemically activated peroxy radicals formed in reactions 2, or hydrogen-atom migration in the alkoxy radicals formed in reactions 3. To address the possible impact of isotope scrambling reactions on the determined branching ratio, model calculations (using the model described in the Appendix with scrambling reactions added) were performed. In these calculations the branching ratio was set at 0.75, and profiles similar to the one shown in Figure 3 were calculated at different scrambling rate constants. The model calculations show that the reduction of the observed branching ratio to 0.5 over the whole time range of the observations requires very fast exchange reaction on the order of 10-8 cm3 molecule-1 s-1, which is unphysical. Even with a very fast exchange reaction with a rate constant of 10-10 cm3 molecule-1 s-1 (which is hardly probable for a molecule-molecule reaction) the determined branching ratio would vary from 0.74 to 0.5 over the 5-30 ms range. In simple terms, low initial concentrations of free radicals lead to a low concentration of the products, which makes bimolecular processes involving the reaction products too slow to play a role. Possible isotopic scrambling in heterogeneous processes on the reactor wall cannot play any significant role due to the limit imposed on the wall rate constant by diffusion through the laminar boundary layer. Estimating the boundary layer thickness as δ = 1 mm, for 200 Torr of N2, and the reactor internal radius R = 1.2 cm, the diffusion-limited rate constant for a wall reaction is kD = 2D/ δR ≈ 2 0.8/0.1 1.2 = 13 s-1. Even assuming that the heterogeneous isotopic scrambling proceeds with the fastest diffusion-limited rate, the characteristic time for this process of ca. 80 ms is much longer than the reaction time and cannot lead to complete isotope scrambling. Besides, if such a process played a role, it would lead to the time dependence of the determined branching ratio in Figure 3. In fact, the remarkable independence of the extracted branching ratio on the reaction time (such as in Figure 3) is proof of the negligible role of isotope scrambling processes, both homogeneous and heterogeneous. The possibility of migration of hydroxyl between the two sites in β-hydroxypropyl radicals formed in reaction 1 is not an issue

specific for the current study. The overall rate constant of reaction 1 is in the high-pressure limit at 296 K and 200 Torr.31,32 The characteristic falloff pressure is ca. 0.7 Torr. At 200 Torr the energized adducts are stabilized in collisions with bath gas molecules, at ca. 0.7 Torr, about one-half of the energized adducts dissociate back to reactants, i.e., the stabilization rate is equal to the dissociation rate. Estimating the stabilization rate at this pressure as 2 106 s-1 (βc = 0.2, ZLJ = 4 10-10 cm3 molecule-1 s-1, [M] = 2.5 1016 molecules cm-3), one concludes that at pressures of ca. 3 Torr the isomerization rate of ca. 8 106 s-1 would compete with the collisional stabilization. The barrier for isomerization via the van der Waals complex is about 2 kcal/mol lower than the dissociation energy,9 and such an increase seems to be quite feasible. Therefore, somewhat higher pressures should be required to stabilize the adduct before isomerization. For propene, this is not expected to have a significant impact at 200 Torr; however, at low pressures of a few Torr used in conventional flow systems this might lead to an even larger role of the isomerization of the chemically activated complex. At low pressures, when (microcanonical) isomerization of chemically activated complex is faster than the collisional stabilization, the branching ratio would reflect the (microcanonical) equilibrium between the two isomers, rather than the branching ratio for the initial attachment. Therefore, comparison of the branching ratios for hydroxyl attachment to propene and smaller or similar size molecules measured at high and low pressures should be done with caution. The branching ratio for OH addition to 1-butene determined using mass spectrometry over the pressure range 2-5 Torr of βa = 0.85 ( 0.106 differs significantly from the current determination for propene. In addition, recently the branching ratio in reaction 1 was evaluated based on the mass spectrum fragmentation pattern.33 A lowpressure (0.8-3 Torr) flow system was employed; the result (β1a= 72 ( 16%) is different from that obtained in the current work and close to the result of Cvetanovic4 (ca. 65%). It is not clear whether the indicated possible pressure effects might be responsible for this difference. The nascent peroxy radicals formed in reactions 2a and 2b carry initially about 38 kcal/mol internal energy (ca. 33 kcal/mol from the O2 addition, ca. 3 kcal/mol from thermal energy of the reactants, and about 2 kcal/mol from the internal H bond in the β-hydroxy-alkylperoxy), which, potentially, might lead to a chemically activated reaction that results in a fast exchange of the 18O and 16O in the radicals. Although this process cannot be completely excluded, it appears implausible as it would require simultaneous rupture and formation of several chemical bonds. However, a reliable assessment of the importance of this hypothetical process would require a theoretical kinetics study beyond the scope of this work. Finally, a possibility exists of fast enough hydrogen-atom migration in alkoxy radicals formed in reactions 3. These radicals might possess some internal energy. In addition, H-atom transfer can occur by tunneling. Currently, the data available is not sufficient to reliably assess the role of this potential process. Additional theoretical studies are required to evaluate the impact of this and other potential intramolecular isotope exchange, isomerization, and bath gas pressure effects in this complex system.

’ CONCLUSIONS The measured branching ratio for attachment channels E7 is consistent with the most recent theoretical calculations.9 2505



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According to the calculations, both channels have transition states with equal “negative barriers” (-1.8 kcal), i.e., the ground states of the transition states are located below the reactants level. In such cases the reaction rate is controlled by the partition functions of the reactants (which are the same in this case) and, essentially, by the numbers of states in the transition states.34,35 Given that the two isomers of the hydroxypropyl radical formed in channels 1a and 1b are very similar and the energies of the transition states are the same, the measured branching ratio (ca. 50%) appears to be quite reasonable. Current measurements confirmed a minor role of the abstraction channel 1c at ambient temperature. The upper estimate (

Branching Ratios in the Hydroxyl Reaction with Propene - The Journal

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