12364
J. Phys. Chem. 1996, 100, 12364-12368
Kinetic Studies of OH Reactions with a Series of Acetates A. El Boudali, S. Le Calve´ , G. Le Bras, and A. Mellouki* Laboratoire de Combustion et Syste` mes Re´ actifs, CNRS and UniVersite´ d’Orle´ ans, 45071 Orle´ ans Cedex 2, France ReceiVed: February 29, 1996; In Final Form: May 9, 1996X
Absolute rate constants have been measured for the gas phase reactions of hydroxyl radicals with a series of aliphatic acetates: methyl acetate (k1), ethyl acetate (k2), n-propyl acetate (k3), n-butyl acetate (k4), and n-pentyl acetate (k5). Experiments were carried out using the pulsed laser photolysisslaser induced fluorescence technique over the temperature range 243-372 K. The obtained kinetic data were used to derive the Arrhenius expressions, and since the Arrhenius plots were slightly curved they were fitted, using a three-parameter expression. The Arrhenius expressions obtained are: k1 ) (0.53 ( 0.09) × 10-12 exp[-(128 ( 102)/T] ; k2 ) (0.48 ( 0.09) × 10-12 exp[(397 ( 103)/T]; k3 ) (1.03 ( 0.12) × 10-12 exp[(370 ( 69)/T] ; k4 ) (2.10 ( 0.28) × 10-12 exp[(299 ( 81)/T], and k5 ) (2.75 ( 0.46) × 10-12 exp[(302 ( 102)/T] (in units of cm3 molecule-1 s-1). At room temperature, the rate constants obtained (in units of 10-12 cm3 molecule-1 s-1) were as follows: methyl acetate, 0.32 ( 0.03; ethyl acetate, 1.67 ( 0.22; n-propyl acetate, 3.42 ( 0.26 ; n-butyl acetate, 5.52 ( 0.51; n-pentyl acetate, 7.34 ( 0.91. Our results are compared with the previous determinations and discussed in terms of structure-activity relationships.
Introduction Volatile organic compounds (VOCs) are emitted into the atmosphere by various anthropogenic or natural sources. One class of VOCs which is used to a large extent in industry, particularly as solvents and in the manufacture of perfumes and flavorings, is acetates. These compounds are also produced in nature through vegetation. A substantial proportion of these VOCs could escape then to the atmosphere where they are available for photochemical transformation. Another source of acetates is the tropospheric degradation of some oxygenated compounds. For example, methyl acetate is produced from methyl tert-butyl ether (MTBE)1,2 and tert-amyl ether (TAME),3 and ethyl acetate from ethyl tert-butyl ether (ETBE).4 MTBE, TAME, and ETBE are used as additives to unleaded gasoline to increase the octane rating.5 In the troposphere, acetates can be removed by various processes, including reactions with hydroxyl (OH), nitrate (NO3) radicals, and ozone, or they can be photolyzed by solar radiation. Their degradation initiated by the OH radical is the most important. Reactions with the NO3 radical,6 and with ozone7 as well as photolysis,8 are slow processes, thus negligible in the atmospheric degradation of these VOCs. The atmospheric oxidation of these oxygenated compounds initiated by the OH radicals could contribute to the formation of ozone and other components of the photochemical smog in urban areas. Together with mechanistic information on the overall oxidation process, accurate kinetic data are needed at atmospheric temperatures, for the reactions of OH radicals toward VOCs such as the compounds reported here, in order to assess the impact of anthropogenic and biogenic VOCs on air quality. In this work, we report absolute rate constant data for the reactions of the OH radical with five aliphatic acetates: methyl acetate, ethyl acetate, n-propyl acetate, n-butyl acetate, and n-pentyl acetate, in the temperature range 243-372 K: * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, July 1, 1996.
S0022-3654(96)00621-1 CCC: $12.00
OH + CH3C(O)OCH3 f products
k1
(1)
OH + CH3C(O)OC2H5 f products
k2
(2)
OH + CH3C(O)OC3H7 f products
k3
(3)
OH + CH3C(O)OC4H9 f products
k4
(4)
OH + CH3C(O)OC5H11 f products
k5
(5)
The temperature dependent rate constant data of the present study are the first to be reported for reactions 3 and 5, and the second to be reported for reactions 1, 2, and 4. The lack of such data is reflected by the absence of recommended values in the available data evaluations.9-11 The purpose of this work was then to establish or better define the temperature dependence of the rate constants for the above reactions. Experimental Section The pulsed laser photolysis-laser induced fluorescence (PLP-LIF) apparatus and procedures used to measure k1-k5 have been described in previous publications from this laboratory.12,13 Therefore, only information necessary to understand this work is presented here. The reaction cell, made of Pyrex, could be heated or cooled to regulate the temperature by flowing through its jacket a liquid (ethanol or water) from a thermostated bath. OH radicals were produced by photolysis of H2O2 at λ ) 248 nm (KrF excimer laser). The OH temporal concentration profiles were obtained by pulsed laser induced fluorescence using a Nd:YAG pumped frequency-doubled dye laser which was triggered at a variable delay time after the photolysis pulse. The probe pulse excites the Q11, Q11′, and R23 lines in the (1,0) band of the (A2Σ+, V′ ) 1) r (X2Π, v′′ ) 0) transition of the OH radical at around 282 nm. The OH precursor (H2O2), the acetate, and the inert gas (helium) were flowed vertically through the cell mutually orthogonal to the photolysis and probe laser beams. Fluorescence was collected orthogonal to both laser beams and was © 1996 American Chemical Society
Kinetic Studies of OH Reactions focused onto a photomultiplier tube (PM) through a lens system and a band pass filter centered at 309.4 nm (fwhm ) 7.6 nm). The PM tube was positioned below the reaction volume and coaxial with the gas flow. For the OH radical kinetic studies, the output pulse from the PM tube was integrated for a preset period by a gated charge integrator. The integrated signal was digitized and sent to a microcomputer for averaging and analysis. Typically, the signals from 100 probe laser shots were averaged to obtain one data (concentration, time) point. The detection limit under these conditions was about 2 × 108 molecule cm-3 (S/N ) 1). An OH concentration vs time profile was obtained by averaging signals for delay times from about 10 µs to 10 ms using a delay time generator. In kinetic runs, typically 9-14 delays were sampled to map out an OH profile over at least three lifetimes. The reaction mixture was flowed slowly through the cell, so that each photolysis/probe sequence interrogated a fresh gas mixture and reaction products did not build up in the cell. The helium carrier gas (UHP certified to >99.9995% (Alphagas)) was used without purification. The 50 wt % H2O2 solution obtained from Prolabo was concentrated by bubbling helium through the solution to remove water for several days prior to use and constantly during the course of the experiments. It was admitted into the reaction cell by passing a small flow of helium through a glass bubbler containing H2O2. Methyl acetate (99%), ethyl acetate (99.8%), n-propyl acetate (99%), and n-pentyl acetate (99%) were from Aldrich; n-butyl acetate (>99%) was from Fluka. These compounds were further purified by repeated freeze, pump, and thaw cycles and fractional distillation before use. For the kinetic measurements, the studied acetate was premixed with helium in a 10 L glass light-tight bulb to form a 0.3-8% mixture at a total pressure of ≈850 Torr. All the gases flowed into the reactor through Teflon tubing. The gas mixture containing the acetate, the photolytic precursor (H2O2), and the bath gas (approximately 100-120 Torr of helium) were flowed through the cell with a linear velocity ranging between 5 and 20 cm s-1. The concentrations of reactants and the bath gas were calculated from their mass flow rates, the temperature, and the pressure in the reaction cell. All flow rates were measured with mass flow meters calibrated by measuring the rate of pressure increase in a known volume. The cell pressure was measured with a capacitance manometer connected at the cell entrance. Results and Discussion Reactions 1-5 were studied under pseudo-first-order conditions using a large excess of acetate over OH. The acetate concentration ranges were (in 1014 molecule cm-3) 1.2-52.1, 0.28-28.52, 0.68-31.12, 0.25-8.68, and 0.23-2.76 for methyl acetate, ethyl acetate, n-propyl acetate, n-butyl acetate, and n-pentyl acetate, respectively. The OH initial concentration ([OH]0]) was always less than 6 × 1011 molecule cm-3, the H2O2 concentrations and the photolysis laser fluence ranges being (0.5-1.5) × 1014 molecule cm-3 and 2-15 mJ per pulse, respectively. Under these conditions, the OH concentration time profile followed the pseudo-first-order rate law over at least three lifetimes:
[OH]t ) [OH]0 e-k′t where k′ ) ki[Xi] + k′0 Xi refers to the acetate in reaction i (i ) 1-5), and ki is the rate constant for the reaction of OH with the acetate (i). The decay rate, k′0, is the first-order OH decay rate in the absence of the acetate. The value of k′0 is essentially the sum of the reaction
J. Phys. Chem., Vol. 100, No. 30, 1996 12365
Figure 1. Plots of k1′-k5′ vs acetate concentration at room temperature. The lines represent the linear least-squares fitting.
rate of OH with its precursor (H2O2), and the diffusion rate of OH out of the detection zone. Plots of (k′-k′0) versus the acetate concentration obtained at room temperature for the different acetates are shown in Figure 1. In all cases the data show excellent linearity, and values of ki were derived from the least-squares fit of the straight lines. The quoted errors for ki determined in this work include 2σ from the least-squares analysis and the estimated systematic error 5% (due to uncertainties in measured concentrations). Systematic errors were minimized under our experimental conditions. Possible contributions to the measured rate constants from secondary reactions of OH with the products of reactions 1-5 were significantly reduced by using a high range (102104) of [acetate]/[OH] ratios which did not show any difference in measured k values. Also rate constants were shown to be independent of variations in the gas flow rates through the reactor or changes in the total pressure of the system. All the compounds studied were purified to better than 99%, and hence loss of OH radicals by reaction with impurities in the gas mixtures is expected to be insignificant. The five investigated acetates do not absorb at 248 nm, the wavelength at which H2O2 was photolyzed to generate OH radicals.8 Hence reaction of OH with photofragments of the acetates cannot contribute to OH loss. As expected variations in the photolysis fluence had no effect on the determined rate constants. However, artifact was suspected with the low-volatile n-pentyl acetate compound. Samples of n-pentyl acetate in helium could be sticky at the wall of the glass bulb where it was stored at room temperature. Rate constants measured under these conditions were effectively found to be lower by 10-20% than the value obtained when the bulb and the Teflon line were heated at around 50 °C. Hence, k5 was measured under these experimental conditions. Rate constants for the reactions of OH radicals with the aliphatic acetates were determined over the temperature range 243-372 K. The experimental conditions and the measured values of the rate constant are listed in Tables 1 and 2. The obtained data are summarized in Table 3. The measured rate constants are shown plotted in Arrhenius form in Figure 2. The plots show a slight negative temperature dependence of the rate constant for all the acetates, except for methyl acetate, for which a slight positive temperature dependence was observed. Besides, all the plots appear to exhibit slight curvature and are better represented by a three-parameter expression of the form
12366 J. Phys. Chem., Vol. 100, No. 30, 1996
El Boudali et al.
TABLE 1: Reactions OH + Methyl Acetate (1) and OH + Ethyl Acetate (2): Summary of Experimental Conditions and Measured k1 and k2 T (K)
[methyl acetate] (1014)a
1013 × (k1 ( 2σ)b
[ethyl acetate] (1014)a
1012 × (k2 ( 2σ)b
243 253 263 273 273 283 298 298 298 298 298 323 348 369
3.35-49.90 3.51-52.09 3.23-49.86 3.08-47.43
3.54 ( 0.24 3.24 ( 0.14 3.27 ( 0.15 3.19 ( 0.12
1.52-22.62 1.31-28.52
2.60 ( 0.14 2.30 ( 0.05
1.64-24.27 3.01-43.67 3.18-43.07 3.34-44.54 1.24-17.05 1.54-23.21 1.46-21.29 1.32-20.60
3.26 ( 0.08 3.18 ( 0.10 3.09 ( 0.14c 3.17 ( 0.16d 3.15 ( 0.26e 3.40 ( 0.14 3.78 ( 0.11 4.05 ( 0.19
0.81-15.13 0.28-5.67 1.10-24.03 0.77-13.89 1.74-22.56 1.55-26.57 0.73-13.37
2.05 ( 0.10 2.26 ( 0.10e 1.81 ( 0.06 1.74 ( 0.09 1.60 ( 0.14 1.71 ( 0.08d 1.76 ( 0.08c
0.73-13.04 0.64-12.11 0.55-11.14
1.66 ( 0.15 1.55 ( 0.14 1.52 ( 0.10
a Units of molecule cm-3. b Units of cm3 molecule-1 s-1. c Variation of the photolysis laser fluence (decrease by a factor of 3). d Variation of flow velocity (decrease by a factor of 3). e Experiments carried out at 40 Torr.
k ) A′T2 e-E/RT. This expression, with the exponent value of 2 for temperature, has been shown to well represent the curvature of Arrhenius plots for reactions of OH with organic compounds. Both Arrhenius and three-parameter expressions of the rate constants are given in Table 3. Comparison with Previous Results. The rate coefficients for reactions 1-5 obtained in the present work and in previous studies are reported in Table 3. Comparison can be made for k1-k5 at room temperature and also for k1, k2, and k4 as a function of temperature. For k3 and k5, the present work provides the first temperature dependence data. Agreement between the data obtained at room temperature in this study and relative or absolute rate measurements is in general good, while some discrepancies are observed between the temperature dependent data. For reaction 1, the k1 values are in good agreement, except that of Campbell and Parkinson14 which was obtained by a relative method and is significantly lower. However, as
discussed by Atkinson,15 the rate constants obtained by Campbell and Parkinson are suspect due to questions concerning the validity of the experimental technique used. For reaction 2, there is a good agreement between the different values of k2 at room temperature. Wallington et al.16 have determined the temperature dependence of k1 and k2 over the range 240-440 K. Because of the observed nonlinearity of the Arrhenius plots, they used only the kinetic data obtained over the range 296440 K to derive the Arrhenius expressions given in Table 3 along with our results in the temperature range 298-369 K for comparison. Within these temperature ranges, there is a reasonable agreement between the Arrhenius parameters of k1 obtained by Wallington et al.16 and our work. However, in the range 240-296 K, Wallington et al. observed a significant negative temperature dependence of k1 (E/R ) -(450 ( 45)), while our k1 data exhibits a near zero temperature dependence (E/R ) -(103 ( 118)) in the same temperature range. At 240 K, our value is nearly 37% lower than that obtained by Wallington et al. For k2, there is also some difference between both studies in the range 296-440 K: the temperature dependence of k2 from Wallington et al. is slightly positive, while ours is slightly negative in the range 298-369 K (see Table 3). This difference could be explained by the different extent of the temperature ranges used to derive the Arrhenius parameters in both studies, the Arrhenius plots being nonlinear (the temperature range investigated by Wallington et al. was larger than in the present study). Besides, in the range 240296 K the negative temperature dependence of k2 is higher in the study of Wallington et al. (E/R ) -(974 ( 145)) than in ours (E/R ) -(574 ( 80)). For reaction 3 involving n-propyl acetate, the room temperature rate constant obtained in this work is in excellent agreement with that obtained by Wallington et al.16 and Williams et al.21 while those of Winer et al.19 and Kerr and Stocker20 are slightly higher and lower, respectively. For n-butyl acetate, there are two groups of k4 values at room temperature which differ by ca. 30%. Hartmann et al.22 and Wallington et al.16 reported a value around 4.2 × 10-12, while more recently Williams et al.21 obtained 5.71 × 10-12 (in cm3 molecule-1 s-1). Our measurements confirm this latest value.
TABLE 2: Reactions OH + n-Propyl Acetate (3), OH + n-Butyl Acetate (4), OH + n-Pentyl Acetate (5): Summary of Experimental Conditions and Measured k3, k4 and k5 T (K) 258 263 263 273 273 283 298 298 298 298 298 298 298 298 313 333 333 333 353 372 372
[n-butyl acetate] (1014)a
1012 × (k4 ( 2σ)b
0.41-5.96 0.39-5.20 0.39-5.74
6.96 ( 0.23 7.08 ( 0.18 6.39 ( 0.23
3.71 ( 0.11 3.43 ( 0.06 3.40 ( 0.09 3.40 ( 0.13c 3.20 ( 0.10d 3.32 ( 0.15e
0.29-4.54 0.44-5.72 0.37-5.32 0.25-4.24 0.35-4.71 0.30-4.50 0.71-8.68 0.46-2.90
6.01 ( 0.20 5.57 ( 0.16 5.44 ( 0.09 5.46 ( 0.10 5.61 ( 0.23 5.71 ( 0.21c 5.53 ( 0.22d 6.15 ( 0.15e
0.99-12.69 0.83-10.68
3.18 ( 0.12 2.99 ( 0.06
0.30-4.20 0.28-3.87 0.34-4.11
5.20 ( 0.18 5.00 ( 0.14 5.12 ( 0.09
0.82-10.13 0.83-9.61 0.87-10.51
2.98 ( 0.11 2.93 ( 0.12 2.91 ( 0.05
0.28-3.70 0.26-3.51 0.31-3.68
5.00 ( 0.11 5.07 ( 0.35 5.05 ( 0.18
[n-propyl acetate] (1014)a
1012 × (k3 ( 2σ)b
1.18-14.90 0.78-13.26
4.56 ( 0.09 4.37 ( 0.15
0.72-12.74
4.03 ( 0.10
0.97-12.63 0.68-11.86 0.95-12.08 1.00-13.49 2.04-25.24 2.96-31.12
[n-pentyl acetate] (1014)a
1012 × (k5 ( 2σ)b
0.34-2.56 0.34-2.61 0.33-2.76 0.28-2.27 0.29-2.26 0.26-2.02 0.29-2.19 0.32-2.46 0.34-2.36 0.60-4.33 0.37-3.45 0.31-2.30 0.23-1.58 0.29-2.16 0.26-1.91 0.27-2.08 0.24-1.65 0.26-1.92
8.92 ( 0.58 9.01 ( 0.46 8.19 ( 0.31 7.00 ( 0.25 7.26 ( 0.19 7.77 ( 0.17 7.13 ( 0.32 7.55 ( 0.54 7.10 ( 0.34c 7.07 ( 0.42d 6.89 ( 0.33e 7.07 ( 0.16 6.76 ( 0.30 6.67 ( 0.22 6.13 ( 0.35f 6.29 ( 0.12 6.35 ( 0.39 7.08 ( 0.34
a Units of molecule cm-3. b Units of cm3 molecule-1 s-1. c Variation of the photolysis laser fluence (decrease by a factor of 3). d Variation of flow velocity (decrease by a factor of 3). e Experiments carried out at 40 Torr. f Variation of the photolysis laser fluence (decrease by a factor of 4).
Kinetic Studies of OH Reactions
J. Phys. Chem., Vol. 100, No. 30, 1996 12367
TABLE 3: Comparison of OH Reaction Rate Coefficientsa,b with Previous Work molecule CH3C(O)OCH3
T, (K)
ka (10-12 cm3 molecule-1 s-1)
292 296 298 298
0.17 ( 0.05 0.341 ( 0.029 0.385 ( 0.016 0.322 ( 0.026
n
292 296 296 298
305 303 296 297 298
CH3C(O)OC5H11
298 296 297 298
297 298
techniquec
ref [14] [16] [17] this work this work this work
343 ( 74 128 ( 102 -(455 ( 62)
298-369 243-369 243-369
RR FP-RF RR LP-LIF LP-LIF LP-LIF
296-440 298-369 243-369 243-369
RR FP-RF FP-RF LP-LIF LP-LIF LP-LIF
[14] [18] [16] this work this work this work
-(370 ( 69) -(991 ( 34)
258-372 258-372
RR RR FP-RF RR LP-LIF LP-LIF
[19] [20] [16] [21] this work this work
594 ( 126
298-516
2
3.25 ( 0.28 2.10 ( 0.28 (2.84 ( 0.41) × 10-6
-(155 ( 56) -(299 ( 81) -(926 ( 44)
298-372 263-372 263-372
LP-RF FP-RF RR LP-LIF LP-LIF LP-LIF
[22] [16] [21] this work this work this work
2
2.75 ( 0.46 (3.32 ( 0.70) × 10-6
-(302 ( 102) -(968 ( 66)
273-372 273-372
RR LP-LIF LP-LIF
[21] this work this work
1.84 ( 0.37 1.7 ( 0.2 1.51 ( 0.14 1.67 (0.22
4.2 ( 0.9 2.50 ( 0.25 3.45 ( 0.34 3.42 ( 0.87 3.42 ( 0.26 2
CH3C(O)OC4H9
T range (K) 296-440
2 CH3C(O)OC3H7
E/Ra,b (K) 260 ( 150
0.83 ( 0.35
2 CH3C(O)OC2H5
Aa,b (10-12 cm3 molecule-1 s-1)
4.3 ( 0.8 4.15 ( 0.30 5.71 ( 0.94 5.52 ( 0.51
1.02 ( 0.12 0.53 ( 0.09 (0.82 ( 0.17) × 10-6
2.3 ( 0.2 0.98 ( 0.20 0.48 ( 0.09 (0.73 ( 0.21) × 10-6
1.03 ( 0.12 (1.42 ( 0.16) × 10-6 31 ( 7
7.53 ( 0.48 7.34 ( 0.91
131 ( 28 -(163 ( 133) -(397 ( 103) -(986 ( 76)
Errors are those given by the authors. For our data, the uncertainties for A and E/R were given by ∆A ) 2AσlnA and ∆E/R ) 2σE/R for the Arrhenius forms. c Key: LP-LIF, laser photolysis-laser induced fluorescence; FP-RF, flash photolysis-resonance fluorescence; RR, relative rate. a
b
Figure 2. Plots of k1-k5 vs 1/T. The solid lines represent the threeparameter least-squares fits to the individual data points for each acetate. The dashed lines correspond to the Arrhenius parameter fits. The error bars of the individual points are 2σ and do not include estimated systematic errors.
Williams et al. indicated that the difference between their measurements and those of Hartmann et al. and Wallington et al. could be due to a pressure effect on k4. The total pressures at which the experiments were carried out were 11-5016,22 and 760 Torr,21 respectively. Our results do not support this explanation since our experiments were performed at low pressure (35-100 Torr). Hartmann et al.22 have also investigated the temperature dependence of k4, and a significant disagreement between their Arrhenius parameters (both A and
Figure 3. Plots of k1, k2 and k4 vs 1/T; comparison with previous measurements.
E/R) and those reported in this study is observed as shown in Figure 3. Our values at room temperature and 372 K are, respectively, 22% higher and 24% lower than those obtained by Hartmann et al. It is worth noting that the temperature ranges considered in both studies were different. Finally, for the reaction of OH with n-pentyl acetate, the study of Williams et al.21 gives a room temperature rate constant in excellent agreement with that determined in this work. For this reaction
12368 J. Phys. Chem., Vol. 100, No. 30, 1996 and reaction 3, our temperature dependent values of the rate constants are the first to be reported and cannot then be compared with literature. Trends in the OH + Acetate Reaction Rate Constants. Our determination of OH reactivity toward aliphatic acetates basically supports the observations made previously by different groups presented above. We confirm that OH radical reacts by H-atom abstraction predominantly with the alkoxy end of the acetates rather than with the acetyl end, and comparison of the room temperature rate coefficients shows an increase in the reactivity from methyl acetate to n-pentyl acetate. Even for methyl acetate, the H-atom abstraction from the methoxy end is dominant, since the rate constant for the H-atom abstraction from the CH3 group of the acetyl end is estimated to be around 1 × 10-13 cm3 molecule-1 s-1 in reference to the rate constant for the OH reaction with CH3C(O)CH3 (k ) 2 × 10-13 cm3 molecule-1 s-1 at 298 K9) which also contains CH3 groups with a carbonyl group in the R position. This estimated rate constant may even be lower considering the recent determination of the rate constant of the OH reaction with peroxy acetyl nitrate (CH3C(O)O2NO2) which also contains a CH3 group adjacent to a carbonyl group (k < 3 × 10-14 cm3 molecule-1 s-1 at 298 K23). On the basis of such group reactivity, the structureactivity relationship (SAR) of Atkinson24,25 was applied to the rate constant calculation for reactions of OH with organic molecules including acetates. In the updated SAR data base,25 the substituent factors of the -C(O)OR and -OC(O)R ester groups were calculated to be F(-C(O)OR) ) 0.74 and F(OC(O)R) ) 1.6 at 298 K. Using these values, the SAR gives the following calculated rate constants which compare well with the experimental ones (in parentheses): k1 ) 0.32 (0.32), k2 ) 1.76 (1.67), k3 ) 3.3 (3.4), k4 ) 4.7 (5.5), and k5 ) 6.1 (7.3) (in units of 10-12 cm3 molecule-1 s-1 at 298 K). Temperature Dependence of the OH + Acetate Reaction Rate Constants. Except for k1 which is slightly positive temperature dependent, the temperature coefficient of the other measured rate constants is negative and about the same (-300 K e E/R e -400 K) within the temperature range used in this work (see Table 3). We also observed a nonlinearity of the Arrhenius plots for the five studied reactions. The plots for all reactions exhibit a negative temperature dependence at low temperature, while a positive temperature dependence (reaction 1) or a nearly constant value is observed in the high-temperature range of the studies. Such behavior for k2-k5 seems to anticipate some slight positive temperature dependence at higher temperature, corresponding to a direct H-atom transfer mechanism which occurs for OH + alkanes reactions. In contrast, the temperature dependences provide some support for the occurrence of an indirect H-atom transfer mechanism at low temperature with formation of a long-lived bound adduct between the OH radical and the acetates. Such a mechanism has been already suggested for OH + ether reactions.26,27 Atmospheric Implication. Concerning the atmospheric implication, the rate constant data obtained in the present study
El Boudali et al. contribute to better define the tropospheric lifetimes of the studied acetates which react predominantly with the OH radical. With a typical tropospheric OH concentration of 1 × 106 molecule cm-3 the following tropospheric lifetimes (τ ) 1/ki[OH]) are estimated: 36 days, 7 days, 81 h, 50 h, and 38 h for methyl acetate, ethyl acetate, n-propyl acetate, n-butyl acetate, and n-pentyl acetate, respectively. Acknowledgment. The Authors acknowledge the French Ministry of Environment and European Commission for support, Dr. S. Te´ton for some assistance, and Dr P. Dagaut for performing the GC/FID analysis of the acetate samples. References and Notes (1) Tuazon, E. C.; Carter, W. P. L.; Aschmann, S. M.; Atkinson, R. Int. J. Chem. Kinet. 1991, 23, 1003-1015. (2) Smith, D. F.; Kleindienst, T. E.; Hudgens, E. E.; McIver, C. D.; Bufalini, J. J. Int. J. Chem. Kinet. 1991, 23, 907-924. (3) Smith, D. F.; McIver, C. D.; Kleindienst, T. E. Int. J. Chem. Kinet. 1995, 27, 453-472. (4) Smith, D. F.; Kleindienst, T. E.; Hudgens, E. E.; McIver, C. D.; Bufalini, J. J. Int. J. Chem. Kinet. 1992, 24, 199-2154. (5) Guidance on Estimating Motor Vehicle Emission Reductions from the Use of AlternatiVe Fuels and Fuel Blends; U.S. EPA Report No. EPAAA-TSS-PA-87-4, U.S. EPA: Ann Arbor, MI 1988. (6) Langer, S.; Ljungstrom, E.; Wangberg, I. J. Chem. Soc., Faraday Trans. 1993, 89, 425-431. (7) Atkinson, R.; Carter, W. P. L. Chem. ReV. 1984, 84, 437-470. (8) Calvert, J.; Pitts, J. N., Jr. Photochemistry; Wiley: New York, 1966. (9) Atkinson, R. J. Phys. Chem. Ref. Data 1989, Monographe No. 1. (10) Atkinson, R. J. Phys. Chem. Ref. Data 1994 Monograph No. 2. (11) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, R. F., Jr. Kerr, J. A.; Troe, J. J. Phys. Chem. Ref. Data 1992 Vol. 21, No. 6. (12) Mellouki, A.; Te´ton, S.; Laverdet, G.; Quilgars, A.; Le Bras, G. J. Chim. Phys. Phys.-Chim. Biol. 1994, 91, 473-487. (13) Mellouki, A.; Te´ton, S.; Le Bras, G. Int. J. Chem. Kinet. 1995, 27, 791-805. (14) Campbell I. M.; Parkinson, P. E. Chem. Phys. Lett. 1978, 53, 385387. (15) Atkinson, R. Chem. ReV. 1986, 85, 69-201. (16) Wallington, T. J.; Dagaut, P.; Liu, R.; Kurylo, M. J. Int. J. Chem. Kinet. 1988, 20, 177-186. (17) Smith, D. F.; McIver, C. D.; Kleindienst, T. E. Int. J. Chem. Kinet. 1995, 27, 453-472. (18) Zetzsch, C. cited in ref 9. (19) Winer, A. M.; Lloyd, A. C.; Darnall, K. R.; Atkinson, R.; Pitts, J. N., Jr. Chem. Phys. Lett. 1977, 51, 221-226. (20) Kerr, J. A.; Stocker, D. W. J. Atmos. Chem. 1986, 4, 253-262. (21) Williams, C. C.; O’Rji, L. N.; Stone, D. A. Int. J. Chem. Kinet. 1993, 25, 539-548. (22) Hartmann, D.; Gedra, A.; Rha¨sa, D.; Zellner, R. Proceedings of the 4th European Symposium on the Physico-Chemical BehaVior of Atmospheric Pollutants, 1986; Riedel: Dordrecht, Holland, 1987; pp 225235. (23) Talukdar, R. K.; Burkholder, J. B.; Schmoltner, A. M.; Roberts, J. M.; Wilson, R.; Ravishankara, A. R. J. Geophys. Res. 1995, 97, 1416314173. (24) Atkinson, R. Int. J. Chem. Kinet. 1987, 19, 799-828. (25) Kwok, E. S. C.; Atkinson, R. Atmos. EnViron. 1995, 29, 16851695. (26) Nelson, L.; Rattigan, O.; Neavyn, R.; Sidebottom, H.; Treacy, J.; Nielsen, O. J. Int. J. Chem. Kinet. 1990, 22, 1111-1126. (27) Te´ton, S.; Mellouki, A.; Le Bras, G.; Sidebottom, H. Int. J. Chem. Kinet., 1996, 28, 291-297.
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