Absolute Rate Coefficient of the Gas-Phase Reaction between

Oct 24, 2013 - Ngoc Duy Vu, Victor Khamaganov, Vinh Son Nguyen, Shaun A. Carl, and Jozef Peeters* .... Anh T. Le , Gregory E. Hall , Trevor J. Sears...
1 downloads 0 Views 1MB Size
Download PDF
Article pubs.acs.org/JPCA

Absolute Rate Coefficient of the Gas-Phase Reaction between Hydroxyl Radical (OH) and Hydroxyacetone: Investigating the Effects of Temperature and Pressure Ngoc Duy Vu, Victor Khamaganov,† Vinh Son Nguyen, Shaun A. Carl, and Jozef Peeters* The Department of Chemistry, University of Leuven, Celestijnenlaan 200F, 3001 Leuven, Belgium S Supporting Information *

ABSTRACT: The rate coefficient (k1) of the reaction between hydroxyl radical and hydroxyacetone, which remained so far controversial, was determined over the temperature range 290−500 K using pulsed-laser photolysis coupled to pulsed-laser induced fluorescence (PLP-PLIF). Hydroxyl radical was generated by pulsed photolysis of H2O2 at 248 nm. The results show that at a pressure of 50 Torr He, the rate coefficient obeys a negative temperature dependence k1(T) = (1.77 ± 0.19) × 10−12 exp((353 ± 36)/T) cm3 molecule−1 s−1 for temperatures between 290 and 380 K, in good agreement with the results of Dillon et al. (Phys. Chem. Chem. Phys. 2006, 8, 236) at 60 Torr He. However, always at 50 Torr He but for the higher temperature range 410−500 K, a positive temperature dependence was found: k1(T) = (1.14 ± 0.25) × 10−11 exp(−(378 ± 102)/T) cm3 molecule−1 s−1, close to the expression obtained by Baasandorj et al. (J. Phys. Chem. A 2009, 113, 10495) for pressures of 2 and 5 Torr He but at lower temperatures, 280−360 K, where their k1(T) values are well below these of Dillon et al. and of this work. Moreover, the rate coefficient k1(301 K) determined as a function of pressure, from 10 to 70 Torr He, shows a pronounced decrease once the pressure is below ∼40 Torr He, thus explaining the disparity between the higher-pressure data of Dillon et al. and the lower-pressure results of Baasandorj et al. The pressure dependence of k1 and of its temperature-dependence below ∼400 K is rationalized by the reaction proceeding via a hydrogen-bonded prereactive complex (PRC) and a submerged transition state, such that at high pressures collisionally thermalized PRCs contribute additional reactive flux over and through the submerged barrier. The high-pressure rate coefficient data both of Dillon et al. and of this work over the combined range 230−500 K can be represented by the theory-based expression k1(T) = 5.3 × 10−20 × T2.6 exp(1100/T) cm3 molecule−1 s−1.

■

INTRODUCTION The occurrence of hydroxyacetone (HYAC) in the atmosphere originates from biomass burning and other biogenic sources. HYAC formation in biomass burning was reported by Christian et al.1 who measured gas and particle emissions from 47 fuel types. Their results showed that HYAC is the third most abundant oxygenated volatile organic compound (OVOC) generated from biomass fuels sampled in Indonesia and is emitted in large portions for instance from burning rice straw (about 6.5% relative to CO). An important source of HYAC is the OH-initiated oxidation of methacrolein (MACR), which is a major primary product of the OH-initiated oxidation of isoprene. Crounse et al.2 showed that HYAC is formed together with CO and OH via a 1,4-H shift of the peroxy radical resulting from terminal OH addition to MACR. The yields of HYAC in MACR oxidation were reported by Fu et al.3 (39%) and Galloway et al.4 (39.5%). HYAC is also formed directly from the OH-initiated oxidation of isoprene at high NO levels. Measurements by Karl et al.,5 Paulot et al.6 and Galloway et al.4 showed that the first-generation product yield of HYAC from isoprene oxidation is close to 3% and contributes a major part of the HYAC present in the atmosphere. According to the mechanism of direct HYAC production in isoprene oxidation first proposed by Dibble7 and later revised by Peeters et al.,8 a © 2013 American Chemical Society

secondary, dihydroxy-peroxy radical from isoprene undergoes a very fast enolic 1,6-H shift leading to a tertiary peroxy radical that reacts fast with NO yielding an oxy radical that decomposes quickly into HYAC and glyoxal. In the atmosphere, the main fate of HYAC is oxidation by the OH radical. Given its fairly high global source strength of several tens of Tg/yr, HYAC can affect the oxidative capacity and the trace-gas composition of the atmosphere. HYAC is also of interest within the context of secondary organic aerosol formation and cloud processing.3,4 Accurate information on the kinetics of the reaction between HYAC and OH radical is therefore required for improving global atmospheric chemistry models. The rate coefficient k1 of the HYAC + OH reaction and its temperature dependence is still quite controversial. The earliest determination, k1 = (3.0 ± 0.3) × 10−12 cm3 molecule−1 s−1 at 298 K was reported by Dagaut et al.9 who used flash photolysis and monitored OH by resonance fluorescence. Rate coefficients close to this value were reported later by several other groups. Orlando et al.10 measured k1 by a relative-rate method at a Received: August 1, 2013 Revised: October 21, 2013 Published: October 24, 2013 12208

dx.doi.org/10.1021/jp407701z | J. Phys. Chem. A 2013, 117, 12208−12215

The Journal of Physical Chemistry A

Article

α-H from the −CH2OH group, which may lower the energy of the transition state. The energy of the most stable prereactive complex (PRC) relative to the initial reactants was computed to be −5.7, −4.63, and −4 to −5 kcal/mol by Dillon et al.,13 Galano,15 and Baasandorj et al.,14 respectively, though these values pertain to different complexes. The energy of the exit-channel transition state relative to the reactants (E0) was computed to be +3, −1.1, and −2.5 to +2.3 kcal/mol depending on calculation levels and on the TS conformer involved, by Dillon et al.,13 Galano,15 and Baasandorj et al.,14 respectively. This work aims to remeasure the rate coefficient of the reaction between OH and HYAC over both a wide temperature range (290−500 K) and a wide pressure range (10−100 Torr He), (i) to obtain clarity regarding the value of the rate coefficient in atmospheric conditions and (ii) to elucidate the discrepancies outlined above between Dillon’s and Baasandorj’s experimental results. To aid in rationalizing the results, the experimental work is complemented by electronic structure computations on this reaction at the M06-2X/6-311++G(3df,2p) level of theory.22

pressure of 750 Torr of synthetic air, finding k1(298 K) = (2.5 ± 0.6) × 10−12 and (3.5 ± 0.7) × 10−12 cm3 molecule−1 s−1 with methanol and ethanol, respectively, as reference compound. Chowdhury et al.,11 in a PLP-PLIF study generating OH by 193 nm laser photolyis of HYAC, obtained a rate coefficient of at room temperature k1 = (2.8 ± 0.2) × 10−12 cm3 molecule−1 s−1 at pressures of 10−90 Torr Ar. Butkovskaya et al.12 used a turbulent flow reactor coupled to a chemical ionization mass-spectrometer to obtain k1(298 K) = (3.17 ± 0.22) × 10−12 cm3 molecule−1 s−1 at a pressure of 200 Torr N2. The most extensive and recent k1(T) determinations were performed by Dillon et al.,13 and Baasandorj et al.,14 both over fairly wide temperature ranges. The highly detailed PLPPLIF study by Dillon et al.13 at a pressure of 60 Torr He and over the temperature range from 233 to 363 K found a slightly negative temperature dependence described by k1(T) = (2.15 ± 0.3) × 10−12 exp((305 ± 10)/T) cm3 molecule−1 s−1, and a k1(298 K) result of (5.95 ± 0.5) × 10−12 cm3 molecule−1 s−1, which is about twice as great as all earlier reported values. In contrast, Baasandorj et al.,14 using a discharge-flow system and resonance fluorescence detection of OH, reported for pressures of 2 and 5 Torr He a slightly positive temperature dependence over the range 280−350 K: k1(T) = (1.88 ± 0.75) × 10−11 exp(−(545 ± 60)/T) and k1(298 K) = (3.02 ± 0.28) × 10−12 cm3 molecule−1 s−1, the latter in good agreement with the earlier determinations. The reason for the discrepancy between Dillon’s k1(298 K) and the other data, as well as the opposite trend in temperature dependence compared to Baasandorj et al.14 is unclear. Dillon et al.13 excluded errors from overestimating the absorption cross section of HYAC, the effect of impurities, and the regeneration of OH. Baasandorj et al.14 speculated that the difference in pressures might be a source of the discrepancy between their work and that of Dillon et al.13 According to Orlando et al.,10 Dillon et al.,13 Galano,15 and Baasandorj et al.,14 the overall reaction between OH and HYAC occurs via (indirect) H abstraction pathways. The by far predominant channel is the abstraction of a more-weakly bonded secondary α-H atom from the alcohol functionality:

■

METHODOLOGIES PLP-PLIF System. The kinetics of the reaction between HYAC and OH was studied using a pulsed laser photolysis− laser induced fluorescence setup (PLP-PLIF). Gas mixtures of HYAC/He, H2O2/He, and the bath gas (He) were introduced through calibrated mass flow controllers into the heatable reaction chamber described earlier23,24 and were mixed while passing through a main fused silica tube exiting in the reaction chamber 1 cm upstream of the irradiated and probed volume. All walls in contact with the admitted HYAC were pyrex, stainless steel, PFA, or fused silica. The pressure in the reactor was controlled by a throttle valve located between the reactor and an 80 m3/h rotation vacuum pump and monitored with a 100 Torr capacitance manometer (Barocel 600, Datametrics). The reaction chamber was equipped with an oxidized-SiCcoated ceramic tube surrounded by Ni/Cr resistive wire coils to heat the gas mixture when needed. OH radicals were directly produced from H2O2 photolysis using a pulsed 248 nm excimer laser beam (10 ns), at low pulse energy in the range of 0.5−1.5 mJ/(pulse cm2), with a repetition rate of 10 Hz. The OH concentration was monitored by laser-induced fluorescence: a delayed pulsed laser beam at 281.997 nm from a Nd:Yagpumped dye laser was used as probe beam, exciting the OH, A2Σ(υ′ = 1) ← X2Π(υ = 0) transition. The OH decay profile was constructed by changing incrementally the time delay of the probe laser beam compared to the pump laser beam by using a PC timer card controlled also by the 10 Hz master generator that triggered the pump laser. The OH fluorescence, A2Σ(υ′ = 1) → X2Π(υ = 1) at 308 nm, was collected at right angles to the excimer beam axis by a 32 mm focal length biconvex lens, passed through a 310 ± 10 nm interference filter (Andover Corp.), and detected by a photomultiplier (Hamamatsu R955). The output signal of the PMT was gateintegrated by a boxcar (Stanford Research System SR250) over the fluorescence decay lifetime of OH(A), digitized by an A/D converter ADC-12 (PICO technology Ltd.) and then acquired on a PC for further analysis. The boxcar output signals were averaged over the last 10 samples, which amounted to an effective integration time constant of 100 μs. The zero-line was measured by recording the signal before the excimer laser was fired. Typically, each OH decay profile was averaged over 20

OH + CH3C(O)CH 2OH → CH3C(O)C•HOH + H 2O (R.1a)

whereas very minor channels are the abstractions of H atoms from the CH3 and OH groups: OH + CH3C(O)CH 2OH → •CH 2C(O)CH 2OH + H 2O (R.1b)

OH + CH3C(O)CH 2OH → CH3C(O)CH 2O• + H 2O (R.1c) 15

Galano predicted that at ambient temperature the reaction channel R.1a contributes more than 99%. Electronic structure calculations by both Dillon et al.13 and Galano15 showed that these reactions occur via prereactive complexes (PRC), involving a hydrogen bond between the hydroxyl-H and the alcohol-O or carbonylO of HYAC, similar to the reaction between OH and acetone16−20 and that between OH and ethanol:21 HYAC + OH ⇌ PRC → TS → products

Thus, there is a barrier-free entrance channel separating the reactants from the final transition state to products. The Hbond can be partly preserved while the hydroxyl-O abstracts an 12209

dx.doi.org/10.1021/jp407701z | J. Phys. Chem. A 2013, 117, 12208−12215

The Journal of Physical Chemistry A

Article

Computational Method. Optimized geometries and energies of the reactants, prereaction complexes and transition states of the title reaction have been calculated at the highperformance M06-2X/6-311++G(3df,2p) level of theory.22 This recent DFT functional, developed for reactions of organic molecules, accounts properly for London dispersion effects, which are important in particular for hydrogen-bonded systems, as the structures at hand.

repeated scans to reduce random uncertainties associated mainly with laser beam energy fluctuations. An overview of the PLP-PLIF system is shown in Figure 1.

■

RESULTS Experimental Results. The fluence of the 248 nm pump laser beam was kept in the range of 0.5−1.5 mJ/(pulse cm2) to generate (1−4) × 1010 OH molecule−1 cm−3 from H2O2 introduced at concentrations less than 1 × 1014 molecules cm−3. These conditions ensure that the contribution of the OH self-reaction to the total OH loss is negligible and that OH regeneration in the photolyzed HYAC/H2O2/He mixtures as discussed by Dillon et al.13 is negligible. In principle, OH regeneration might occur by subsequent reactions of HO2 from OH + H2O2 with peroxy radicals formed from HYACp h o t o l y s i s f r a g m e n t r a d i c a l s ( C H 2 O H , CH 3 CO , CH3COCHOH) and trace amounts of O2 in the reactor. It is obvious that the regeneration rate of OH depends on the fluence of the pump laser beam, and the concentrations of HYAC and O2. In these experiments, the low laser fluences and HYAC concentrations below 1 × 1015 molecules cm−3 ensured that the total fragment radical concentrations are smaller than 1 × 1011 molecules cm−3, such that the rate of any conceivable reaction between these fragment radicals or derived peroxys was at most 0.5% of the OH decay rate over the entire 1−5 ms decay, precluding any interfering OH-regeneration. This is confirmed by the monoexponential OH decays observed, usually over a factor of about 30, as exemplified in Figure 2.

Figure 1. Schematic diagram of the PLP-PLIF experimental setup.

Chemicals; Reactant Concentration Determinations. HYAC (95%) was obtained from Alfa Aesar. As HYAC is hygroscopic, dehydration by vacuum pumping for several hours was carried out before use. Measured equilibrium vapor pressures of the purified liquid HYAC were in excellent agreement with the experimental vapor pressures obtained by Petitjean et al.25 Gaseous HYAC samples were daily prepared by diluting HYAC (0.3−0.5%) in He into a 10 L pyrex bulb, shielded by Al-foil. The concentrations of HYAC were derived from the partial pressures of HYAC and He accurately measured using calibrated Barocel capacitance manometers. The HYAC mole fraction of the gas mixtures in the pyrex bulb was found to be stable within better than 5% during at least 3 days, as verified by the constancy of measured OH-decay rates (see below) over such a period for a given mixture flow rate into the reactor. Also, the evolution of repeatedly measured OH decay rates just after admitting a given HYAC flow showed that the HYAC adsorption/desorption equilibrium on the flow-line walls took about 5 min to establish; after this saturation period, the OH decay rates remained perfectly constant. It was ascertained that after saturation of the walls no loss of HYAC occurred in the entire flow line by verifying that the OH decay rate divided by the reactant concentration remained constant when the residence time of the admitted gas mixture was varied in the flow line and main silica tube by factors ≥2. The total absence of any observable HYAC losses evidences the reliability of the HYAC concentrations in the reactor as derived here from accurately measured pressures, flow rates, and temperature. It might be noted that in situ measurements of [HYAC] necessarily rely on a calibration that ultimately depends too on partial pressure determinations. H2O2 (Chem-Lab, 35%) was concentrated up to 85 wt % by vacuum pumping. The concentration of H2O2 was monitored by both 266 nm absorption measurement and determination of the specific density. Ultrahigh purity He (Praxair, 99.9996%) was used as bath gas.

Figure 2. Examples of LIF OH signal decay profiles observed following pulsed 248 nm photolysis of HYAC/H2O2/He mixtures containing different [HYAC]; temperature 301 K; pressure 50 Torr He; total flow 200 sccm. [HYAC]: (a) 0.4 × 1014 molecules cm−3; (b) 0.8 × 1014 molecules cm−3; (c) 1.2 × 1014 molecules cm−3.

The initial 250 μs portions of the semilogarithmic OH-signal versus t plots as in Figure 2, showing curvature due to the 100 μs integration time constant of the boxcar output (see above), were always disregarded in the analysis. Under these conditions, the decay of OH as a function of time t is described by the firstorder rate law: [OH]t = [OH]0 exp(−k′t )

(1)

or in terms of the OH signal (It): It = I0 exp( −k′t )

(2)

where 12210

dx.doi.org/10.1021/jp407701z | J. Phys. Chem. A 2013, 117, 12208−12215

The Journal of Physical Chemistry A

Article

k′ = k1[HYAC] + k 2[H 2O2 ] + kd

(3)

with k′ the total pseudo-first-order rate constant (being constant due to large excess of HYAC and H2O2 compared to OH), k1 and k2 are the bimolecular rate coefficients of the reactions of OH with HYAC and H2O2, respectively, and kd is the rate of diffusional loss of OH out of the probed volume. For a given k1 determination, i.e., at a single temperature and pressure, values of k′ are measured for a series of HYAC concentrations, ranging from 0.3 × 1014 to 12 × 1014 molecules cm−3, at a fixed concentration of H2O2. For each [HYAC], k′ is obtained by an exp(−k′t) fit to the observed intensity It versus reaction time t. The slope of k′ versus [HYAC], at constant [H2O2], yields the bimolecular k1; Figure 3 gives examples of such plots for 301, 350, and 383 K.

Figure 4. Arrhenius plot of bimolecular rate coefficient k1 of the reaction OH + HYAC measured in this work. Error bars are the statistical 2σ uncertainties.

However, our k1 values at temperatures greater than ∼410 K and up to 500 K show a positive trend with T described by k1 = (1.14 ± 0.25) × 10−11 exp(−(378 ± 102)/T) cm3 molecule−1 s−1, which is coincident with the expression of Baasandorj et al.14 obtained for the lower T-range of 280−350 K but at far lower pressures of 2−5 Torr He. The transition in the temperature dependence of the rate coefficient observed in this study occurs in the region around 400 K. To verify possible pressure dependence of the rate coefficient, k1 was measured also at different pressures from 10 to 70 Torr He at 301 K. The obtained data are presented in Table 2 and Figure 5. The results show that at pressures P ≥ 45

Figure 3. Plots of the pseudo-first-order decay constants, k′, versus [HYAC] at different temperatures. [HYAC] = (0.31 to 3.2) × 1014 molecules cm−3; total pressure = 50 Torr He. Data at 350 and 383 K have been offset by 300 and 500 s−1, respectively.

Table 2. Second-Order Rate Coefficients of the Reaction OH + HYAC at Different He Pressures

The measured second-order rate coefficient values k1(T) over the range 290−500 K at a pressure of 50 Torr He are presented in Table 1 and Figure 4. The Figure 4 data show an unusual temperature dependence of the rate coefficient. Below ∼380 K, a negative temperature dependence of k1 was obtained that can be expressed as k1 = (1.77 ± 0.19) × 10−12 exp((353 ± 36)/T) cm3 molecule−1 s−1, in excellent agreement with the results of Dillon et al.13 at 60 Torr for T = 233−363 K. Table 1. Second-Order Rate Coefficients of the Reaction OH + HYAC at Different Temperatures T, K

[HYAC], 1014 molecules cm−3

P, Torr (He)

k1, 10−12 cm3 molecule−1 s−1

293 294 301 309 319 335 350 358 373 383 410 429 438 460 485 494 498

0.4−3.17 0.43−3.01 0.4−3.6 0.38−3.0 0.42− 3.19 0.7−2.8 0.37−3.2 0.34−2.68 0.37−4.09 0.31−2.51 0.59−3.55 0.74−2.45 0.52−3.14 0.5−3.0 0.46−2.79 0.46−2.77 0.42−2.13

50 50 50 50 50 50 50 50 60 50 50 50 100 100 80 100 50

6.17 ± 0.2 5.60 ± 0.26 5.74 ± 0.32 5.49 ± 0.24 5.15 ± 0.34 5.28 ± 0.22 4.93 ± 0.24 4.79 ± 0.2 4.51 ± 0.3 4.35 ± 0.2 4.5 ± 0.26 4.89 ± 0.6 4.71 ± 0.24 5.31 ± 0.3 5.13 ± 0.38 5.59 ± 0.2 5.16 ± 0.2

P, Torr (He)

[HYAC], 1014 molecules cm−3

k1, 10−12 cm3 molecule−1 s−1

10 10 15 20 25 30 35 40 45 50 60 70

2.24−5.04 2.14−5.42 2.36−10.2 2.13−8.6 1.36−8.21 2.82−8.51 1.91 −11.45 2.53−10.2 1.24−6.18 0.4−3.6 1.89−7.6 1.09−5.49

4.09 ± 0.62 3.91 ± 0.9 4.4 ± 0.7 4.69 ± 0.24 4.95 ± 0.24 5.3 ± 0.44 5.3 ± 0.2 5.76 ± 0.48 5.36 ± 0.46 5.74 ± 0.32 5.81 ± 0.36 5.79 ± 0.48

Torr He, the k1 values approach an asymptote or high-pressure limit. When the pressure is decreased, the rate coefficient becomes gradually lower, to attain a low-P limit that is somewhat less than half the high-P value. Using the best fit k1 =

Figure 5. Rate coefficient k1 of the HYAC + OH reaction at 301 K in the pressure range 10−70 Torr He. 12211

dx.doi.org/10.1021/jp407701z | J. Phys. Chem. A 2013, 117, 12208−12215

The Journal of Physical Chemistry A

Article

Figure 6. Optimized geometries of the HYAC reactant, one of the lowest prereactive complexes, and the connected, lowest transition state computed at the M06-2X/6-311++G(3df,2p) level.

Figure 7. Schematic energy diagram of the reaction between OH and HYAC via formation of a prereactive complex: (a) low pressure regime; (b) high-pressure regime. R: reactants. PRCa: prereactive complex. TSa: transition state for H-abstraction to form the products. c: capture reaction. r: reverse reaction. f: forward reaction. st: collisional stabilization. a: collisional activation. t: tunneling reaction.

2.0 × 10−12 + 3.44 × 10−13 P/(1 + 0.074 P), extrapolation to 2 and 5 Torr He gives values 2.6 × 10−12 and 3.2 × 10−12 cm3 molecule−1 s−1, respectively, close to Baasandorjs’ results. Computational Results. Four different multiple-hydrogenbonded HYAC−OH prereactive complexes (PRCs), some with enantiomeric conformers, have been identified. Optimized geometries and energies relative to the reactants of all PRCs and transitions states, as well as some other structures, obtained at the M06-2X/6-311++G(3df,2p) level of theory,22 are displayed in Figure S1 (Supporting Information), and the most relevant stuctures for the discussion are also shown in Figure 6. The energies relative to the separated reactants, always including vibration zero-point energies (VZPE), are all in the range −4.9 to −5.2 kcal/mol. The internal hydrogen bond that exists in the HYAC reactant between the alcohol-H and the carbonylO, is preserved in all but one of these PRCs, with an H--O bond distance quasi-equal to that in the reactant. No transition state could be found between the reactants and any of these PRCs, all of which are quite loose structures with several low-frequency vibration modes and/or nearly free internal rotation modes. The lowest-lying transition state for product formation, TSa shown in Figure 6, connects PRCa with the products CH3C(O)C•HOH + H2O of reaction channel R.1a. TSa conserves the two H-bonds in PRCa, though the one between the hydroxyl-H and alcohol-O is weakened, and the energy of TSa is computed to be −1.29 kcal/mol, such that this TS is submerged. Note that the present TSa energy value is close to Galano’s −1.1 kcal/mol.18 Moreover, it is compatible with the −0.6 kcal/mol found by Xu and Lin21 for the strikingly similar α-H-abstraction TS in their high-level theoretical study of the analogous ethanol + OH reaction, with PRC at −5.1 kcal/mol. The schematic PES of Figure 7 in the

next section also shows the energies of the separated reactants, PRCa and TSa. Transition states were also localized for the abstractions of the methyl-H and the terminal alcohol-H by hydroxyl (Figure S1, Supporting Information). As expected, their energies are considerably higher: +2.01 and +0.71 kcal/mol above the separated reactants, respectively. Because they are also tighter than TSa, the corresponding channels R.1b and R.1c will be much slower than R.1a and nearly negligible at ambient temperatures. Finally, an even higher-lying transition state, at +6.33 kcal/mol (Figure S1, Supporting Information), was found for OH-addition to form the CH3C(OH)(O•)CH2OH adduct that should rapidly decompose into •CH3 + HOCH2C(O)OH. Clearly, the addition can in no way compete with the H-abstraction R.1a, except at flame temperatures.

■

DISCUSSION The difference between the k1 values and their T-dependence obtained on one hand by Dillon et al.13 and in this work, and on the other hand by Baasandorj et al.,14 as well as the transition in k1 T-dependence from negative to positive over the 290−500 K range observed in this study, can both be ascribed to the particular mechanism of the title reaction, which proceeds through a prereactive complex and via a submerged TS, a type of mechanism for which Peeters and Vereecken predicted pressure dependence of the rate coefficient and of its trend with temperature.26 Figure 7 displays the schematic PES of the reaction, with energy data as computed in this work: HYAC + OH ⇌ PRC → TS → products 12212

dx.doi.org/10.1021/jp407701z | J. Phys. Chem. A 2013, 117, 12208−12215

The Journal of Physical Chemistry A

Article

PRCa, accessed via a barrier-free entrance channel, separates the reactants from the transition state to products. The partially conserved H-bond while the hydroxyl-O abstracts an α-H from the −CH2OH group depresses the energy of TSa and keeps it below the reactant energy. Generally, the presence of an attractive entrance channel to a PRC combined with a prominent but submerged exit-channel transition state complicates the kinetic analysis, especially in situations where one is not able to invoke equilibrium between reactants and prereactive complexes. Such reactions should exhibit a complex temperature dependence as shown by Xu and Lin,21 and Galano,15 and pressure dependence for the overall rate coefficient is expected.26 In the further qualitative discussion below, first the pressure dependence of the rate coefficient will be rationalized. The reaction rate hinges on the internal-energy distribution of the initially vibrationally excited prereactive complexes and its modification during the PRC lifetime by collisions when they occur. At very low pressures, Figure 7a, only PRCs with fixed energies above the reactant and hence well above the TS level are present, and the only fates are (i) fast redissociation r over the very loose variational bottleneck back to the reactants and (ii) slower forward reaction f well above the tight transition state. The net, forward rate will therefore be much slower than the initial capture rate c. At high enough pressures on the other hand, prereactive complexes are very rapidly stabilized by collision and equally fast collisionally reactivated, resulting in a thermal Boltzmann energy-distribution of the PRCs. The large steady-state population of thermalizd PRCs in the well, i.e., with energies below the reactant level and below the TS level, will increase the total reactive flux as (i) additional PRCs are present, with energies between the reactants and TS levels, that can cross over the barrier to products and (ii) the stabilized PRCs with energies below the TS level contribute to increased product formation by tunneling through the H-abstraction barrier, t. At the same time, the PRC population above the reactant level will remain nearly the same as at very low pressure, given that the high-P rate coefficient (ca. k1 = 6 × 10−12 cm3 molecule−1 s−1 at 300 K) remains well below the expected total capture rate for the two H-bond acceptors of order of 5 × 10−11 cm3 molecule−1 s−1,19 such that the redissociation step r remains dominant and its rate therefore almost unchanged. As a result, for the case at hand, the fraction of PRC formed that effectively yield products and hence the effective rate coefficient will be larger at high pressures as measured in this work and shown in Figure 5. The ratio of the thermalized PRC with energy between the reactant and TS levels, over the PRC with energy above the reactants, increases fairly rapidly with decreasing temperature, as does also the thermal tunneling factor, whereas at very low pressures as in the work of Baasandorj et al.,14 where all PRC have energies well above the TS, tunneling is only marginal. As a result, the highpressure and low-pressure rate coefficients are expected to diverge more as temperature decreases, in agreement with the higher-pressure k1 data of Dillon et al.13 and of this work on one hand, and the low-pressure results of Baasandorj et al.14 on the other hand (Figure 8). Conversely, however, once the temperature exceeds about 400 K, the tunneling factor tends to its limiting value of 1, as computed by Galano.15 At the same time, as T increases, the fraction of PRC with energies between the reactant and TS levels relative to those with energies above the reactants becomes smaller and the contribution to the reactive flux of the latter, high-energy PRCs becomes dominant.

Figure 8. Arrhenius plot of the rate coefficient (k1) of the reaction OH + HYAC: data of this work compared with earlier results. The theorybased fit for high pressures, k1(T) = 5.3 × 10−20 × T 2.6 × exp(1100/ T) (solid curve), is discussed in the text.

Therefore, the high-P and low-P rate coefficients should gradually approach one another at higher T, again consistent with the observations referred to above. Another factor potentially contributing to the reduction of the rate coefficient in the absence of collisions during the PRC lifetime, is the increase of the effective exit barrier due to the higher rotational energy of the more compact TSa compared to the looser entrance bottleneck structure. A remaining question regarding the pressure-dependence of the rate coefficient is that the high pressure regime appears to be attained already at about 50 Torr He, which implies that the lifetime of the PRCs would be rather long, of order of several nanoseconds, which is somewhat puzzling for PRCs with stabilities of only ∼5 kcal/mol. The likely rationalization is that there is a large number of various PRC isomers/conformers (Figure S1, Supporting Information), which can quickly interconvert over low barriers, and in addition that these PRC isomers are very loose structures with several very-lowfrequency modes resulting in unusually high densities of internal states, which together greatly prolong their lifetimes to redissociation. Finally, the T-dependence in the high-pressure regime is addressed qualitatively, for which we refer to and borrow from the high-level computational study of the CH3CH2OH + OH reaction by Xu and Lin.21 This reaction was found to proceed likewise through a PRC involving an H-bond between the hydroxyl radical and the −CH2OH moiety, and via a connecting transition state for H-abstraction of an α-H from the −CH2OH function by the OH, quasi-identical in nearly all aspects to PRCa and TSa, respectively, of this work. The computed PRC energy for the ethanol + OH reaction was −5.1 kcal/mol, comparable to the PRCa stability, whereas the reported TS energy was −0.6 kcal/mol, which is +0.7 kcal/mol higher than our TSa, compatible with the experimental k(ethanol + OH) being twice lower than our and Dillon’s k1 at ambient temperature. The variational transition-state analysis by Xu and Lin21 yields a theoretical high-pressure rate coefficient expression, k(ethanol + OH)(T) = 9.11 × 10−20 × T 2.58 exp(748/T) cm3 molecule−1 s−1, implying a slightly negative T-dependence for lower T, due to the negative TS energy and to tunneling, but a positive T-dependence for T > 300 K, largely because of the TST partition function ratio QTS/ (QOHQethanol) showing a strong increase at higher T on account of the four low-frequency transitional modes. Given the similarity in PRC and TS, in particular regarding the transitional modes, and considering that the partition functions of the other internal modes roughly cancel out in numerator 12213

dx.doi.org/10.1021/jp407701z | J. Phys. Chem. A 2013, 117, 12208−12215

The Journal of Physical Chemistry A

Article

around 1 day 16 h, which is about twice less than the lifetime calculated by Orlando et al.10 In reality, there are also heterogeneous losses of HYAC due to aerosol uptake and wet or dry deposition. Hence, the true lifetime of HYAC should be shorter than the estimation above.

and denominator of the TST partition function ratio, the only essential difference for the TST expression for the HYAC + OH reaction is its lower TS energy of −1.3 compared to −0.6 kcal/mol for ethanol + OH. Therefore, one expects an a priori T-dependence of k1 in the high-P regime that can be approximated by A × T 2.6 exp(1100/T). This expression, with only parameter A optimized for a best fit to the highpressure data both of Dillon et al.13 and of this work, gives k1(T) = 5.3 × 10−20 × T 2.6 exp(1100/T) cm3 molecule−1 s−1, is displayed in Figure 8, and can be seen to capture quite well the observed change in temperature-trend over the 200−500 K range from negative to positive, with expected minimum k1 at about 420 K. The above therefore rationalizes why the k1 values and their T-dependence obtained by Baasandorj et al.14 at 2−5 Torr differ from those of Dillon et al.13 and of this work at pressures ≥50 Torr. However, a puzzling discrepancy remains between the earlier room temperature k1 measurements in the range (2.5−3.5) × 10−12 cm3 molecule−1 s−1 reported by Dagaut et al.,9 Orlando et al.10 at 750 Torr of synthetic air, Chowdhury et al.11 at 10−90 Torr Ar, and Butkovskaya et al.12 at 200 Torr N2, and on the other hand the nearly twice higher values of Dillon and of this work. For potential reasons for this difference, we refer first to Dillon et al.13 and Baasandorj et al.,14 who discussed among others possible errors originating from impurities, errors in absorption cross section measurement, and the loss of HYAC by absorption on the wall. Further, we concur with Dillon et al.13 that Dagaut et al.9 did not give sufficient detail about their experiment for a rigorous assessment. Chowdhury et al.11 created OH by 193 nm laser photolysis of HYAC, which, however, could produce radical species in large quantities leading to a complex chemistry, as evidenced by the unusually large intercept of ca. 5000 s−1 of their k′ versus [HYAC] plot. Orlando et al.10 using a relative rate method, obtained two quite different k1 values, 2.5 × 10−12 with methanol as reference and 3.5 × 10−12 cm3 molecule−1 s−1 with ethanol, and the plots of the HYAC decay versus that of the reference compound, in particular for methanol, show very large scatter. In general, OH regeneration is a possible cause for underestimating rate coefficients of OH reactions. In this respect, whereas Dillon et al.13 found identical rate coefficients for the reactions of HYAC with OH and OD, Butkovskaya et al.12 reported k1 for OD about 30% higher than that for OH, indicating that considerable OH was regenerated from HYAC product radicals. Butkovskaya et al.12 corrected for OH reformation but estimated it at only about 10% in their OH/ HYAC/O2/NO reaction system at 298 K; it is not clear whether the recently established efficient OH regeneration by reaction of HO2 with acylperoxy and acetonylperoxy radicals27 could have been important in their complex system. Atmospheric implications. The excellent agreement between our result and of Dillon et al.13 for atmospheric temperatures and “high” pressures suggests that the rate coefficient of the reaction between hydroxyl radical and hydroxyacetone increases with altitude in the troposphere, where the pressure is well within the high-pressure regime for the OH + HYAC reaction. Assuming an average temperature of the troposphere of 260 K and hence an average rate coefficient k1 of 6.9 × 10−12 cm3 molecule−1 s−1, which can be taken from Dillon et al.13 because their study and the present work are in perfect agreement over the entire range of overlap (293−363 K), and adopting an average OH concentration of 1 × 106 radicals cm−3, the estimated tropospheric lifetime of HYAC is

■

CONCLUSIONS In this work, the rate coefficient k1 of the title reaction OH + HYAC was measured over the temperature range 290−500 K at 50 Torr He using pulsed-laser photolysis coupled to pulsedlaser induced fluorescence. The results show a transition in temperature dependence of k1. At temperatures below ∼380 K, a negative temperature dependence was determined that can be expressed as k1 = (1.77 ± 0.19) × 10−12 exp((353 ± 36)/T) cm3 molecule−1 s−1, whereas at temperatures from ∼410 to 500 K, a positive temperature dependence was found given by k1 = (1.14 ± 0.25) × 10−11 exp(−(378 ± 102)/T) cm3 molecule−1 s−1. Moreover, the rate coefficient at 301 K was measured over the pressure range from 70 down to 10 Torr He and found to show a pronounced decrease as the pressure becomes lower than ∼45 Torr He. All these observations, including the seemingly conflicting results of the previous studies of Dillon et al.13 and Baasandorj et al.,14 can be explained by the pertaining reaction mechanism that involves a hydrogen-bonded prereactive complex (PRC) and a submerged TS. At temperatures below about 400 K, high pressures open up new pathways to the products by thermalized PRC with energies below the reactants level going over and tunneling through the barrier for the rate-controling H-abstraction. The decrease of the tunneling factor when the temperature rises leads to a negative temperature dependence at high pressures, whereas the Tdependence is positive both at low pressures and at high enough temperatures where the tunneling factor tends to unity. Thus, our findings also reconcile the seemingly disparate k1 results of Dillon et al.13 with those of Baasandorj et al.14 whereas they also support the higher k1 value of ca. 6 × 10−12 cm3 molecule−1 s−1 in tropospheric conditions of Dillon et al.,13 as opposed to the about twice lower earlier determinations of several groups. Finally, the theory-based expression k1(T) = 5.3 × 10−20 × T 2.6 exp(1100/T) cm3 molecule−1 s−1, adapted from the computational TST study of Xu and Lin21 on the analogous ethanol + OH reaction, is found to closely represent the Tdependence of the high-pressure k1 measurements both of Dillon et al.13 and of this work over the combined range 230− 500 K.

■

ASSOCIATED CONTENT

S Supporting Information *

Structures and energies of optimized geometries of HYAC reactant, prereactive complexes, transition states, and OH adducts. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*J. Peeters: e-mail, [email protected]; telephone, +32(0)16327382. Present Address †

V. Khamaganov: Synchrotron SOLEIL, DESIRS Beamline L’Orme des Merisiers, St. Aubin - BP 48 91192 Gif-sur-Yvette CEDEX, France. 12214

dx.doi.org/10.1021/jp407701z | J. Phys. Chem. A 2013, 117, 12208−12215

The Journal of Physical Chemistry A

Article

Notes

(14) Baasandorj, M.; Griffith, S.; Dusanter, S.; Stevens, P. S. Experimental and Theoretical Studies of the Kinetics of the OH + Hydroxyacetone Reaction as a Function of Temperature. J. Phys. Chem. A 2009, 113, 10495−10502. (15) Galano, A. Theoretical Study on the Reaction of Tropospheric Interest: Hydroxyacetone + OH. Mechanism and Kinetics. J. Phys. Chem. A 2006, 110, 9153−9160. (16) Vandenberk, S.; Vereecken, L.; Peeters, J. The Acetic-Acid Forming Channel in the Acetone + OH Reaction: A Combined Experimental and Theoretical Investigation. Phys. Chem. Chem. Phys. 2002, 4, 461−466. (17) Yamada, T.; Taylor, P. H.; Goumri, A.; Marshall, P. The Reaction of OH with Acetone and Acetone-d6 from 298 to 832 K: Rate Coefficients and Mechanism. J. Chem. Phys. 2003, 119 (20), 10600−10606. (18) Raff, J. D.; Stevens, P. S.; Hites, R. A. Relative Rate and Product Studies of the OH-Acetone Reaction. J. Phys. Chem. A 2005, 109, 4728−4735. (19) Caralp, F.; Forst, W.; Henon, E.; Bergeat, A.; Bohr, F. Tunneling in the Reaction of Acetone with OH. Phys. Chem. Chem. Phys. 2006, 8, 1072−1078. (20) Shannon, R. J.; Taylor, S.; Goddard, A.; Blitz, M. A.; Heard, D. E. Observation of a Large Negative Temperature Dependence for Rate Coefficients of Reactions of OH with Oxygenated Volatile Organic Compounds Studied at 86−112 K. Phys. Chem. Chem. Phys. 2010, 12, 13511−13514. (21) Xu, S.; Lin, M. C. Theoretical Study on the Kinetics for OH Reactions with CH3OH and C2H5OH. Proc. Combust. Inst. 2007, 31, 159−166. (22) Zhao, Y.; Truhlar, D. G. The M06 Suite of Density Functionals for Main Group Thermochemistry, Thermochemical Kinetics, Noncovalent Interactions, Excited States, and Transition Elements: Two New Functionals and Systematic Testing of Four M06-Class Functionals and 12 Other Functionals. Theor. Chem. Acc. 2008, 120, 215−241. (23) Carl, S. A.; Sun, Q.; Vereecken, L.; Peeters, J. Absolute Rate Coefficients of the Reaction HCCO + NO over the Range T=297− 802 K. J. Phys. Chem. A 2002, 106, 12242−12247. (24) Du, L.; Carl, S. A. Absolute Rate Coefficient and Mechanism of Gas Phase Reaction of Ketenyl Radical and SO2. J. Phys. Chem. A 2012, 116, 10074−10081. (25) Petitjean, M.; Reyès-Pérez, E.; Pérez, D.; Mirabel, P.; Calvé, S. L. Vapor Pressure Measurements of Hydroxyacetaldehyde and Hydroxyacetone in the Temperature Range (273 to 356) K. J. Chem. Eng. Data 2010, 55, 852−855. (26) Peeters, J.; Vereecken, L. Hydrogen Abstraction by Hydroxyl through H-bonded Complexes: Pressure Dependence, 19th International Symposium on Gas Kinetics, Orleans, France, July 2006. (27) Hasson, A. S.; Tyndall, G. S.; Orlando, J. J. A Product Yield Study of the Reaction of HO2 Radicals with Ethyl Peroxy (C2H5O2), Acetyl Peroxy (CH3C(O)O−2), and Acetonyl Peroxy (CH3C(O)CH2O2) Radicals. J. Phys. Chem. A 2004, 108, 5979−5989.

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was carried out in part in the frame of the BIOSOA project within the Science for Sustainable Development of the Belgian Science Policy Office. Ngoc Duy Vu is indebted to the Ministry of Education and Training of Vietnam for granting a Ph.D. scholarship. The authors also acknowledge support from the BOF programme, GOA project 60A/2008/05. The authors thank Christophe Coeck, Paul Wijnants, and Rita JungbluthHendrickx for technical assistance.

■

REFERENCES

(1) Christian, T. J.; Kleiss, B.; Yokelson, R. J.; Holzinger, R.; Crutzen, P. J.; Hao, W. M.; Saharjo, B. H.; Ward, D. E. Comprehensive Laboratory Measurements of Biomass-burning Emissions: 1. Emissions from Indonesian, African, and Other Fuels. J. Geophys. Res. 2003, 108 (D23). (2) Crounse, J. D.; Knap, H. C.; Ørnsø, K. B.; Jørgensen, S.; Paulot, F.; Kjaergaard, H. G.; Wennberg, P. O. Atmospheric Fate of Methacrolein. 1. Peroxy Radical Isomerization Following Addition of OH and O2. J. Phys. Chem. A 2012, 116, 5756−5762. (3) Fu, T. M.; Jacob, D. J.; Wittrock, F.; Burrows, J. P.; Vrekoussis, M.; and Henze, D. K. Global Budgets of Atmospheric Glyoxal and Methylglyoxal, and Implications for Formation of Secondary Organic Aerosols. J. Geophys. Res. 2008, 113 (D15303). (4) Galloway, M. M.; Huisman, A. J.; Yee, L. D.; Chan, A. W. H.; Loza, C. L.; Seinfeld, J. H.; Keutsch, F. N. Yields of Oxidized Volatile Organic Compounds during the OH Radical Initiated Oxidation of Isoprene, Methyl Vinyl Ketone, and Methacrolein under High-NOx Conditions. Atmos. Chem. Phys. 2011, 11, 10779−10790. (5) Karl, T.; Guenther, A.; Turnipseed, A.; Tyndall, G.; Artaxo, P.; Martin, S. Rapid Formation of Isoprene Photo-Oxidation Products Observed in Amazonia. Atmos. Chem. Phys. 2009, 9, 7753−7767. (6) Paulot, F.; Crounse, J. D.; Kjaergaard, H. G.; Kroll, J. H.; Seinfeld, J. H.; Wennberg, P. O. Isoprene Photooxidation: New Insights into the Production of Acids and Organic Nitrates. Atmos. Chem. Phys. 2009, 9, 1479−1501. (7) Dibble, T. S. Intramolecular Hydrogen Bonding and Double HAtom Transfer in Peroxy and Alkoxy Radicals from Isoprene. J. Phys. Chem. A 2004, 108 (12), 2199−2207. (8) Peeters, J.; Nguyen, T. L. Unusually Fast 1,6-H Shifts of Enolic Hydrogens in Peroxy Radicals: Formation of the First-Generation C2 and C3 Carbonyls in the Oxidation of Isoprene. J. Phys. Chem. A 2012, 116, 6134−6141. (9) Dagaut, P.; Liu, R.; Wallington, T. J.; Kurylo, M. J. Kinetic Measurements of the Gas-Phase Reactions of OH Radicals with Hydroxy Ethers, Hydroxy Ketones, and Keto Ethers. J. Phys. Chem. 1989, 93, 7838−7840. (10) Orlando, J. J.; Tyndall, G. S.; Fracheboud, J. M.; Estupinan, E. G.; Haberkorn, S.; Zimmer, A. The Rate and Mechanism of the GasPhase Oxidation of Hydroxyacetone. Atmos. Environ. 1999, 33, 1621− 1629. (11) Chowdhury, P. K.; Upadhyaya, H. P.; Naik, P. D.; Mittal, J. P. ArF Laser Photodissociation Dynamics of Hydroxyacetone: LIF Observation of OH and Its Reaction Rate with the Parent. Chem. Phys. Lett. 2002, 351, 201−207. (12) Butkovskaya, N. I.; Pouvesle, N.; Kukui, A.; Mu, Y.; Lebras, G. Mechanism of the OH-Initiated Oxidation of Hydroxyacetone over the Temperature Range 236−298 K. J. Phys. Chem. A 2006, 110, 6833− 6843. (13) Dillon, T. J.; Horowitz, A.; Hoelscher, D.; Crowley, J. N.; Vereecken, L.; Peeters, J. Reaction of HO with Hydroxyacetone (HOCH2C(O)CH3): Rate Coefficients (233−363 K) and Mechanism. Phys. Chem. Chem. Phys. 2006, 8, 236−246. 12215

dx.doi.org/10.1021/jp407701z | J. Phys. Chem. A 2013, 117, 12208−12215