Kinetic and Dynamic Investigations of OH Reaction ... - ACS Publications

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Kinetic and Dynamic Investigations of OH Reaction with Styrene Joeson Cho, Masoud Roueintan, and Zhuangjie Li* Department of Chemistry and Biochemistry, California State University-Fullerton, Fullerton, California 92834, United States S Supporting Information *

ABSTRACT: The kinetics of hydroxyl radical reaction with styrene has been studied at 240−340 K and a total pressure of 1−3 Torr using the relative rate/discharge flow/mass spectrometry technique. In addition, the dynamics of the reaction was also studied using the ab initio molecular orbital method. The reaction was found to be essentially pressure independent over 1−3 Torr at both 298 and 340 K. At 298 K, the average rate constant was determined, using four different reference compounds, to be kstyrene+OH = (5.80 ± 0.49) × 10−11 cm3 molecule−1 s−1. At 240−340 K, the rate constant of this reaction was found to be negatively dependent on temperature with an Arrhenius expression determined to be kstyrene+OH = (1.02 ± 0.10) × 10−11 exp[(532 ± 28)/T] cm3 molecule−1 s−1. Observation of mass spectral evidence of adduct products and their respective fragment ions suggests that the reaction proceeds with addition of the OH to the vinyl carbons of the styrene molecule. Ab initio calculations of both the addition and the abstraction pathways predict that the addition pathways are more energetically favorable because of large exothermicity and essentially barrierless transition state associated with the additions, which is consistent with the experimental observations. Using the styrene + OH rate constant determined at 277 K in the present work, the atmospheric lifetime of styrene was estimated to be 4.9 h.

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INTRODUCTION Styrene is widely used in the chemical industry to produce common goods such as polystyrene plastics, synthetic rubbers, and resins. It is emitted into the atmosphere by number of sources including solvents, adhesives, tobacco smokes, and automobile exhausts, and it has been ranked as the 16th most released chemical into the atmosphere in the United States in 2005.1−12 Styrene has been detected at the parts per billion (ppb) levels in ambient atmosphere, with typical abundances ranging from 0.06 to 45 ppb, which leads to a growing concern about its contribution to air pollution because it has been categorized as a hazardous air pollutant in the 1990 Clean Air Act.1,2,4−13 Styrene can lead to the formation of ground level ozone in the presence of high NO x (NO + NO 2 ) concentrations, with an overall ozone impact of 65% of the weighted average of all volatile organic compound (VOC) emissions.14 Furthermore, styrene can result in the formation of secondary organic aerosol when reacting with ozone under dry and ammonia-free conditions.15 Styrene is known to be toxic to humans, and exposure to styrene can cause damages to the central nervous and reproductive systems. It is also listed as a potential carcinogen.1,16 Thus, it is important to understand the chemical transformation of styrene in the atmosphere in order to better model the quality of air loaded with this compound in polluted areas.

Styrene is primarily eliminated from the atmosphere by a reaction with atmospheric oxidants such as OH, O3, NO3, and Cl. The reaction of styrene and the hydroxyl radical is expected to be the major daytime sink because of high reactivity and abundance of OH radicals in the atmosphere, C6H5CHCH 2 + OH → products

(1)

There were only limited numbers of kinetics studies available in literature for this reaction. Atkinson and Aschmann reported a rate constant of (5.87 ± 0.15) × 10−11 cm3 molecule−1 s−1 for reaction 1 at 740 Torr and 296 K using the relative rate method with reactant detection by gas chromatography equipped with flame ionization detector.17 Bignozzi et al. also measured the rate constant of reaction 1 at 298 K and 760 Torr using the same technique and reported a similar value of (5.3 ± 0.5) × 10−11 cm3 molecule−1 s−1.18 Baulch et al. studied the kinetics of reaction 1 at 295 K and 0.46−4.5 Torr using a discharge flowresonance fluorescence technique and reported a positive pressure dependence of rate constant for this reaction in a range of (1.2−4.3) × 10−11 cm3 molecule−1 s−1.19 To the best of our knowledge, there has been no temperature dependence study of the rate constant reported for the reaction 1, and the atmospheric lifetime of styrene has been estimated on the basis of room temperature kinetic data. Since most removal of VOCs tropospheric OH occurs at temperatures below 298 K, it is necessary to obtain kinetic information at tropospheric temperatures to better model the atmospheric chemistry Received: February 7, 2014 Revised: September 5, 2014 Published: September 12, 2014

© 2014 American Chemical Society

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dx.doi.org/10.1021/jp501380j | J. Phys. Chem. A 2014, 118, 9460−9470

The Journal of Physical Chemistry A

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Figure 1. Experimental arrangement of the RR/DF/MS system used for the kinetic and product study of styrene + OH reaction at 1−3 Torr and 240−340 K (1a), and for study of the potential effect of secondary reaction on the kinetic results (1b).

involving styrene, and estimate the atmospheric lifetime of this pollutant. The reaction mechanism of reaction 1 was previously studied by Bignozzi et al. and Tuazon et al.18,20 Bignozzi et al. proposed that the OH radical mainly adds to the terminal carbon of the vinyl group of the styrene molecule,18 whereas Tuazon et al. suggested that the addition takes place on both carbons of the vinyl group,20

Tuazon et al. also reported that the major products of the styrene oxidation process in the presence of O2 and NO are benzaldehyde and formaldehyde.20 In this paper, we will report our kinetic study of reaction 1 at 1−3 Torr and 240−340 K, and our product study at 1 Torr and 298 K using the relative rate/discharge flow/mass spectrometry (RR/DF/MS) technique. We will then comment on the atmospheric lifetime of styrene on the basis of the findings from this study. We will also report our ab initio molecular orbital calculations of this reaction and compare the theoretical results with our experimental observation of the products of reaction 1.

C6H5CHCH 2 + OH → C6H5C(OH)HCH 2 (1.1)

C6H5CHCH 2 + OH → C6H5CHC(OH)H 2 (1.2) 9461



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EXPERIMENTAL SECTION The detailed RR/DF/MS experimental apparatus for acquiring kinetic parameters of the reaction of volatile organic compounds (VOCs) with hydroxyl radical has been described previously,21−26 hence the RR/DF/MS technique is briefly discussed in the present work. A schematic arrangement of experimental apparatus is shown in Figure 1, in which Figure 1a illustrates the kinetics investigation scheme and Figure 1b depicts the experimental setup to examine for potential effect of secondary reactions on the kinetic results. The flow reactor is made of a Pyrex tube with a length of 80 cm and an internal diameter of 5.08 cm. The inside of the flow reactor is covered with a sheet of poly(tetrafluoroethylene) (TFE, 0.79 mm thick) to minimize the wall loss of the OH radicals and contamination of the internal surface by reaction products. The flow reactor is wrapped with a Pyrex jacket to allow for variation of temperature by circulating a fluid through the jacket using a fluid circulator (Neslab ULT-80). Either water or methanol was pumped through the Pyrex jacket for reactor temperatures above or below 298 K, respectively. Kinetics measurements were carried out four to six times at each temperature on different days under the same experimental conditions to verify the consistency of the kinetics results. A steady-state flow inside the flow reactor was maintained by using a mechanical pump (Edwards E2M175). The vacuum chamber sheltering the mass spectrometer was differentially pumped in two stages by two 6” diffusion pumps equipped with a liquid nitrogen baffle. The ultimate pressure in the second stage was 99%), propanal (>99%), naphthalene (99%), and isoprene (>99%) were purchased from Aldrich Chemical Co. Inc. Deuterium oxide (>99%) and styrene (>99%) were purchased from Thermo Fisher Scientific Inc. All chemicals were used as received. Deionized water was employed as the OH precursor.

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THEORETICAL APPROACH All computations were carried out using the Gaussian 09 program.28 The structure of reactants, prereaction complexes, products, and transition states involved in reaction 1 and subsequent reactions were optimized using the Møller−Plesset perturbation theory truncated at second-order (MP2) in conjunction with both 6-31G(d,p) and 6-311G(d,p) basis sets 9462



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parameters such as temperature, pressure, and flow rates. The average of these two rate constants was kstyrene+OH = (5.90 ± 0.48) × 10−11 cm3 molecules−1 s−1. It is important to carry out the rate constant measurement using different reference compounds for a reaction when relative rate technique is employed for the kinetics investigations, partly because some of the reference reactions have themselves been measured by relative-rate methods and the derived rate constants could be in error as the chain continues. With the usage of different references, the consistency of the kinetic results within the experimental uncertainty would give confidence for the accuracy of the rate constant measurement of the reaction, which connects reliably to original absolute rate constant measurements. For the relative rate technique to be valid, it requires that the decay of styrene and reference compounds are only associated with reactions 1 and 3. However, there was a concern in the present work that the products from reactions 1 and 3 may react with styrene and the reference compounds, contributing to decay of these reactants. To address this concern, several testing experiments were carried out using the experimental apparatus shown in Figure 1b, which allowed the products from reaction 1 or 3 to directly interact with the reference compound or styrene, respectively. It was found that the changes of mass spectral intensities of the reference and target compound were less than 1.3% and 1.7% when products of reaction 1 and 3 interacted with the reference and target compound, respectively. This suggested that the decay of styrene and the reference compounds were not significantly influenced by the primary reaction products. The potential contribution of reaction of styrene and reference compound with atomic oxygen and hydrogen to the decay of these reactants was also assessed in the present work using the Runge−Kutta method, which numerically solves the differential equations defined by the rate law for chemical reactions given in Table 1S as Supporting Information.24 The atomic oxygen and hydrogen can be generated by the selfreaction of hydroxyl radicals and subsequent reaction of hydroxyl radical with atomic oxygen, respectively. Our calculation results indicated that the concentration of atomic oxygen and hydrogen in the reactor was less than 2.4 × 1011 and 4.1 × 1010 molecules cm−3, respectively, and that the interaction between the reactants and the atomic hydrogen and oxygen accounted for less than 1% of the decay of both styrene and reference compound during 25 ms under our experimental conditions. Thus, the majority of the decay of styrene and reference compound was due to the reaction of the hydroxyl radical with the target and reference compounds, respectively, and the rate constant of reaction 1 determined in the present work is considered to be unaffected by secondary reactions, and hence reliable. Table 1 summarizes the rate constants determined for reaction 1 at 298 K and 1−3 Torr in the present work along with available values from literature. Our average rate constant of kstyrene+OH = (5.80 ± 0.49) × 10−11 cm3 molecules−1 s−1 at 298 K was determined by using isoprene, 1,3-butadiene, naphthalene, and propanal as the reference compounds. Within experimental uncertainties, our average kstyrene+OH agrees with the rate constant of kstyrene+OH = (5.87 ± 0.15) × 10−11 reported by Atkinson and Aschmann,17 and of kstyrene+OH = (5.3 ± 0.5) × 10−11 cm3 molecules−1 s−1 reported by Bignozzi et al.,18 but significantly higher than the value of kstyrene+OH = (1.97 ± 0.32) × 10−11 cm3 molecules−1 s−1 reported by Baulch et al.19 Our

(MP2/6-31G(d,p) and MP2/6-311G(d,p)). Frequency calculations for each of the species were performed at the same levels of theory. Intrinsic reaction coordinate (IRC) calculations were carried out at the MP2/6-31G(d,p) level of theory to check that the transition states are connected to appropriate reactants and products on the potential energy surface. Single-point energy calculations were performed at PMP4 level of theory in conjunction with 6-311++G(2d,2p) basis set (PMP4/6-311+ +G(2d,2p)) using the geometries optimized at MP2/6311G(d,p) level. Zero-point energy (ZPE) calculated at the MP2/6-311G(d,p) level of theory was added to the single-point energy results (MP4/6-311++G(2d,2p)//MP2/6-311G(d,p) + ΔZPE) to give the best estimate of the relative energetics for the reactions being examined in the present work.

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RESULTS AND DISCUSSION A. Pressure Dependence of kstyrene+OH. Figure 2 presents typical kinetic data of reaction 1 at 298 K and a total pressure of about 1 Torr using isoprene and 1,3-butadiene as reference compounds.

Figure 2. Typical kinetic data of styrene + OH reaction taken at ∼1.0 Torr and 298 K using isoprene (red circle) and 1,3-butadiene (blue circle) as the reference compounds. The straight line is a result of linear regression fit of each data set. The initial concentration of styrene, isoprene, and 1,3-butadiene was 4.3 × 1013, 7.9 × 1013, and 8.7 × 1013 molecules cm−3, respectively. The OH concentration was varied from (0−6) × 1013 molecules cm−3.

It can be seen that in a range of 0−45% reactant consumptions, the decay of the target and reference compounds follows the relationship expressed in eq 4. A total of 88 and 41 experimental data points were collected at 1 Torr and 298 K using isoprene and 1,3-butadiene as reference compounds in the present work, and a linear regression of these data points generated a slope of kstyrene+OH/kisoprene+OH and kstyrene+OH/k1,3‑butadiene+OH to be 0.605 ± 0.013 and 0.815 ± 0.021, respectively. The reference OH rate constants are known as kisoprene+OH = (1.03 ± 0.13) × 10−10 and k1,3‑butadiene+OH = (6.82 ± 0.60) × 10−11 cm3 molecules−1 s−1 at 298 K and 1 Torr,23,24 and the rate constant of reaction 1 was then determined as kstyrene+OH = (6.23 ± 0.80) × 10−11 and (5.56 ± 0.51) × 10−11 cm3 molecules−1 s−1 at 1 Torr and 298 K using isoprene and 1,3-butadiene as the reference compounds, respectively. The quoted error bars were taken as 2σ, which have accounted for the scattering of the data, the uncertainties of the reference OH rate constants, and the experimental 9463



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Table 1. Summary of Rate Constant (in cm3 molecule−1 s−1) of Styrene + OH Reaction as a Function of Pressure at Room Temperature and 340 K along with Obtainable Experimental Data in Literature Ptotal (Torr) 1.0 2.0 3.0 1.0 1.0 2.0 3.0 1.0 2.0 3.0 1.0 2.0 3.0 1.0 2.0 3.0 1.0 2.0 3.0 740 760 0.46 1.0 2.0 2.83 4.5

T (K) 298 298 298 340 298 298 298 298 298 298 340 340 340 298 298 298 340 340 340 296 298 295 295 295 295 295

reference compound isoprene isoprene isoprene isoprene 1,3-butadiene 1,3-butadiene 1,3-butadiene naphthalene naphthalene naphthalene naphthalene naphthalene naphthalene propanal propanal propanal propanal propanal propanal isoprene isooctane na na na na na

0.605 0.596 0.603 0.561 0.815 0.813 0.810 2.013 1.980 2.095 2.645 2.466 2.465 3.169 3.145 3.120 2.770 2.648 2.650 na na na na na na na

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.013 0.011 0.012 0.016 0.021 0.017 0.044 0.096 0.107 0.096 0.125 0.288 0.209 0.252 0.329 0.168 0.135 0.158 0.180

techniquec

k1(×1011)

slope a

(88) (103) (92) (68) (41) (46) (26) (59) (76) (51) (92) (34) (41) (52) (38) (43) (40) (36) (44)

6.23 6.14 6.21 4.82 5.56 5.54 5.52 5.23 5.15 5.23 5.42 5.06 5.05 6.17 6.12 6.07 4.82 4.61 4.61 5.87 5.3 1.20 1.97 3.81 4.10 4.31

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.82 0.80 0.82 0.91 0.51 0.50 0.57 0.74 0.78 0.84 0.60 0.78 0.66 1.36 1.42 1.29 0.91 0.89 0.90 0.15 0.5 0.17 0.32 0.25 0.60 0.45

b

RR/DF/MS RR/DF/MS RR/DF/MS RR/DF/MS RR/DF/MS RR/DF/MS RR/DF/MS RR/DF/MS RR/DF/MS RR/DF/MS RR/DF/MS RR/DF/MS RR/DF/MS RR/DF/MS RR/DF/MS RR/DF/MS RR/DF/MS RR/DF/MS RR/DF/MS DP/GC DP/GC RR RR RR RR RR

ref this this this this this this this this this this this this this this this this this this this 14 15 17 17 17 17 17

work work work work work work work work work work work work work work work work work work work

a

The number in parentheses represents number of data points collected at corresponding temperature and pressure. bThe error bar was taken as 2σ for all reported rate constants in the present work. cNotation: DP, direct photolysis, GC, gas chromatography, RR, relative rate.

Figure 3. Summary of pressure dependence of the rate constant for the styrene + OH.

difference in pressure dependence studies of reaction 1 between Baulch et al. and the present work are currently unclear. Since there were some uncertainties associated with the pressure dependence of the OH + isoprene reaction, it is possible that these uncertainties could have impacted our kinetics results for

kinetics results also suggest that the rate constant for reaction 1 is essentially pressure independent at 1−3 Torr. This observation is different from that reported by Baulch et al., who observed a positive pressure dependence in the range of 0.46 to 4.5 Torr for reaction 1.19 The exact cause for the 9464



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Table 2. Rate Constant of Styrene + OH Reaction As a Function of Temperature at 1 Torr T (K) 240 260 277 298 320 340

ref compound(s)

slope

isoprene isoprene isoprene four compoundsc isoprene three compoundse

0.633 ± 0.022 (46) 0.619 ± 0.016 (66) 0.603 ± 0.016 (105) n/a 0.571 ± 0.019 (72) n/a a

k1 (×1011)

technique

9.37 ± 1.15b 7.99 ± 0.99 6.99 ± 0.90 5.80 ± 0.49d 5.33 ± 0.73 5.02 ± 0.43f

RR/DF/MS RR/DF/MS RR/DF/MS RR/DF/MS RR/DF/MS RR/DF/MS

ref this this this this this this

work work work work work work

a

The number in parentheses represents number of data points collected at corresponding temperature. bThe error bar was taken as 2σ for all reported rate constants. cIsoprene, 1,3-butadiene, naphthalene, and propanal. dAverage rate constant calculated from rate constants determined using isoprene, 1,3-butadiene, naphthalene, and propanal as reference compounds in the present work. eIsoprene, naphthalene, and propanal. fAverage rate constant calculated from rate constants determined using isoprene, naphthalene, and propanal as reference compounds in the present work.

Figure 4. Arrhenius plot of styrene + OH reaction at 240−340 K along with other experimental data from literature.

also listed in Table 1, which suggest that the rate constant for reaction 1 is also essentially pressure independent at 1−3 Torr and 340 K. This observation indicates that the high pressure limit could have also been reached at 1 Torr at 340 K for reaction 1. Figure 3 summarizes the available rate constants of styrene + OH reaction as a function of pressure. B. Temperature Dependence of kstyrene+OH . The measurement of the rate constant of reaction of styrene with OH was also conducted at 240, 260, 277, 298, 320, and 340 K under a total pressure of 1 Torr using isoprene as the reference compound in the present work, and the results are summarized in Table 2 and Figure 4. Our kinetics results indicate that the rate constant of reaction OH with styrene is negatively dependent on temperature at 240−340 K, suggesting that this reaction proceeds with the addition of the OH radicals to the styrene molecule. In addition, the kstyrene+OH values obtained at 1 Torr and 340 K using naphthalene, propanal, and isoprene were consistent within experimental uncertainties, which gave additional confidence to our temperature-dependent kinetic results. A linear least regression fit of our kinetic data in Table 2 yielded an Arrhenius expression of kstyrene+OH = (1.02 ± 0.10) × 10−11 exp[(532 ± 28)/T] cm3 molecule−1 s−1. The lifetime of styrene can be estimated assuming that primary removal of styrene in the troposphere occurred via the

reaction 1. To further examine our pressure dependence kinetic results, a separate kinetic study for reaction 1 was conducted at 1−3 Torr and 298 K using both propanal and naphthalene as the reference compounds. These were chosen because the reaction of propanal + OH involves hydrogen abstraction which should be pressure independent, and the reaction of naphthalene + OH was recently determined to be essentially pressure independent in this pressure range.26,29 The results are also given in Table 1. It can be seen from Table 1 that there is little variation in kstyrene+OH values when both propanal and naphthalene was used as the reference compound, which in turn implied that kstyrene+OH is pressure independent. Our kinetics results are therefore consistently pointing to a conclusion that reaction 1 is essentially pressure independent at 1−3 Torr, and it follows that the high pressure limit of this reaction could have been reached at 1 Torr and 298 K. The pressure dependence kinetics study was also carried out at 340 K for reaction 1 using naphthalene and propanal as the reference compound in the present work. Since the measured knaphthalene+OH under the same conditions was found to be essentially pressure independent at 340 K and kpropanal+OH undergoes a hydrogen abstraction pathway, a rate constant of knaphthalene+OH = (2.05 ± 0.21) × 10−11 and kpropanal+OH = (1.74 ± 0.32) × 10−11 cm3 molecules−1 s−1 at 1 Torr was used to calculate kstyrene+OH at 340 K at 1−3 Torr.26,29 The results are 9465



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those for styrene + OD occur at m/z = 89, 90, 91, 92, 93, 94, 106, 107, 108, 119, 120, 123, and 124. Although reaction 1 proceeds with the addition of OH radical to styrene, as suggested by the negatively temperature dependent rate constant,

styrene + OH reaction. The atmospheric lifetime of styrene was then calculated using the following equation30 1 τstyrene = kstyrene + OH(277K)[OH]avg (5) where τstyrene is the atmospheric lifetime of styrene, kstyrene+OH is the rate constant of reaction 1 at 277 K, and [OH]avg is the average atmospheric OH concentration. Using [OH]avg = (8.1 ± 0.9) × 105 molecules cm−3,30 and the rate constant obtained at 277 K in this work, the upper limit of the atmospheric lifetime of styrene was calculated to be 4.9 h. The atmospheric lifetime of styrene is expected to be shorter due to the additional losses of styrene via reactions with other atmospheric oxidants, such as atomic chlorine and ozone during the day time, and nitrate radicals during the nighttime.31 C. Product Study of Reaction 1. Identification of products from reaction 1 may provide useful information about the mechanism of this chemical process. To study the products of reaction 1, the mass spectra of styrene + H2O, styrene + OH, and styrene + OD were collected. Figure 5

where R = H for styrene + OH, and D for styrene + OD, throughout this work direct styrene−OH and styrene−OD adducts were not observed at m/z = 121 and 122, respectively, for the P1b or P2b products shown in reactions 6.1 and 6.2. In the mass spectrum of styrene + OH (blue profile), the peak at m/z = 122 is assigned to Ph−C(H)OH−CH3+ and Ph−CH2− CH2OH+, the formation of which is still currently unclear. Likewise, the observation of the mass spectral peak at m/z = 124 from the styrene + OD reaction (red profile) is assigned to Ph−C(H)OD−CH3+ and Ph−CHD−CH2OD+, respectively. The addition of OH to the C2 carbon of the styrene molecule was evidenced by the mass spectral peaks at m/z = 107 (Ph−CH2O+) and 106 (Ph−CHO+). The hydroxyl radical addition to the C2 carbon is supported with replacement of OH by OD and observation of ion peaks at m/z = 108 (Ph− CHOD+) and 107 (Ph−CDO+) in the styrene + OD mass spectrum (red profile). The ion peak at m/z = 106 in the styrene + OH mass spectrum could arise from an intramolecular migration of the deuterium to form P1d, as shown in reaction 7.

The hydroxyl radical was also found to add to the C1 carbon, as evidenced by the mass fragment peaks at m/z = 89−94 in Figure 5. The peak at m/z = 91 and m/z = 92 in the styrene + OH mass spectrum (blue profile) are assigned to PhCH2+ and PhCH3+, respectively. The PhCH2+ may be resulted from the C1−C2 bond rupture in the Ph−CH2−CH2OH+ species. The PhCH3+ could be generated due to the intramolecular migration of the hydrogen from the hydroxyl group to C2 carbon in the Ph−CH2−CH2OH+ species followed by the C1− C2 bond rupture,

Figure 5. Mass spectra of styrene, styrene + OH, and styrene + OD. All mass spectra were collected in the absence of the reference compound. The styrene spectral profile is in black, whereas the spectral profiles of styrene + OH and styrene + OD are in blue and red, respectively. The initial concentrations of styrene and OH/OD were approximately 4.0 × 1013 and 3.0 × 1013 molecules cm−1, respectively.

shows the mass spectra of reactants and products of reaction 1, in which the styrene + H2O spectrum (black profile) was obtained in the absence of atomic fluorine to eliminate the production of the hydroxyl radical. The spectrum profile was found unchanged while the microwave discharge was turned “on” and “off” without passing F2 molecules through the microwave cavity, indicating that the effect of the microwave discharge was negligible. The styrene + OH and styrene + OD mass spectra were recorded by introducing H2O and D2O into the sliding injector in the presence of the atomic fluorine, respectively. The product spectra of reaction 1 were then generated by subtracting the spectrum of styrene + H2O and styrene + D2O from that of styrene + OH (blue profile) and styrene + OD (red profile), respectively. It can be seen from Figure 5 that the major peaks for styrene + OH occur at m/z = 91, 92, 106, 107, 120, and 122, while

The peaks at m/z = 92 and m/z = 94 in the styrene + OD mass spectrum (red profile) are assigned to Ph−CHD+ and Ph−CHD2+, which can be interpreted in the same way as that for interpretation of the formation of PhCH2+ and Ph−CH3+ by simply replacing the OH by OD in the P2.1 species. The peaks at m/z = 91 and m/z = 93 can result from the stripping of one hydrogen and one hydrogen plus one deuterium from the Ph− CHD2+, respectively. The intramolecular migration of deuterium to C2 in the styrene−OD adduct was also considered as an additional pathway for the formation of the fragment ion peak at m/z = 92 (Ph−CHD+), 9466



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Figure 6. Summary of best estimated (at PMP4/6-311++G(2d,2p)//MP2/6-311G(d,p) + ΔZPE level of theory) energetics (in kcal mol−1) of styrene + OH system.

of theory was then considered to be appropriate to characterize the structural parameters for species associated with reaction 1 and subsequent reactions. There are two major possible reaction pathways for the reaction of styrene with OH radicals: (1) the addition of OH to the vinyl group of styrene, as evidenced by the product mass spectra as shown in Figure 5, and (2) abstraction of a hydrogen atom from the vinyl group by the OH radical. The reaction pathways of addition of OH to the aromatic ring and abstraction of hydrogen atom from the aromatic ring were not considered in the present work since there was no experimental evidence suggesting these pathways in the reaction of styrene with OH radicals.18,20 Different products may be resulted from each of the pathways due to vinyl C1 and C2 carbons and the hydrogen atoms bonded to these carbons. Our computational results indicated that the OH addition to styrene began with the formation of a prereaction complex,

The relative energetics of reactions 7 and 9 has been calculated in the present work, and will be discussed next in section D. D. Ab Initio Investigation of the Reaction Pathways of Reaction 1. To obtain further insight of the reaction of the hydroxyl radical with styrene, ab initio computations were carried out to examine the dynamics of this chemical reaction. The geometries of styrene and OH radical optimized at the MP2/6-311G(d,p) level of theory are given in Figure 1S as Supporting Information. As shown in Figure 1S, the calculated rotational constants of styrene were in very good agreement with the experimental values determined by Caminati et al., with differences less than 1.5%.32 The MP2/6-311G(d,p) level 9467



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styrene molecule, in which two hydrogen bonds were formed between the oxygen with H9 and H12 with a bond length of 3.223 and 2.651 Å, respectively. The C1-styrene−OH adduct (P2b) was 19.3 kcal mol−1 more stable than the prereaction complex. The C1−C2 bond length is expected to increase from 1.347 Å in the complex to 1.495 Å in the C1-styrene−OH adduct (P2b). This long C1−C2 distance weakens this bond and allows the C1−C2 bond to be readily ruptured upon electron bombardment during the ionization. Alternately, the Ph−CH2 (P2d) and CH2O (P2e) species can be formed by the intramolecular hydrogen migration of H18 from the oxygen to the C2 carbon to form Ph−CH2CH2O (P2c) species followed by the rupture of the C1−C2 bond of P2c via a transition state with an activation energy of 30.9 kcal mol−1. This process is similar to that of the OH addition to C2, potentially giving rise to our experimental observation of a mass spectral peak at m/z = 91 in Figure 5. The dissociation of the C1-styrene−OH (P2b) and C2styrene−OH (P1b) adducts into Ph−CH (P2f) + CH2OH (P2g) and Ph−C(H)OH (P1f) + CH2 (P1g) is predicted to be endothermic by 66.7 kcal mol−1 and 79.8 kcal mol−1, respectively, which is the least likely reaction pathways for the subsequent reaction of the C1-styrene−OH (P2b) and C2styrene−OH (P1b) adducts. The abstraction processes were predicted to be the less favorable pathways in comparison to the addition processes. The abstraction of H11, H10, and H9 by the OH radical was predicted to have an activation energy barrier of 5.6 (TS3), 7.7 (TS4), and 7.2 (TS5) kcal mol−1, respectively. Unlike the addition process, the hydroxyl radical abstracted the hydrogen from styrene without the formation of a prereaction complex. Finally, our calculation results indicated that the abstraction of the H11, H10, and H9 leads to a decrease of C1−C2 bond length from 1.344 to1.281 Å (P3), 1.288 Å (P4), and 1.283 Å (P5), respectively.

which further configured to yield the styrene−OH adducts through a transition state. Subsequent reactions of the styrene− OH adducts were also investigated in the present work to help understand our experimental observation of the products. Figures 2S−4S given as Supporting Information show the geometries optimized at MP2/6-311G(d,p) level of theory for the prereaction complexes, transition states, and products involved in the hydroxyl radical addition to, and extraction of a hydrogen from the C2 and C1 carbons of the vinyl group of the styrene molecule, respectively, in which Figures 2S and 3S also show the optimized geometries of the transition states and products of subsequent rearrangement and dissociation reaction of the styrene−OH adducts. The total energy for species involved in reaction 1 and subsequent reactions is given in Table 2S as Supporting Information, and the relative energy referenced to reactants for reaction 1 and subsequent reactions are given in Table 3S as Supporting Information and summarized in Figure 6. The vibrational frequencies and zero-point energies for species involved in these chemical processes are given in Table 4S as Supporting Information. All the stable reactants and products species have only positive vibrational frequencies, indicating that they are located at the minimum on potential surfaces. There is one imaginary vibrational frequency for each of the transition state complexes, from which the vibrational vector connects the appropriate reactant and product along the reaction pathway, as confirmed by IRC calculations. Our calculation results indicate that the addition of OH radical onto C2 carbon initially forms a prereaction complex (P1a), as shown in Figure 2S in the Supporting Information. The hydrogen, H18, of the OH radical was oriented toward the double bond of the vinyl group, while there were two hydrogen bonds formed between oxygen and both H11 and H16, with a bond length of 3.173 and 2.835 Å, respectively. The prereaction complex (P1a) was 2.3 kcal mol−1 more stable than the reactants. The C2-styrene−OH adduct (P1b) was formed from the P1a complex through a transition state (TS1a) with an activation energy barrier of 3.2 kcal mol−1, and the overall addition of OH to C2 process is predicted to be exothermic by 23 kcal mol−1. The C1−C2 bond length of the C2-styrene−OH adduct (P1b) is longer than that of styrene by about 0.15 Å. This change of C1−C2 double bond character to single bond made the C2-styrene−OH adduct (P1b) easier to dissociate, which accounts for our experimental observation of the mass spectral peak at m/z = 107 in Figure 5. The intramolecular migration of the hydrogen atom in the hydroxyl radical (H18) from oxygen to the C1 carbon was predicted to associate with an activation energy of 30.7 kcal mol−1. Our calculation results suggest that intramolecular migration further weakens the C1− C2 bond by further increasing the C1−C2 bond length from 1.493 to1.547 Å, and the hydrogen migration product (P1c) is expected to dissociate into more stable products of benzaldehyde (P1d) and methyl radical (P1e) through a transition state with an activation energy of 10.2 kcal mol−1. This process may have given rise to our experimental observation of the mass spectral peak at m/z = 106 in Figure 5. The addition of hydroxyl radical onto the C1 carbon was found to be the most energetically favorable pathway due to an essentially overall barrierless transition state involved for this process, as predicted by our calculation results. Similar to the addition of OH radical onto C2, a prereaction complex, which is 2.3 kcal mol−1 more stable than the reactant, was also predicted to be formed for the addition of OH onto C1 of the

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SUMMARY The kinetics of the styrene + OH reaction has been investigated at 240−340 K under a total pressure of 1−3 Torr using the RR/DF/MS technique. The average rate constant at 298 K was determined to be kstyrene+OH (298 K) = (5.80 ± 0.49) × 10−11 cm3 molecules−1 s−1 at 1 Torr using isoprene, 1,3-butadiene, naphthalene, and propanal as the reference compounds. Within the experimental uncertainties, our rate constant is in excellent agreement with reported values by Atkinson and Aschmann, and Bignozzi et al. The rate constant of styrene reaction with OH was found to be essentially independent of pressure at 1−3 Torr, suggesting that the falloff region of the styrene + OH reaction may be below 1 Torr. At 240−340 K, the rate constant of reaction 1 is found to be negatively dependent on temperature, with an Arrhenius expression determined to be kstyrene+OH = (1.02 ± 0.10) × 10−11 exp[(532 ± 28)/T] cm3 molecule−1 s−1. The negative dependence of the rate constant on temperature suggests the addition of the hydroxyl radical onto styrene, and evidence for the addition of the OH radical to both carbon atoms in the vinyl group of styrene was observed in the present work. On the basis of kstyrene+OH at 277 K, the lifetime of styrene was determined to be 4.9 h. Our ab initio calculation results suggest that the addition of the hydroxyl radical to the vinyl group of styrene is the most favorable pathway for the reaction of OH radicals with styrene, which agrees with the experimental observation. Our 9468



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calculation results also suggest that the addition process begins with the formation of the prereaction complex.

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ASSOCIATED CONTENT

S Supporting Information *

Reaction scheme for chemical model simulation (Table 1S), optimized geometries for reactants, prereaction complexes, transition states, and products involved in reaction 1 and subsequent reactions (Figures 1S − 4S), total energies and ZPE of species involved in the reaction of styrene + OH system (Table 2S), relative energetics of the reaction of styrene + OH system (Table 3S), and the calculated vibrational frequencies of the species in the reaction of styrene + OH system (Table 4S). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. Tel.: (657)278-3585. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported in part by California State University Fullerton (CSUF) Senior Faculty Research Grant. REFERENCES

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