Article pubs.acs.org/JPCA
Experimental and Theoretical Study of the Kinetics of the OH + Propionaldehyde Reaction between 277 and 375 K at Low Pressure Paul E. Carey, Jr. and Philip S. Stevens* School of Public and Environmental Affairs and Department of Chemistry Indiana University, Bloomington, Indiana 47405, United States S Supporting Information *
ABSTRACT: Measurements of the rate constant for the reaction of OH radicals with propionaldehyde as a function of temperature were performed using lowpressure discharge-flow tube techniques coupled with laser-induced fluorescence detection of OH radicals. The measured room-temperature rate constant of (1.51 ± 0.22) × 10−11 cm3 molecules−1 s−1 at 4 Torr was generally lower but in reasonable agreement with previous absolute and relative rate studies at higher pressures. Measurements as a function of temperature resulted in an Arrhenius expression of (2.3 ± 0.4) × 10−11 exp[(−110 ± 50)/T] cm3 molecules−1 s−1 between 277 and 375 K at 4 Torr. The observed temperature dependence at low pressure is in contrast to previous measurements of a negative temperature dependence at higher pressures. Ab initio calculations of the potential energy surface for this reaction suggest that the primary reaction pathway involves the formation of a hydrogen-bonded prereactive complex, which could account for the difference in the observed temperature dependence at lower and higher pressures.
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INTRODUCTION Propionaldehyde (CH3CH2CHO) is an important aldehyde in the chemistry of the atmosphere with both natural and anthropogenic sources.1,2 Primary sources of aldehydes to the atmosphere include vehicle exhaust associated with incomplete combustion, while secondary sources include the atmospheric oxidation of volatile organic compounds (VOCs).3,4 Propionaldehyde can be produced through the atmospheric oxidation of several VOCs such as propane and 2-pentanol,5,6 and ambient mixing ratios of propionaldehyde on the order of 37 ppb have been measured in Los Angeles and between 1 and 3 ppb in forested and rural areas.1,7 Photolysis of aldehydes can be an important source of free radicals, particularly in polluted regions of the atmosphere, 8 and the contribution of propionaldehyde as a radical source depends on the relative rate of photolysis with loss by reaction with OH radicals. Reaction of propionaldehyde with OH radicals is believed to proceed through hydrogen abstraction from the α-carbon:5,9 CH3CH 2CHO + OH → CH3CH 2CO + H 2O
The CH3CH2C(O)O radical produced in reaction 3 likely decomposes to form CO2 and CH3CH2 radicals, eventually leading to the formation of acetaldehyde.10 There have been several measurements of the rate constant for the OH + propionaldehyde reaction at room temperature using both absolute and relative rate techniques, with reported rate constants between 1.7 and 2.1 × 10−11 cm3 molecules−1 s−1.9,11−17 However, there has been only one reported measurement of the temperature dependence for this reaction. Thévenet et al. reported an Arrhenius expression of (5.3 ± 0.5) × 10−12 exp[(405 ± 30)/T] cm3 molecules−1 s−1 over the temperature range 240−372 K.16 The reported negative temperature dependence for this reaction is similar to that observed for the reaction of OH radicals with other aldehydes and suggests a complex mechanism and not a simple bimolecular elementary reaction.18,19 There have also been several theoretical studies focusing on the reaction of OH with aldehydes, including formaldehyde, acetaldehyde, and propionaldehyde.5,18 The results of these studies suggest that the most probable reaction pathway involves the abstraction of a hydrogen from the α-carbon through the formation of a stable prereactive complex, similar to that found for the hydrogen abstraction reactions of OH with other compounds.20−23 However, there have been few studies of the impact of these prereactive complexes on the
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The CH3CH2CO radical product reacts rapidly with O2 to form a peroxy radical, which can subsequently react with NO to form an alkoxy radical, or react with NO2 to form peroxypropionyl nitrate (PPN): CH3CH 2CO + O2 → CH3CH 2C(O)O2
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CH3CH 2C(O)O2 + NO → CH3CH 2C(O)O + NO2
Received: May 30, 2015 Revised: July 28, 2015
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CH3CH 2C(O)O2 + NO2 ↔ CH3CH 2C(O)O2 NO2 © XXXX American Chemical Society
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DOI: 10.1021/acs.jpca.5b05179 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A
system was gated to further reduce laser scatter and background signals. The OH signal sensitivity was typically between 4 and 10 × 10−9 counts s−1 cm3 molecules−1, and with backgrounds of 50−300 counts s−1, the minimum detectable limit for OH was approximately 8 × 108 molecules cm−3 (S/N = 1, 10 s integration). For these experiments, pseudo-first-order conditions were used, and concentrations of OH radicals were typically kept below 2 × 1011 molecules cm−3. Propionaldehyde (Aldrich, 97%) was purified by several freeze−pump−thaw cycles, and dilute mixtures in He were prepared by distillation into a volume-calibrated reservoir resulting in concentrations of approximately 0.4−8%. Propionaldehyde was added to the flow tube through a 6 mm o.d. glass injector coated with halocarbon wax, and the reaction time was varied by changing the position of the injector in the reaction zone relative to the detector. The concentration of propionaldehyde in the flow tube was determined by measuring the pressure drop in the calibrated reservoir over time.
pressure dependence of these reactions. Recently, Vu et al. found that the OH + hydroxyacetone reaction displays a strong pressure dependence for pressures below 40 Torr, which they attributed to the impact of a prereactive complex on the reaction kinetics.24 This study presents absolute measurements of the rate constant for the OH + propionaldehyde reaction at 298 K and 4 Torr using discharge-flow tube techniques. In addition, measurements of the temperature dependence of the rate constant for this reaction between 277 and 375 K at 4 Torr are presented. Theoretical calculations of the potential energy surface for this reaction are also presented to provide some additional insights into the mechanism of this reaction.
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EXPERIMENTAL METHODS Experiments were done using the discharge-flow technique combined with laser-induced fluorescence (LIF) detection of OH. The system used in this study is similar to those described in detail elsewhere,25−27 and a schematic of the system is illustrated in Figure 1. The flow tubes used in this study were
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COMPUTATIONAL METHODS Theoretical calculations were performed using the Gaussian 03 software package28 on the Indiana University IBM BladeCenter HS21/iDataPlexdx340 Quarry Cluster system. The geometries and frequencies of the reactants, prereactive complex, transition states, and products were optimized using Becke’s threeparameter hybrid method employing the LYP correction functional (B3LYP) with the 6-311++G(2d,2p) basis set and using second-order Møller−Plesset perturbation theory (MP2) with the 6-311++G(2d,2p) basis set. Single-point energy calculations on the MP2/6-311++G(2d,2p) optimized geometries were also done by using fourth-order Møller−Plesset perturbation theory with single, double, triple, and quadruple excitations (MP4(SDTQ)) using the 6-311++G(2d,2p) basis set.
Figure 1. Schematic diagram of the discharge-flow system used in this study.
100 cm long with an inner diameter of 2.5 cm and were coated with halocarbon wax (Halocarbon Corporation) to minimize the loss of OH radicals on the walls of the reactor. The temperature of the system was controlled using heating tape wrapped around the reaction zone for temperatures above room temperature and cooled by circulating liquid-nitrogen cooled ethanol through a jacket surrounding the main reaction zone for experiments below room temperature. The pressure of the system was measured by an MKS Baratron capacitance manometer located in the center of the reaction zone and controlled by varying the bulk flow of helium (Indiana Oxygen, 99.995%) using an MKS 1179 flow controller. Flow velocities between 10 and 12 m s−1 were achieved using a Leybold D16B mechanical pump. OH radicals were produced using the H· + NO2 → OH· + NO reaction. H atoms were produced using a microwave discharge of H2 (Indiana Oxygen, 99.999%), and an excess of NO2 (1% in UHP helium, Matheson Tri-Gas) was added 2 cm downstream from the discharge. OH radicals were detected by laser-induced fluorescence using the frequency-doubled output of a Lambda Physik dye laser pumped by a Spectra Physics diode-pumped Nd:YAG laser. OH radicals were excited using the A-X (1,0) band via the Q1(1) transition at 282 nm, and the fluorescence in the A-X (0,0) band at 308 nm was detected using a photomultiplier tube equipped with photon counting electronics placed at a right angle to the laser beam (Hamamatsu H6180-01). An interference filter (10 nm bandpass, 20% transmittance, Esco Products) centered at 308 nm located in front of the photomultiplier tube isolated the OH fluorescence from direct scatter from the laser. The detection
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EXPERIMENTAL RESULTS
Figure 2 shows a series of pseudo-first-order decay plots for the OH + propionaldehyde reaction in the presence of varying concentrations of propionaldehyde. The pseudo-first-order rate constant for a given propionaldehyde concentration was calculated from a weighted linear least-squares fit (based on the precision of each measurement)29 of the slope of the
Figure 2. Sample pseudo-first-order decays of OH as a function of reaction distance for the OH + propionaldehyde reaction at 298 K and 4 Torr at various propionaldehyde concentrations. B
DOI: 10.1021/acs.jpca.5b05179 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A logarithm of the background-corrected OH fluorescence signal versus reaction distance and corrected for axial diffusion and OH radical loss on the injector under the plug-flow approximation:30 k1 = k1decay(1 + (k1decayD)/v 2) − k injector
temperature, with the quoted uncertainty equal to twice the standard error (2σ) of the weighted least-squares fit of the data. The room-temperature rate constant for the OH + propionaldehyde reaction was found to be (1.51 ± 0.22) × 10−11 cm3 molecules−1 s−1. This value is generally lower but in reasonable agreement with the previous measurements at room temperature and at pressures between 50 and 760 Torr using both relative rate and absolute techniques (Table 2 and Figure 4).9,11−17 The results reported here are in reasonable agreement with the measurements at 100 Torr of Semmes et al., who reported a rate constant of (1.71 ± 0.24) × 10−11 cm3 molecule−1 s−1 by generating OH radicals using flash photolysis and monitoring the decay of OH using resonance fluorescence.14 Previous measurements at higher pressures (299− 760 Torr) using relative rate techniques are generally greater, with reported values ranging from 1.8 to 2.1 × 10−11 cm3 molecule−1 s−1 (Table 2). Thévenet et al. measured a roomtemperature rate constant of (2.0 ± 0.3) × 10−11 cm3 molecule−1 s−1 using pulsed flash photolysis with laser-induced fluorescence detection of OH and found no dependence of the rate constant on pressure between 50 and 200 Torr at 298 K.16 However, preliminary measurements of the rate constant for the OH + propionaldehyde reaction at 2 Torr and 298 K resulted in a rate constant of (9.2 ± 1.0) × 10−12 cm3 molecule−1 s−1, suggesting the possibility of a strong pressure dependence for this reaction (Table 1). Thus, the lower value reported here at 4 Torr compared to previous measurements may be due to a pressure dependence for this reaction below 20 Torr. Future work will involve additional measurements of the rate constant for this reaction as a function of both temperature and pressure to accurately determine the pressure dependence of this reaction. The measurements at temperatures between 277 and 375 K and 4 Torr are also summarized in Table 1, and an Arrhenius plot of the measured rate constants versus inverse temperature is shown in Figure 4. A weighted least-squares fit of the measurements as a function of temperature results in the Arrhenius expression (2.3 ± 0.4) × 10−11 exp[(−110 ± 50)/T] cm3 molecules−1 s−1. The observed temperature dependence reported here is in contrast to the results of Thévenet et al., who found that the rate constant displayed a negative temperature dependence (Figure 4). They reported an
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k1decay
In this equation, is the uncorrected pseudo-first-order rate constant, D the OH diffusion coefficient in He (0.145 T3/2 /PTorr cm2 s−1),31 v the average flow velocity (10−12 m s−1), and kinjector the rate of loss for OH radicals on the moveable injector measured in the absence of propionaldehyde (
