Effect of Structure on the Rate Constants for Reaction of NO3 Radicals

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Effect of Structure on the Rate Constants for Reaction of NO3 Radicals with a Series of Linear and Branched C5-C7 1-Alkenes at 296 ( 2 K Sara M. Aschmann† and Roger Atkinson*,†,‡ †

Air Pollution Research Center and ‡Department of Environmental Sciences and Department of Chemistry, University of California, Riverside, California 92521, United States

bS Supporting Information ABSTRACT: Rate constants for the gas-phase reactions of NO3 radicals with 13 linear and branched C5-C7 1-alkenes, CH2dCHR, where R = alkyl, have been measured at 296 ( 2 K and atmospheric pressure of air by a relative rate method. 1-Butene was used as the reference compound, and the rate constants obtained (in units of 10-14 cm3 molecule-1 s-1) were as follows: 1-pentene, 1.50 ( 0.07; 1-hexene, 1.80 ( 0.08; 1-heptene, 2.06 ( 0.14; 3-methyl-1-butene, 1.39 ( 0.04; 3-methyl-1-pentene, 1.39 ( 0.03; 4-methyl-1-pentene, 1.52 ( 0.04; 3,3-dimethyl-1-butene, 1.54 ( 0.03; 3-methyl1-hexene, 1.65 ( 0.08; 4-methyl-1-hexene, 1.86 ( 0.08; 5-methyl-1-hexene, 2.14 ( 0.08; 3,3-dimethyl-1-pentene, 1.44 ( 0.07; 3,4dimethyl-1-pentene, 1.49 ( 0.10; and 4,4-dimethyl-1-pentene, 1.37 ( 0.06; where the indicated errors are two least-squares standard deviations and do not include uncertainties in the rate constant for the reference compound 1-butene. These rate constants increase along the series 1-butene < 1-pentene < 1-hexene < 1-heptene, and this is attributed to inductive effects. For a given carbon number, the rate constants depend on the position and degree of branching, and the observed trend of measured rate constants with position and degree of branching in the alkyl substituent group R correlates well with steric hindrance as calculated by McGillen et al. (Phys. Chem. Chem. Phys. 2008, 10, 1757).

’ INTRODUCTION Alkenes are the dominant class of volatile organic compound (VOC) emitted into the atmosphere from vegetation,1 and they comprise ∼10% of non-methane VOCs in urban atmospheres dominated by anthropogenic sources.2 In the atmosphere, alkenes react with OH radicals, NO3 radicals, O3, and Cl atoms,2-4 with the dominant loss process depending on time and location.2 These electrophilic reactions proceed mainly (OH and NO3 radicals and Cl atoms) or totally (O3) by initial addition to the CdC bond(s) of the alkene.2-4 Studies of the reactions of OH and NO3 radicals and O3 with alkenes show that the kinetics of these reactions are influenced by inductive and steric effects.2,5-7 The effect of inductive effects is evident from the increase in the rate constants for the reactions of alkenes with OH and NO3 radicals and O3 with the number of alkyl substituents around the CdC bond,2-7 while steric effects lead to a decrease in the rate constant over that predicted in the absence of steric effects.6,7 Recent kinetic studies of the reactions of OH and NO3 radicals with a series of 1-alkenes [CH2dCHR] and 2-methyl1-alkenes [CH2dC(CH3)R], where R = (CH2)nCH3, show that the rate constants initially increase with the carbon number of the substituent group, n, and then attain a plateau at n g 7.8-10 In contrast, the room-temperature rate constants for the reactions of O3 with the same series of 1-alkenes and 2-methyl-1-alkenes show little effect of substituent group carbon number n on the rate constants,10 and in this case the increasing inductive effect of longer alkyl substituent group chain length is offset by steric effects due to the longer substituent group.5,7 Steric effects and, for cyclic alkenes, ring r 2011 American Chemical Society

strain effects appear to be much more pronounced in the O3 reactions than in the corresponding OH and NO3 radical reactions.6,7,10-12 Recently, McGillen et al.7 have proposed and tested a structure -activity relationship (SAR) involving inductive and steric effects to successfully predict rate constants for the reactions of O3 with aliphatic alkenes and dienes, with the steric effects being calculated by use of geometric factors.7 This SAR successfully predicted the rate constants for 48 alkenes and polyenes, with rate constants ranging over 3 orders of magnitude, to within a factor of 2.3.7 To explore the interplay of inductive and steric effects in other alkene reactions, in this work we have measured rate constants at 296 ( 2 K for the reactions of NO3 radicals with 13 linear and branched C5-C7 1-alkenes. The NO3 þ 1-alkene reactions were chosen because H-atom abstraction by NO3 from the alkyl substituent group(s) in alkenes is minor or negligible10 (in contrast to the corresponding OH radical reactions13), and the linear 1-alkene reactions with NO3 radicals showed the largest effect of substituent group length (i.e., of n) on the rate constant (a factor of 3 from n = 1 to n = 13).10

’ EXPERIMENTAL METHODS All experiments were carried out in ∼7000 L Teflon chambers equipped with a Teflon-coated fan to ensure rapid mixing of Received: November 19, 2010 Revised: January 5, 2011 Published: February 9, 2011 1358

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reactants during their introduction into the chamber. Rate constants for the reactions of NO3 radicals with the 1-alkenes were obtained by a relative rate technique,10 in which the relative disappearance rates of the 1-alkenes and a reference compound (whose NO3 radical reaction rate constant is reliably known) were measured in the presence of NO3 radicals. Provided that the 1-alkenes and the reference compound were removed only by reaction with NO3 radical, then ! " ½1-alkenet0 k1 ln -Dt ¼ ln ½1-alkenet k2

! # ½reference compoundt0 -Dt ½reference compoundt

ðIÞ where [1-alkene]t0 and [reference compound]t0 are the initial concentrations of the 1-alkene and reference compound at time t0, respectively; [1-alkene]t and [reference compound]t are the corresponding concentrations at time t; Dt is the factor that accounts for the small amount of dilution caused by the additions of N2O5 to the chamber (Dt = 0.0026 per N2O5 addition to the chamber); and k1 and k2 are the rate constants for reactions 1 and 2, respectively: ð1Þ NO3 þ1-alkene f products NO3 þreference compound f products

ð2Þ

Hence plots of {ln ([1-alkene]t0/[1-alkene]t) - Dt} against {ln ([reference compound]t0/[reference compound]t)-Dt} should be straight lines with zero intercept and slopes of k1/k2. NO3 radicals were generated from the thermal decomposition of N2O5,14 and NO2 was also included in the reactant mixtures.10 1-Butene was used as the reference compound because its rate constant for reaction with NO3 radicals is reliably known3,4 and it could be analyzed by the same sampling and analysis procedure as for the other 1-alkenes studied. The initial reactant concentrations were: 1-alkene and 1-butene, ∼2.4  1013 molecules cm-3 each, and NO2, (0.48-1.4)  1014 molecules cm-3. Three or, in one experiment, four additions of N2O5 [each addition corresponding to (1.6-4.7)  1013 molecules cm-3 of N2O5 in the chamber] were made to the chamber during each experiment. The concentrations of the 1-alkenes (including 1-butene) were measured during the experiments by gas chromatography with flame ionization detection (GC-FID). Gas samples were collected from the chamber into 100 cm3 volume all-glass gastight syringes and transferred via a 1 cm3 gas sampling loop onto a 30 m DB-5 megabore column initially held at -25 °C and then temperature-programmed to 200 °C at 8 °C min-1. Replicate prereaction analyses in the chamber in the dark (a total of 65 sets of replicate analyses; 21 sets for 1-butene and 3 or 4 sets for each other 1-alkene studied) showed agreement for a given 1-alkene in a given experiment ranging from 0.1% to 4.4% and with an average difference of 1.2% (0.9% for 1-butene). The analytical uncertainties tended to increase as the carbon number increased and decrease with the degree of branching, and were hence highest for 1-heptene and the methyl-1-hexenes. In most experiments, two alkenes (of different carbon numbers to avoid any interference in the GC analyses) were present in addition to 1-butene (see Table S1 in the Supporting Information). Two experiments were also carried out with 3,3-, 3,4-, and 4,4dimethyl-1-pentene present (but no 1-butene), in order to directly measure the relative reactivities of these three dimethyl-1-pentenes. The initial concentrations and procedures were as described above. In addition, reactions of 3-methyl-1-butene and

Figure 1. Plots of eq I for the reactions of NO3 radicals with 4,4dimethyl-1-pentene (44DM1P), 3-methyl-1-butene (3M1B), 1-pentene, 1-hexene, and 1-heptene, with 1-butene as the reference compound. For each 1-alkene, the data from three experiments (see Table S1, Supporting Information) are combined. The data for 3M1B, 1-pentene, 1-hexene, and 1-heptene have been displaced vertically by 0.10, 0.20, 0.30, and 0.40 unit, respectively, for clarity.

Figure 2. Plots of eq I for the reactions of NO3 radicals with 3-methyl-1pentene, 3,3-dimethyl-1-pentene (33DM1P), 3,3-dimethyl-1-butene (33DM1B), and 4-methyl-1-hexene, with 1-butene as the reference compound. For each 1-alkene, the data from three experiments (see Table S1, Supporting Information) are combined. The data for 33DM1P, 33DM1B, and 4-methyl-1-hexene have been displaced vertically by 0.10, 0.20, and 0.30 unit, respectively, for clarity.

3-methyl-1-pentene with NO3 radials were carried out with GCFID analyses before and after reaction, using sample collection onto Tenax solid adsorbent10 to investigate formation of methyl vinyl ketone, a possible product of any H-atom abstraction reaction. The chemical used, and their stated purities, were as follows: 3-methyl-1-butene (95%), 1-pentene (99%), 3-methyl-1-pentene (99%), 4-methyl-1-pentene (98%), 4,4-dimethyl-1-pentene (99%), 1-hexene (99%), 1-heptene (g99%), and methyl vinyl ketone (99%), Aldrich; 3,3-dimethyl-1-butene (99.6%), 3,3dimethyl-1-pentene (99%), 3,4-dimethyl-1-pentene (99%), 3-methyl-1-hexene (95%), 4-methyl-1-hexene (99%), and 1359

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Table 1. Rate Constant Ratios k1/k2 and Rate Constants k1 for the Reactions of NO3 Radicals with a Series of C5-C7 1-Alkenes at 296 ( 2 K 1014  k1 (cm3 molecule-1 s-1) 1-alkene 1-pentene

Figure 3. Plots of eq I for the reactions of NO3 radicals with 3,4dimethyl-1-pentene (34DM1P), 4-methyl-1-pentene, 3-methyl-1-hexene (3M1H) and 5-methyl-1-hexene, with 1-butene as the reference compound. For each 1-alkene, the data from three experiments (see Table S1, Supporting Information) are combined. The data for 4-methyl-1-pentene, 3M1H, and 5-methyl-1-hexene have been displaced vertically by 0.20, 0.40, and 0.50 unit, respectively, for clarity.

5-methyl-1-hexene (99%), ChemSampCo; and 1-butene (99%) and NO (g99.0%), Matheson Gas Products. N2O5 was synthesized as described previously14 and stored under vacuum at 77 K. NO2 was prepared as needed by reacting NO with an excess of O2.

’ RESULTS AND DISCUSSION The experiments carried out and the data obtained are listed in Table S1, Supporting Information. The data from the experiments with 1-butene present are plotted in accordance with eq I in Figures 1-3, and the rate constant ratios k1/k2 obtained from least-squares analyses of these plots are given in Table 1. These rate constant ratios are placed on an absolute basis by use of a rate constant of k2(1-butene) = 1.32  10-14 cm3 molecule-1 s-1 at 296 K,4 and the resulting rate constants k1 are also given in Table 1 together with the available literature data. As shown in Table 1, our rate constants for 1-pentene, 1-hexene, and 1-heptene are in excellent agreement with the relative rate measurements of Canosa-Mas et al.,15 who also used 1-butene as the reference compound, and our rate constant for 1-hexene is also in excellent agreement with that of Mason et al.,10 who used a relative rate method with thiophene as the reference compound (Table 1). In contrast, the absolute rate constants obtained by Martinez et al.16 using a discharge flow technique with laserinduced fluorescence detection of NO3 are factors of 4-5 higher than the rate constants measured in the present work and by Canosa-Mas et al.15 and Mason et al.10 (Table 1). As noted by Canosa-Mas et al.,15 it is possible that secondary reactions of NO3 radicals with reaction products were the cause of the higher rate constants reported by Martinez et al.16 As evident from Table 1, our rate constants for 3,3-, 3,4- and 4,4-dimethyl-1-pentene, obtained from experiments in which only one dimethyl-1-pentene was present in a given experiment (Table S1, Supporting Information), are indistinguishable within

k1/k2a

this workb

literature

1.14 ( 0.05

1.50 ( 0.07

3-methyl-1-butene

1.05 ( 0.03

1.39 ( 0.04

1-hexene

1.36 ( 0.06

1.80 ( 0.08

3-methyl-1-pentene

1.05 ( 0.02

1.39 ( 0.03

4-methyl-1-pentene

1.15 ( 0.03

1.52 ( 0.04

3,3-dimethyl-1-butene 1.17 ( 0.02

1.54 ( 0.03

1-heptene

1.56 ( 0.10

2.06 ( 0.14

3-methyl-1-hexene

1.25 ( 0.06

1.65 ( 0.08

4-methyl-1-hexene

1.41 ( 0.06

1.86 ( 0.08

5-methyl-1-hexene 1.62 ( 0.06 3,3-dimethyl-1-pentene 1.09 ( 0.05

2.14 ( 0.08 1.44 ( 0.07

3,4-dimethyl-1-pentene 1.13 ( 0.07

1.49 ( 0.10

4,4-dimethyl-1-pentene 1.04 ( 0.04

1.37 ( 0.06

ref c

1.48 ( 0.08

15

6.19 ( 0.38d

16

1.81 ( 0.11c

15

9.32 ( 0.71d 2.00 ( 0.16e

16 10

2.01 ( 0.15c

15

Relative to NO3 þ 1-butene. Indicated errors are two least-squares standard deviations. b Placed on an absolute basis by use of a rate constant of k2(1-butene) = 1.32  10-14 cm3 molecule-1 s-1 at 296 K.4 c At 296 ( 1 K, relative to 1-butene. The measured rate constant ratios of 1.12 ( 0.06, 1.37 ( 0.08, and 1.52 ( 0.11 for 1-pentene, 1-hexene and 1-heptene, respectively, are placed on an absolute basis by use of a rate constant of k2(1-butene) = 1.32  10-14 cm3 molecule-1 s-1 at 296 K.4 d Absolute rate measurement at 298 K. e At 296 ( 2 K, relative to thiophene. The measured rate constant ratio of 0.509 ( 0.040 is placed on an absolute basis by use of a rate constant of k2(thiophene) = 3.93  10-14 cm3 molecule-1 s-1.17 a

Figure 4. Plots of eq I for the reactions of NO3 radicals with 3,4dimethyl-1-pentene and 4,4-dimethyl-1-pentene, with 3,3-dimethyl-1pentene as the reference compound. Data were combined from two experiments (see Table S1, Supporting Information).

the two standard deviation measurement uncertainties. The data from two additional experiments in which all three dimethyl1-pentenes were present are plotted in accordance with eq I in Figure 4, and the directly measured rate constant ratios 1360

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Table 2. Rate Constant Ratios for the Reactions of NO3 Radicals with Dimethyl-1-pentenes at 296 ( 2 K k(dimethyl-1-pentene)/k(3,3-dimethyl-1-pentene) dimethyl-1-pentene

k1/k(1-butene)a

calculatedb

measured directlyc

3,3-dimethyl-1-pentene

1.09 ( 0.05

1.00

3,4-dimethyl-1-pentene

1.13 ( 0.07

1.04 ( 0.09

1.06 ( 0.02

4,4-dimethyl-1-pentene

1.04 ( 0.04

0.95 ( 0.06

0.933 ( 0.020

1.00

a

From Table 1. b Calculated from k(dimethyl-1-pentene)/k(3,3-dimethyl-1-pentene) = [k(dimethyl-1-pentene)/k(1-butene)]/[k(3,3-dimethyl-1pentene)/k(1-butene)], by use of the rate constant ratios from Table 1. c Determined from least-squares analyses of the plots of eq I shown in Figure 4. Indicated errors are two least-squares standard deviations.

k(dimethyl-1-pentene)/k(3,3-dimethyl-1-pentene) are listed and compared with the ratios derived from the individual values of k(dimethyl-1-pentene)/k(1-butene) in Table 2. The agreement is excellent, and from the direct measurements the ranking of rate constants is 3,4-dimethyl-1-pentene > 3,3-dimethyl-1pentene > 4,4-dimethyl-1-pentene. As shown previously by Mason et al.,10 the room-temperature rate constants for the reactions of NO3 radicals with a series of 1-alkenes of structure CH2dCH(CH2)nCH3 increase with n, with the rate constant initially increasing approximately linearly with n and then tending to an asymptotic value at n g 9. Measured room-temperature rate constants for the reactions of NO3 radicals with C4-C10 alkanes, which proceed only by H-atom abstraction from the various C-H bonds, are in the range 4.6  10-17 to 4.4  10-16 cm3 molecule-1 s-1,4 factors of 20-200 lower than the rate constants measured here for the 1-alkenes. Furthermore, Mason et al.10 reported, in their Supporting Information, that H-atom abstraction accounts for e6% of the overall NO3 radical reaction with 1-octene at room temperature. H-Atom abstraction from the 1-alkenes studied here should therefore be of no importance, noting that the 1-alkenes studied here most prone to H-atom abstraction would be expected to be those with allylic H-atoms: 3-methyl1-butene, 3-methyl-1-pentene, and 3-methyl-1-hexene. Methyl vinyl ketone [CH3C(O)CHdCH2] is an expected product of the NO3 radical-initiated reactions of 3-methyl-1-butene and 3-methyl-1-pentene, together with other products of H-atom abstraction including CH2dCHC(OH)(CH3)CH2R, CH2d CHC(OOH)(CH3)CH2R, CH2dCHC(OONO2)(CH3)CH2R, HOCH2CHdC(CH3)CH2R, HOOCH2CHdC(CH3)CH2R, HC(O)CHdC(CH3)CH2R, HOCH2 CHdC(CH3 )C(O)R, HOCH2CHdC(CH3)CH(OH)R, HOCH2CHdC(CH3)CH(OOH)R, and HOCH2CHdC(CH3)CH(OONO2)R, where R = H and CH3, respectively.3,4,17 However, we observed no formation of methyl vinyl ketone from the NO3 radical-initiated reactions of 3-methyl-1-butene or 3-methyl-1-pentene, with conservative upper limits to the methyl vinyl ketone formation yields from these reactions of CdC< bond from attack by NO3 radicals, then the observed steric factors are expected to be independent of temperature. Even if they were of the form steric factor = e-d/T, then for the magnitude of the steric effects observed in this work they would change relatively little for the temperature range encountered in the troposphere (for example, from a steric factor of 0.75 at 296 K to 0.65 at 200 K for ΔS = 0.05). High-precision kinetic data over a significant temperature range would then be needed to quantify any temperature dependence of the steric effects for the reactions studied here.

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(6) Johnson, D.; Rickard, A. R.; McGill, C. D.; Marston, G. Phys. Chem. Chem. Phys. 2000, 2, 323. (7) McGillen, M. R.; Carey, T. J.; Archibald, A. T.; Wenger, J. C.; Shallcross, D. E.; Percival, C. J. Phys. Chem. Chem. Phys. 2008, 10, 1757. (8) Aschmann, S. M.; Atkinson, R. Phys. Chem. Chem. Phys. 2008, 10, 4159. (9) Nishino, N.; Arey, J.; Atkinson, R. J. Phys. Chem. A 2009, 113, 852. (10) Mason, S. A.; Arey, J.; Atkinson, R. J. Phys. Chem. A 2009, 113, 5649–5656. (11) Atkinson, R.; Aschmann, S. M.; Carter, W. P. L.; Pitts, J. N., Jr. Int. J. Chem. Kinet. 1983, 15, 721. (12) Treacy, J.; Curley, M.; Wenger, J.; Sidebottom, H. J. Chem. Soc., Faraday Trans. 1997, 93, 2877. (13) Aschmann, S. M.; Arey, J.; Atkinson, R. J. Phys. Chem. A 2010, 114, 5810. (14) Atkinson, R.; Plum, C. N.; Carter, W. P. L.; Winer, A. M.; Pitts, J. N., Jr. J. Phys. Chem. 1984, 88, 1210. (15) Canosa-Mas, C. E.; King, M. D.; McDonnell, L.; Wayne, R. P. Phys. Chem. Chem. Phys. 1999, 1, 2681. (16) Martinez, E.; Caba~ nas, B.; Aranda, A.; Albaladejo, J.; Wayne, R. P. J. Chem. Soc., Faraday Trans. 1997, 93, 2043. (17) Atkinson, R. J. Phys. Chem. Ref. Data 1991, 20, 459. (18) Grosjean, D.; Williams, E. L., II. Atmos. Environ. 1992, 26A, 1395. (19) Aird, R. W. S.; Canosa-Mas, C. E.; Cook, D. J.; Marston, G.; Monks, P. S.; Wayne, R. P.; Ljungstr€ om, E. J. Chem. Soc., Faraday Trans. 1992, 88, 1093. (20) King, M. D.; Canosa-Mas, C. E.; Wayne, R. P. Phys. Chem. Chem. Phys. 1999, 1, 2231. (21) NIST Chemistry WebBook, NIST Standard Reference Database No. 69; http://webbook.nist.gov/chemistry/, 2010. (22) Masclet, P.; Grosjean, D.; Mouvier, G.; Dubois, J. J. Electron Spectrosc. Relat. Phenom. 1973, 2, 225. (23) Kerdouci, J.; Piquet-Varrault, B.; Doussin, J.-F. ChemPhysChem 2010, 11, 3909–3920.

’ ASSOCIATED CONTENT

bS

Supporting Information. One table listing the results of the kinetic experiments carried out. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Telephone: (951) 827-4191.

’ ACKNOWLEDGMENT We thank the National Science Foundation (Grant ATM0650061) for supporting this research. While this research has been supported by this agency, it has not been reviewed by the agency and no official endorsement should be inferred. ’ REFERENCES (1) Guenther, A; Hewitt, C. N.; Erickson, D.; Fall, R.; Geron, C.; Graedel, T.; Harley, P.; Klinger, L.; Lerdau, M.; McKay, W. A; Pierce, T.; Scholes, B.; Steinbrecher, R.; Tallamraju, R.; Taylor, J.; Zimmerman, P. J. Geophys. Res. 1995, 100, 8873. (2) Calvert, J. G.; Atkinson, R.; Kerr, J. A.; Madronich, S.; Moortgat, G. K.; Wallington, T. J.; Yarwood, G. The Mechanisms of Atmospheric Oxidation of the Alkenes; Oxford University Press: New York, 2000. (3) Atkinson, R. J. Phys. Chem. Ref. Data 1997, 26, 215. (4) Atkinson, R.; Arey, J. Chem. Rev. 2003, 103, 4605. (5) Treacy, J.; El Hag, M.; O’Farrell, D.; Sidebottom, H. Ber. BunsenGes. 1992, 96, 422. 1363

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