Kinetics of the Gas-Phase Reactions of Chlorine Atoms with

Apr 18, 2014 - Gas- and Particle-Phase Products from the Chlorine-Initiated Oxidation of Polycyclic Aromatic Hydrocarbons. Matthieu Riva , Robert M. H...
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Kinetics of the Gas-Phase Reactions of Chlorine Atoms with Naphthalene, Acenaphthene, and Acenaphthylene Matthieu Riva,†,‡ Robert M. Healy,§ Pierre-Marie Flaud,†,‡ Emilie Perraudin,†,‡ John C. Wenger,*,§ and Eric Villenave*,†,‡ †

Univ. Bordeaux, EPOC, UMR 5805, F-33405 Talence Cedex, France CNRS, EPOC, UMR 5805, F-33405 Talence Cedex, France § Department of Chemistry and Environmental Research Institute, University College Cork, Cork, Ireland ‡

ABSTRACT: Reactions of polycyclic aromatic hydrocarbons (PAHs) with chlorine atoms may occur in specific areas such as coastal regions and the marine boundary layer. In this work, rate constants for the gas-phase reactions of naphthalene, acenaphthene, and acenaphthylene with chlorine atoms have been measured using the relative rate technique. Experiments were performed at room temperature (293 ± 2 K) and atmospheric pressure in an atmospheric simulation chamber using a proton-transfer reaction mass spectrometer (PTR-MS) to monitor the concentrations of PAHs and the reference compounds (acetone, methanol, 1,3,5-trimethylbenzene, and isoprene) as a function of time. The rate constants obtained in this work were (in units of cm3 molecule−1 s−1) (4.22 ± 0.46) × 10−12, (3.01 ± 0.82) × 10−10, and (4.69 ± 0.82) × 10−10 for naphthalene, acenaphthene, and acenaphthylene, respectively. These are the first measurements of the rate constants for gas-phase reactions of Cl atoms with acenaphthene and acenaphthylene. The rate constant determined in this study for the reaction of naphthalene with Cl atoms is not in agreement with the only other previously reported value in the literature. The results are used to assess the potential role of chlorine atom reactions in the atmospheric oxidation of PAHs.



has also been observed in midcontinental areas.17 Several studies have demonstrated the potential of Cl atoms to be a major oxidant in the marine boundary layer and coastal and industrialized areas.20,21 For instance, Pszenny et al.21 studied the reactivity of selected nonmethane hydrocarbons (NMHCs) to evaluate the significance of Cl atom chemistry on hydrocarbon degradation in coastal areas, using 2.5 × 106 molecules cm−3 as the diurnally averaged concentration of OH radicals and a few 104 molecules cm−3 as the Cl atom concentration. Their results suggest that Cl atoms increase the NMHC degradation rate by 16−30% over that due to OH radical reaction with various transport scenarios. Moreover, the reactivity of Cl atoms with organic compounds may also lead to the formation of secondary organic aerosols and impact on tropospheric ozone production.20,22−24 Most of the Cl oxidation pathways occur via H abstraction and can lead to ozone production, especially in the presence of sufficient amounts of nitrogen oxides (NOx).25 Previous studies of the atmospheric oxidation of PAHs in the gas phase have mainly focused on reactions with OH, NO3, and O3.7−12 In the only previous study of Cl atom reactions with PAHs reported in the literature, Wang et al.26 determined rate constants for naphthalene and a series of alkylnaphthalenes. It

INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental contaminants produced from combustion processes such as the burning of wood and fossil fuels.1,2 Some PAHs are classified by the International Agency for Research on Cancer as carcinogenic and/or mutagenic compounds.3,4 Low molecular weight PAHs such as naphthalene, acenaphthene, and acenaphthylene are among the most abundant PAHs in urban environments.5,6 They are mainly present in the gas phase and can thus be removed from the troposphere by gas-phase reactions with the major atmospheric oxidants, hydroxyl radicals (OH), ozone (O3), and nitrate radicals (NO3).7−11 Chemical reactions with these atmospheric oxidants can produce nitro-PAH, hydroxylated-PAH, and quinones whose toxicity is increased relative to those of the parent compounds.3,12 A considerable fraction of photooxidation products from light PAHs is also able to form secondary organic aerosol.13−15 Chlorine (Cl) atoms are another possible atmospheric oxidant for PAHs. It has been suggested that Cl atoms may be produced from photolysis of molecular chlorine (Cl2) generated by sea salt aerosol reactions.16 The OH-initiated oxidation of hydrochloric acid (HCl) and photolysis of nitryl chloride (ClNO2) are other potentially important sources of Cl atoms.17,18 Indeed, Riedel et al.19 have recently demonstrated that HCl and ClNO2 are dominant primary Cl atom sources for the Los Angeles basin, while significant production of ClNO2 © 2014 American Chemical Society

Received: January 27, 2014 Revised: April 17, 2014 Published: April 18, 2014 3535

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added to ensure that the relative loss of these reference compounds was similar. The reactions of acenaphthene and acenaphthylene with Cl atoms were performed together in the same experiments using isoprene and 1,3,5-trimethylbenzene (1,3,5-TMB) as reference compounds with initial concentrations as follows: [acenaphthene] = 8.5 × 1012, [acenaphthylene] = 8.5 × 1012, [1,3,5-TMB] = 8.8 × 1012, [isoprene] = 7.7 × 1012, and [oxalyl chloride] = 2.7 × 1014. The reactions were initiated by turning on all of the lamps for 20−30 min. Three replicate experiments were performed for each of the naphthalene and acenaphthene/acenaphthylene mixtures. Parent hydrocarbons and reference compounds were monitored in the gas phase during each experiment using a proton-transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS, Kore Technology Ltd.), directly connected to the reaction chamber via a Teflon tube heated to around 5− 10 °C above the temperature of the chamber to prevent any condensation. Details of the instrument are given in Cappelin et al.32 In brief, H3O+ reacts with analyte organic compounds (M) to form the positively charged ions MH+, which are detected using a time-of-flight mass spectrometer. Sampling frequency during the experiments was fixed at 1 min, and the operating range of the mass spectrometer was m/z 0−300. The decay of the PAHs and reference compounds was measured by monitoring their protonated molecular ions (MH+): m/z 129 (naphthalene), m/z 153 (acenaphthylene), m/z 155 (acenaphthene), m/z 33 (methanol), m/z 59 (acetone), m/z 69 (isoprene), and m/z 121 (1,3,5-TMB). The compounds used in this study were naphthalene (Fluka, 99.0%), acenaphthene (Aldrich, 99.0%), acenaphthylene (Aldrich, 99.0%), oxalyl chloride (Aldrich, 98%), benzene (Fluka, 99%), isoprene (Aldrich, 99%), 1,3,5-TMB (Aldrich, 99%), methanol (Aldrich, 99.9%), and acetone (Aldrich, 99.8%).

was also shown that H atom abstraction is the dominant reaction pathway for alkylnaphthalenes. In this work, a relative rate technique has been used to determine rate constants for the reaction of Cl atoms with naphthalene, acenaphthene, and acenaphthylene at room temperature (293 ± 2 K) and atmospheric pressure of air. The results are used to assess the relative importance of Cl atom chemistry to the atmospheric degradation of these light PAHs.



EXPERIMENTAL METHODS Experiments were performed in a 3910 L atmospheric simulation chamber filled to atmospheric pressure with purified air and at room temperature (293 ± 2 K). The chamber has been described in detail elsewhere.27 Briefly, it is a cylinder consisting of a FEP Teflon foil tube closed with aluminum plates, which are also covered with FEP Teflon foil. The chamber is surrounded by 16 lamps (Philips TL12, 40 W) with an emission maximum at 310 nm and 12 lamps (Philips TL05, 40 W) with an emission maximum at 360 nm. The chamber is also equipped with fans to ensure rapid mixing of reactants. Cl atoms were generated by the photolysis of oxalyl chloride (C2O2Cl2).28,29 C2O2 Cl 2 + hν → 2Cl + 2CO

(1)

PAH + Cl → products

(2)

reference + Cl → products

(3)

Rate constants for the reactions of Cl atoms with the PAHs were determined using the well-established relative rate technique whereby the relative decay rates of the PAHs and selected reference compounds were measured in the presence of Cl atoms. Providing that reaction with Cl atoms was the only loss process for the PAHs and reference compounds, then ln



[PAH]0 [reference]0 k = 2 ln [PAH]t k3 [reference]t

RESULTS AND DISCUSSION Validation of the Experimental Method. Validation of the experimental method was performed by comparing the relative reactivity of the two reference compounds measured during the experiments with that expected from the rate constants reported in the literature. A relative rate plot for the reactions of Cl with acetone and methanol, the two reference compounds used in the naphthalene experiments, is shown in Figure 1. Using the slope of the line of best fit through the data points and the recommended30 rate constant of k(Cl + methanol) = 5.50 × 10−11 cm3 molecule−1 s−1 yields a value of k(Cl + acetone) = (2.59 ± 0.33) × 10−12 cm3 molecule−1 s−1, which is slightly higher than the recommended30 value of 2.10 × 10−12 cm3 molecule−1 s−1 but in the middle of the range of values previously reported for this rate constant in the literature.31 The corresponding relative rate plot (not shown) for 1,3,5TMB and isoprene, the two reference compounds used in the acenaphthene/acenaphthylene experiments, was used to determine a value of k(Cl + 1,3,5-TMB) = (1.94 ± 0.13) × 10−10 cm3 molecule−1 s−1, which is in reasonable agreement with the only previously reported value of Wang et al.26 A summary of the intercomparison between measurements and literature is provided in Table 1 and confirms that the experimental approach employed in this work is satisfactory for determination of relative rate constants for the reactions of organic compounds with Cl atoms.

where [PAH]0 and [reference]0 are the initial concentrations of PAH and the reference compound, respectively. [PAH]t and [reference]t are the corresponding concentrations at time t, and k2 and k3 are the rate constants for reactions 2 and 3, respectively. Plots of ln([PAH]0/[PAH]t) versus ln([reference]0/[reference]t) should be linear and pass through the origin. The slope of the plot gives the ratio k2/k3, and provided k3 is well-known, the value of the desired rate constant, k2, can be determined. Routine tests were performed to measure possible loss of the PAHs and reference compounds due to photolysis and deposition on the chamber walls. The results showed that the reference compounds and PAHs did not undergo wall loss or photolysis over the relatively short time scale of these experiments. PAHs and reference compounds were introduced into the chamber by passing dry purified air over a heated Pyrex glass bulb containing a known amount of the liquid or solid compound. For the experiments involving naphthalene, methanol and acetone were used as the reference compounds, and benzene was added as a OH radical scavenger.26 The initial concentrations of reactants during the naphthalene experiments were (in units of molecules cm−3): [naphthalene] = 7.0 × 1012, [acetone] = 4.2 × 1013, [methanol] = 7.4 × 1012, [benzene] = 7.7 × 1014, and [oxalyl chloride] = 2.7 × 1014. Note that, because acetone is less reactive than methanol,30,31 more was 3536

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Figure 1. Relative rate plot for the reaction of Cl atoms with acetone using methanol as the reference compound (open triangles; circles and squares represent data points obtained from three different experiments).

Figure 2. Relative rate plots for the reaction of Cl atoms with naphthalene using methanol as the reference compound and benzene as the OH radical scavenger (triangles; circles and squares represent data points obtained from three different experiments). Open symbols (in black) correspond to directly measured values, while full symbols (in red) correspond to the corrected values that represent loss of naphthalene only due to reaction with Cl atoms.

Determination of the Rate Constant for Naphthalene + Cl. The kinetic plots for the reaction of naphthalene with Cl using methanol and acetone as the reference compounds are shown in Figures 2 and 3, respectively. The plots each contain data from three experiments and are linear with near-zero intercepts, as expected. The rate constant ratios obtained were 0.12 ± 0.01 and 2.80 ± 0.26 using methanol and acetone as reference compounds, respectively, where the errors are twice the standard deviation arising from linear regression analysis. Using recommended values for the reference rate constants,30 the resulting “observed” rate constants (in units of 10−12 cm3 molecule−1 s−1) for reaction of naphthalene with Cl atoms are k2 observed = (6.60 ± 0.55) and (5.90 ± 0.54) using methanol and acetone as reference compounds, respectively. Although these values are in good agreement, they are both significantly higher than the only previously published value of (9.1 ± 0.3) × 10−13 cm3 molecule−1 s−1 by Wang et al.,26 who also utilized the relative rate method but with GC-FID as the measurement technique and dichloromethane as the reference compound. Wang et al.26 noted that appreciable levels of OH radicals were produced in this experimental system and therefore added an excess of benzene (which preferentially reacts with OH rather than Cl) as a scavenger for the OH radicals. Although it was conceded that OH radicals might still be contributing to the loss of naphthalene, Wang et al.26 did not attempt to correct for this possibility and simply noted that their measured rate constant should be considered as an upper limit. In this work, benzene was also added as a OH radical scavenger using the same benzene/naphthalene ratio of 100:1 employed by Wang et al.26 However, kinetic analysis indicates that under these conditions, benzene reacts with around 84% of

Figure 3. Relative rate plot for the reaction of Cl atoms with naphthalene using acetone as the reference compound and benzene as the OH radical scavenger (triangles; circles and squares represent data points obtained from three different experiments). Open symbols (in black) correspond to directly measured values, while full symbols (in red) correspond to the corrected values that represent loss of naphthalene only due to reaction with Cl atoms.

the OH radicals produced, naphthalene reacts with 15%, while methanol and acetone account for less than 1%. A small decay

Table 1. Comparison of Rate Constants Determined for the Gas Phase Reactions of Cl Atoms with Acetone and 1,3,5-TMB with Those Reported in the Literature compound acetone 1,3,5-TMB

reference methanol isoprene

slopea

k(experiment)b,c

0.047 ± 0.006 0.45 ± 0.03

k(literature)d −12

(2.59 ± 0.33) × 10 (1.94 ± 0.13) × 10−10

2.10 × 10−12 2.42 × 10−10

a Indicated errors are two least-squares standard deviations. bAt 293 ± 2 K, in units of cm3 molecule−1 s−1. cUsing reference rate constants k(Cl + methanol) = 5.50 × 10−11 cm3 molecule−1 s−1 at 298 K30 and k(Cl + isoprene) = 4.30 ± × 10−10 cm3 molecule−1 s−1 at 298 K.32 dIn units of cm3 molecule−1 s−1; literature rate constants for acetone and 1,3,5-TMB were obtained from refs 30 and 33, respectively.

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of benzene was observed during experiments performed in this work, and the measured loss was used in conjunction with recommended rate constants for reaction of OH with benzene and naphthalene12 to calculate the proportion of naphthalene loss due to reaction with OH. Each experimental data point for naphthalene was corrected for the calculated OH reactivity, thus providing “corrected” plots, which represent the loss of naphthalene only due to its reaction with Cl atoms. These corrected plots of ln([naphthalene]0/[naphthalene]t) versus ln([reference]0/[reference]t) are shown in Figures 2 and 3. The slopes of these plots, k2/k3, are provided in Table 2, along with the determined values of k2 for naphthalene. The indicated errors in k2 do not include the uncertainty in the reference rate constants (k3), which may add up to a further 20% to the error reported in k2. Table 2. Rate Constants Determined at 293 ± 2 K for the Reaction of Naphthalene with Cl Atoms Using Methanol and Acetone As Reference Compounds in the Presence of Benzene as a OH Radical Scavenger k2/k3a 1.82 ± 0.04 0.084 ± 0.030

reference acetone methanol

k3(reference)b,c −12

2.10 × 10 5.50 × 10−11

Figure 4. Relative rate plots for the reaction of Cl atoms with acenaphthylene (red markers) and acenaphthene (green markers), using 1,3,5-TMB as the reference compound (open triangles; circles and squares represent data points obtained from three different experiments).

k2(naphthalene)b (3.82 ± 0.09) × 10−12 (4.62 ± 0.17) × 10−12

a

Corrected for loss of naphthalene due to reaction with OH radicals. Indicated errors are two least-squares standard deviations. bIn units of cm3 molecule−1 s−1. cReference rate constants for methanol and acetone obtained from ref 30.

The average value obtained in this work, k2(naphthalene) = (4.22 ± 0.46) × 10−12 cm3 molecule−1 s−1, is still higher than that reported by Wang et al.26 Although the two rate constants were obtained using the relative rate method under similar reaction conditions, some experimental differences exist and may, at least in part, account for the discrepancy. First, different analytical techniques were employed to measure substrate and reference compounds, and the possibility of undetected interference from reaction products in either case cannot be ruled out. Second, different Cl atom precursors were used in the experiments; oxalyl chloride was employed in this work, while Wang et al.26 used Cl2. Finally, different reference compounds were employed; Wang et al.26 used dichloromethane as the reference compound, but a direct comparison could not be made in the present study because this halogenated hydrocarbon is not detected by the PTR-TOFMS technique. Nevertheless, two reference compounds were used in the current work, and the resulting values of k2(naphthalene) are in fairly good agreement with each other. Determination of the Rate Constants for Acenaphthene + Cl and Acenaphthylene + Cl. Experimental data obtained for the reactions of acenaphthene and acenaphthylene with Cl atoms using 1,3,5-TMB and isoprene as reference compounds are presented in Figures 4 and 5, respectively. The relative rate plots represent results from three experiments and exhibit good linearity with near-zero intercepts. Furthermore, careful analysis of the experimental data showed that there was no evidence to indicate that OH radicals were contributing to the loss of the PAHs or reference compounds in this set of experiments. Indeed, separate experiments of acenaphthylene and acenaphthene oxidation initiated by both Cl atoms and OH radicals were also carried out to determine oxidation products and investigate secondary organic aerosol formation.34 The major oxidation products

Figure 5. Relative rate plots for the reaction of Cl atoms with acenaphthylene (red markers) and acenaphthene (green markers) using isoprene as the reference compound (open triangles; circles and squares represent data points obtained from three different experiments).

identified for the Cl reactions were very different than those generated in the OH radical system. None of the products of the OH-initiated oxidation of the PAHs were detected in these kinetic experiments, leading to the conclusion that the influence of OH radicals on the observed decay of acenaphthylene and acenaphthene was negligible. The rate constant ratios k2/k3 were obtained by least-squares analysis of the data and are presented in Table 3, along with the calculated rate constants, k2, for reaction of acenaphthene and acenaphthylene with Cl atoms. The rate constants obtained using the two reference compounds are in fairly good agreement and yield average values of (3.01 ± 0.82) × 10−10 and (4.69 ± 0.82) × 10−10 cm3 molecule−1 s−1 for the reactions of Cl atoms with acenaphthene and acenaphthylene, respectively. There are, to date, no published literature data on the kinetics of Cl reactions with acenaphthene and acenaphthylene with which to compare our data. However, as indicated below, the rate constants are in line 3538

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Table 3. Rate Constants Determined at 293 ± 2 K for the Reactions of Acenaphthene and Acenaphthylene with Cl Atoms Using Isoprene and 1,3,5-TMB as Reference Compounds k2/k3a

PAH acenaphthene

0.61 1.40 0.96 2.17

acenaphthylene

± ± ± ±

k3(reference)b,c

reference

0.10 0.34 0.08 0.34

isoprene 1,3,5-TMB isoprene 1,3,5-TMB

4.30 2.42 4.30 2.42

× × × ×

−10

10 10−10 10−10 10−10

k2(PAH)b (2.62 (3.39 (4.13 (5.25

± ± ± ±

0.43) 0.82) 0.34) 0.82)

× × × ×

10−10 10−10 10−10 10−10

Indicated errors are two least-squares standard deviations. bIn units of cm3 molecule−1 s−1. cReference rate constants for 1,3,5-TMB and isoprene obtained from ref 26 and 33, respectively.

a

Table 4. Rate Coefficientsa for the Gas-Phase Reaction of Naphthalene, Acenaphthene, and Acenaphthylene with OH, Cl, O3, and NO3 and the Corresponding Tropospheric Lifetimes Calculated Using [OH]12h = 2 × 106 molecules cm−3, [O3]24h = 7 × 1011 molecules cm−3, [Cl] = 1 × 104 molecules cm−3, and [NO3] = 5 × 108 molecules cm−3 PAH naphthalene acenaphthylene acenaphthene a

kOH (× 10−11) 12

2.3 11.011 9.910

lifetime (OH) 6.0 h 1.3 h 1.4 h

kCl (× 10−10) 0.042 4.69b 3.01b

kO3 (× 10−16)

lifetime (Cl)

b

−3 12

2.0 × 10 , 3.9911 1.8 × 10−3,10

275 d 2.4 d 3.8 d

lifetime (O3)

kNO3 (× 10−12)

lifetime (NO3)

82 d 1.0 h 92 d

8.6 × 10−5,12 4.4210 0.4210

270 d 7.5 min 1.3 h

At room temperature, in units of cm3 molecule−1 s−1. bThis work.

Atmospheric Implications. Because photolysis by sunlight is not important for unsubstituted PAHs,12 the atmospheric degradation of acenaphthylene, acenaphthene, and naphthalene is governed by gas-phase reaction with the different atmospheric oxidants (OH, O3, NO3, and Cl). The tropospheric lifetimes with respect to reaction with each of these oxidants may be calculated using the measured rate constants and the following representative concentrations; 12 h average daytime OH radical concentration of 2 × 106 molecules cm−3, 24 h average O3 concentration of 7 × 1011 molecules cm−3, a night-time average NO3 concentration of 5 × 108 molecules cm−3, and an average global Cl concentration of 1 × 104 molecules cm−3 are used.35 The estimated lifetimes are presented in Table 4 and show that Cl atom reactions are of little importance in the atmospheric removal of these PAHs at the global level. However, in marine and coastal regions, the concentration of Cl atoms may be as high as 4 × 105 molecules cm−3.37 The corresponding tropospheric lifetimes with respect to reaction with Cl atoms would therefore be around 6.9 days and 2.3 and 1.4 h for naphthalene, acenaphthene, and acenaphthylene, respectively. Thus, under these conditions, gas-phase reaction with Cl atoms could be an important atmospheric degradation pathway for both acenaphthylene and acenaphthene. Clearly, more work is needed to measure ambient Cl atom concentrations in various environments in order to provide a better assessment of the potential role of this oxidant in the removal of PAHs from the atmosphere.

with expectations based on the observed reactivity of Cl atoms toward PAHs and other reactive organic compounds.21,26 Reactivity Patterns in Cl + PAH Reactions. The experimental data clearly show that the order of reactivity for the PAHs with Cl atoms is acenaphthylene > acenaphthene ≫ naphthalene. The rate constants for acenaphthylene and acenaphthene are around 2 orders of magnitude greater than that for naphthalene, indicating that the cyclopenta-fused ring is by far the most reactive site in these compounds. Furthermore, the rate constant determined for Cl with acenaphthene is very similar to those reported for a series of dimethylnaphthalenes by Wang et al.,26 where the dominant reaction pathway was found to be H atom abstraction from the alkyl substituents. Thus, it is expected that H atom abstraction from the cyclopenta-fused ring is also the principal pathway for reaction of acenaphthene with Cl atoms. The slightly faster reaction rate observed for Cl + acenaphthylene can be attributed to the existence of a reaction pathway involving Cl addition to the unsaturated bond in the cyclopenta-fused ring. As reported for the dimethylnaphthalenes,26 reaction of Cl atoms with the aromatic ring in acenaphthylene and acenaphthene is deemed to be of little importance. Rate constants for reaction of the PAHs with other atmospheric oxidants are compared in Table 4. The order of reactivity with OH and NO3 radicals is the same as that with Cl atoms (acenaphthylene > acenaphthene ≫ naphthalene), suggesting that similarities exist in the overall reaction mechanisms. However, there are also two aspects worth pointing out. First, the rate constants for Cl atom reactions with acenaphthylene and acenaphthene are higher than the corresponding values for OH and NO3 reactions. This finding is consistent with the large body of kinetic data now available for the gas-phase reactions of organic compounds,30,35 where H atom abstraction reactions and oxidant addition to a >CC< bond are usually fastest for Cl and slowest for NO3. Second, naphthalene is different from the other two PAHs because it exhibits greatest reactivity toward OH radicals. Because this reaction is known to occur mainly via OH addition to the aromatic ring,12 this suggests that addition of Cl and NO3 to the aromatic ring is considerably less favorable. In fact, a similar observation has already been reported for benzene.36



AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS The experiments performed at University College Cork were supported by the European Commission through the research project EUROCHAMP 2 (Contract Number 228355). The authors wish to thank the French Agency for Environment and Energy Management (ADEME) and the Aquitaine Region for their financial support. 3539

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dx.doi.org/10.1021/jp5009434 | J. Phys. Chem. A 2014, 118, 3535−3540