Kinetics and products of the reactions of OH radicals with cyclohexene, 1-methyl-1-cyclohexene, cis -cyclooctene, and cis -cyclodecene

Article abstract of DOI:10.1021/jp307217m

Rate constants for the reactions of OH radicals with four C ₆-C₁₀ cycloalkenes have been measured at 297 ± 2 K using a relative rate technique. The rate constants (in units of 10 ^-11 cm³ molecule^-1 s^-1) were cyclohexene, 6.35 ± 0.12; cis-cyclooctene, 5.16 ± 0.15; cis-cyclodecene, 4.18 ± 0.06; and 1-methyl-1-cyclohexene, 9.81 ± 0.18, where the indicated errors are two least-squares standard deviations and do not include uncertainties in the rate constant for the reference compound 1,3,5-trimethylbenzene. In addition, a rate constant of (4.8 ± 1.3) × 10^-11 cm³ molecule^-1 s^-1 was derived for the reaction of OH radicals with 1,6-hexanedial, relative to our measured rate constant for OH + cyclohexene. Analyses of products of the OH + cyclohexene, 1-methyl-1-cyclohexene, and cis-cyclooctene reactions by direct air sampling atmospheric pressure ionization mass spectrometry and/or by combined gas chromatography-mass spectrometry showed the presence of products attributed to cyclic 1,2-hydroxynitrates and the dicarbonyls 1,6-hexanedial, 6-oxo-heptanal, and 1,8-octanedial, respectively. These dicarbonyl products, which are those formed after decomposition of the intermediate cyclic 1,2-hydroxyalkoxy radicals, were quantified as their dioximes, with molar formation yields of 76 ± 10%, 82 ± 12%, and 84 ± 18% from the cyclohexene, 1-methyl-1-cyclohexene, and cis-cyclooctene reactions, respectively. Combined with literature data concerning 1,2-hydroxynitrate formation from OH + alkenes and the estimated fractions of the overall reactions proceeding by H-atom abstraction, 90 ± 12%, 95 ± 13% and 108 ± 20% of the products or reaction pathways from the OH radical-initiated reactions of cyclohexene, 1-methyl-1-cyclohexene, and cis-cyclooctene in the presence of NO are accounted for.

Full text of DOI:10.1021/jp307217m

Article
pubs.acs.org/JPCA
Kinetics and Products of the Reactions of OH Radicals with
Cyclohexene, 1‑Methyl-1-cyclohexene, cis-Cyclooctene, and cis-
Cyclodecene
Sara M. Aschmann,† Janet Arey,*,†,‡ and Roger Atkinson*,†
‡
†Air Pollution Research Center and Department of Environmental Sciences, University of California, Riverside, California 92521,
United States
S
* Supporting Information
ABSTRACT: Rate constants for the reactions of OH radicals
with four C6−C10 cycloalkenes have been measured at 297
2
K using a relative rate technique. The rate constants (in units
of 10−11 cm3 molecule−1 s−1) were cyclohexene, 6.35 0.12;
cis-cyclooctene, 5.16 0.15; cis-cyclodecene, 4.18 0.06; and
1-methyl-1-cyclohexene, 9.81
0.18, where the indicated
errors are two least-squares standard deviations and do not
include uncertainties in the rate constant for the reference
compound 1,3,5-trimethylbenzene. In addition, a rate constant
of (4.8 1.3) × 10−11 cm3 molecule−1 s−1 was derived for the
reaction of OH radicals with 1,6-hexanedial, relative to our
measured rate constant for OH + cyclohexene. Analyses of
products of the OH + cyclohexene, 1-methyl-1-cyclohexene,
and cis-cyclooctene reactions by direct air sampling atmospheric pressure ionization mass spectrometry and/or by combined gas
chromatography−mass spectrometry showed the presence of products attributed to cyclic 1,2-hydroxynitrates and the
dicarbonyls 1,6-hexanedial, 6-oxo-heptanal, and 1,8-octanedial, respectively. These dicarbonyl products, which are those formed
after decomposition of the intermediate cyclic 1,2-hydroxyalkoxy radicals, were quantified as their dioximes, with molar formation
yields of 76 10%, 82 12%, and 84 18% from the cyclohexene, 1-methyl-1-cyclohexene, and cis-cyclooctene reactions,
respectively. Combined with literature data concerning 1,2-hydroxynitrate formation from OH + alkenes and the estimated
fractions of the overall reactions proceeding by H-atom abstraction, 90 12%, 95 13% and 108 20% of the products or
reaction pathways from the OH radical-initiated reactions of cyclohexene, 1-methyl-1-cyclohexene, and cis-cyclooctene in the
presence of NO are accounted for.
RCH(OH)CH(OO•)R′ + NO
INTRODUCTION
■
→ RCH(OH)CH(O•)R′ + NO2
Alkenes and cycloalkenes emitted into the atmosphere from
biogenic1 and anthropogenic2 sources react in the atmosphere
with OH radicals, NO3 radicals, and O3,3,4 with the OH radical
reaction often being the dominant daytime chemical loss
process.3,4 In the presence of NO, such that the dominant
reaction of organic peroxy radicals is with NO, the key
intermediates in the OH radical-initiated transformations of
alkenes are 1,2-hydroxyalkoxy radicals formed by reactions 1, 2,
and 3b.4
(3b)
As for other alkoxy radicals, the 1,2-hydroxyalkoxy radicals
formed in reaction 3b can decompose by C−C bond scission,
isomerize (generally through a 1,5-H shift via a 6-membered
ring transition state), and react with O2, noting that not all of
these reactions may be feasible for a given 1,2-hydroxyalkoxy
radical.5 For the OH radical-initiated reactions of ≥C5 acyclic
alkenes, decomposition and isomerization of the intermediate
1,2-hydroxyalkoxy radicals are competitive, and both dominate
over reaction with O2 (Table 1).5,6 For example, for the mix of
CH3(CH2)5CH(OH)CH2O• and CH3(CH2)5CH(O•)CH2OH
radicals formed from the OH radical-initiated reaction of 1-
octene in the presence of NO, ∼42% undergo decomposition
OH + RCHCHR′ → RCH(OH)C•HR′
(1)
RCH(OH)C•HR′ + O2(+M)
→ RCH(OH)CH(OO•)R′(+M)
(2)
RCH(OH)CH(OO•)R′ + NO(+M)
→ RCH(OH)CH(ONO2)R′(+M)
Received: July 20, 2012
Revised: August 23, 2012
Published: September 12, 2012
(3a)
© 2012 American Chemical Society
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Table 1. Reaction Rates of Linear Alkoxy, Linear 1,2-Hydroxyalkoxy, and Cycloalkoxy Radicals at 298 K with Respect to
Reaction with O2, Decomposition, and Isomerization, Based on Literature Data
RO• reaction rate (s−1) due to
a
RO•
O2 reaction
decomposition
isomerization
b
linear alkoxy
4.7 × 104
4.7 × 104
4.7 × 104
∼3 × 104
∼4 × 106n
c
linear 1,2-hydroxyalkoxy
∼8 × 106
∼4 × 106n
d
cyclic alkoxy
(3.3−25) × 105
(1.4−15) × 105
a
b
n is the number of isomerization pathways possible (for example, n = 1 in 2-heptoxy, n = 2 in 8-tetradecoxy). Reaction rates for a secondary alkoxy
radical formed from an n-alkane, with isomerization occurring from a CH2 group bonded to two CH2 groups.5 Based on data for the reactions of 7-
hydroxy-8-tetradecoxy radicals,6 with the decomposition rate being derived from the product distribution6 assuming an isomerization rate of 8 × 106
c
s−1.5 Data for the reactions of cycloheptoxy, cyclooctoxy, and cyclodecoxy radicals.7
d
with the remainder being assumed to isomerize,6 as shown
below for the CH3(CH2)5CH(O•)CH2OH radical.
presence of NO to assess the fate of the intermediate cyclic 1,2-
hydroxyalkoxy radicals.
CH3(CH2)5CH(O•)CH2OH
→ CH3(CH2)5CHO + •CH2OH
EXPERIMENTAL METHODS
All experiments were carried out at 297
■
2 K and in the
(4)
presence of 735 Torr of purified air. The majority of
experiments were carried out in a ∼7000 L Teflon chamber
equipped with two parallel banks of black lamps for irradiation
(chamber #1; note that, in the remainder of this article, if no
chamber number is stated, it was chamber #1). A few additional
experiments, concerning dark decays of 1,6-hexanedial and 1,8-
octanedial, and the behavior of 1,6-hexanedial during OH +
cyclohexene reactions, were conducted in a similar second
∼7000 L Teflon chamber (chamber #2). OH radicals were
generated from the photolysis of CH3ONO at wavelengths
>300 nm, and NO was included in the reactant mixtures to
avoid formation of O3 and hence of NO3 radicals. In all cases,
the light intensity corresponded to an NO2 photolysis rate of
0.14 min−1.
Kinetic Study of OH + Cycloalkenes. Rate constants for
the reactions of OH radicals with cyclohexene, 1-methyl-1-
cyclohexene, cis-cyclooctene, and cis-cyclodecene were meas-
ured using a relative rate method in which the relative
disappearance rates of the cycloalkenes and a reference
compound (whose rate constant for reaction with OH radicals
is reliably known) were measured in the presence of OH
radicals.7
CH3(CH2)5CH(O•)CH2OH
→ CH3CH2CH2C•HCH2CH2CH(OH)CH2OH
The CH3CH2CH2C•HCH2CH2CH(OH)CH2OH radical re-
acts further to ultimately form CH3CH2CH2CH(ONO2)-
CH2CH2CH(OH)CH2OH and CH3CH2CH2CH(OH)-
CH2CH2C(O)CH2OH.6
(5)
For alkoxy radicals formed subsequent to H-atom abstraction
from n-alkanes, at room temperature, isomerization (if feasible)
dominates over decomposition and reaction with O2 (Table
1).5,8,9 However, we have recently shown that, for the
cycloalkoxy radicals formed from cycloheptane, cyclooctane,
and cyclodecane, at room temperature decomposition and
isomerization are more important than the O2 reactions (Table
1).7 The increase in the cycloalkoxy radical decomposition rates
over those for the corresponding acyclic alkoxy radicals is
expected because of ring-strain in the 7-, 8-, and 10-membered
ring structures.7 For the cyclohexoxy radical, isomerization does
not occur,7,10 and there is no significant increase in
decomposition rate compared to that of the corresponding
acyclic alkoxy radical (as expected, since there is no ring strain
in cyclohexane).7
As shown in Table 1, at room temperature decomposition of
1,2-hydroxyalkoxy radicals formed after OH radical addition to
acyclic (linear) alkenes is ∼102 faster than decomposition of
alkoxy radicals formed from n-alkanes.5 It is therefore expected
that the cyclic 1,2-hydroxyalkoxy radicals formed from OH +
C3−C10 cycloalkenes will dominantly decompose since ring
strain will increase the decomposition rates by approximately an
order of magnitude or more based on our results for the C7−
C10 cycloalkoxy radicals,7 and isomerization rates will be
decreased,7 with no isomerization expected for the cyclic 1,2-
hydroxyalkoxy radical formed from cyclohexene. However, to
date, there have been very few product studies of the OH
radical-initiated reactions of simple cycloalkenes,11−13 with a
previous study from this laboratory reporting a 31 5% molar
formation yield of 6-oxo-heptanal from OH + 1-methyl-1-
cyclohexene in the presence of NO.12
OH + cycloalkene → products
(6)
OH + reference compound → products
(7)
Providing that the only loss process for the cycloalkene and
reference compound was by reaction with OH radicals, then
⎛
⎝
⎞
⎠
⎛
⎝
⎞
⎠
[cycloalkene]t
[cycloalkene]
[reference compound]t
[reference compound]
k6
k7
0
0
⎜
⎜
⎟
⎟
⎜
⎜
⎟
⎟
ln
=
ln
t
t
(I)
where [cycloalkene]t and [reference compound]t are the
0
0
concentrations of the cycloalkene and reference compound,
respectively, at time t0, [cycloalkene]t and [reference com-
pound]t are the corresponding concentrations at time t, and k6
and k7 are the rate constants for reactions 6 and 7, respectively.
Hence, a plot of ln([cycloalkene]t /[cycloalkene]t) against
0
ln([reference compound]t /[reference compound]t) should
0
In this work, we have measured rate constants for the
reactions of OH radicals with cyclohexene, 1-methyl-1-
cyclohexene, cis-cyclooctene, and cis-cyclodecene and inves-
tigated the products of the OH radical-initiated reactions with
cyclohexene, 1-methyl-1-cyclohexene, and cis-cyclooctene in the
have a slope of k6/k7 and zero intercept.
The initial reactant concentrations (in molecules cm−3) were
CH3ONO and NO, ∼2.4 × 1014 each; and cycloalkene and
reference compound, ∼2.4 × 1013 each. 1,3,5-Trimethylben-
zene was used as the reference compound, and irradiations
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OH + 1,6‐hexanedial → products
the concentration of 1,6-hexanedial at time t is given by,14
[1,6‐hexanedial]t = A(e−x − e−Bx
were carried out for up to 8−12 min, resulting in up to 50−73%
of the initially present cycloalkene or reference compound
being consumed by reaction. The concentrations of the
cycloalkenes and reference compound were measured during
the experiments by gas chromatography with flame ionization
detection (GC-FID). Gas samples of 100 cm3 volume were
collected from the chamber onto Tenax-TA adsorbent, with
subsequent thermal desorption at ∼205 °C onto a 30 m DB-
1701 megabore column, initially held at −40 °C and then
temperature programmed to 250 at 8 °C min−1. During each
experiment, the following GC-FID analyses were conducted: at
least two replicate analyses prior to reaction, one analysis after
each of the three irradiation periods and a replicate analysis
after the third (and last) irradiation period. Replicate analyses
of the cycloalkenes and 1,3,5-trimethylbenzene showed that the
measurement uncertainties were typically <3%.
(8)
)
(II)
where A = αk6[cyclohexene]t /(k8 − k6), B = k8/k6, and x =
0
ln([cyclohexene]t /[cyclohexene]t), α is the formation yield of
0
1,6-hexanedial from OH + cyclohexene [reaction 6a], and k6
and k8 are the rate constants for reactions 6a and 8, respectively.
The initial reactant concentrations (molecules cm−3) were
CH3ONO and NO, ∼1.2 × 1014 each; and cyclohexene, (2.23−
2.42) × 1013. Irradiations were carried out for up to 15 min,
resulting in up to 87% of the initially present cyclohexene being
consumed by reaction. Cyclohexene was monitored by GC-FID
analysis of samples collected onto Tenax solid adsorbent, while
1,6-hexanedial was monitored as its dioximes by GC-FID
analysis of PFBHA-coated SPME fibers, as described above.
Product Analysis by Gas Chromatography. OH radical-
initiated reactions of cyclohexene, 1-methyl-1-cyclohexene, and
cis-cyclooctene were carried out to investigate product
formation yields. The initial concentrations (molecules cm−3)
of CH3ONO, NO, and cycloalkene were ∼1.2 × 1014, ∼1.2 ×
1014, and (2.28−2.46) × 1013, respectively, and 2,5-hexanedione
was also included at a concentration of ∼2.4 × 1012 or ∼4.8 ×
1012 molecules cm−3 as an internal standard. Irradiations (with
a single irradiation period per experiment) were carried out for
1.0−2.5 min (cyclohexene), 0.67−2.0 min (1-methyl-1-cyclo-
hexene), and 1.0−4.0 min (cis-cyclooctene), resulting in 7.6−
26.9%, 10.2−24.7%, and 6.6-31.3% reaction of the initially
present cyclohexene, 1-methyl-1-cyclohexene, and cis-cyclo-
octene, respectively.
Product Analysis by API-MS/MS. CH3ONO−NO−air
irradiations of cyclohexene, cyclohexene-d10, and cis-cyclo-
octene were carried out during which the chamber contents
were sampled through a 25 mm diameter × 75 cm length Pyrex
tube at ∼20 L min−1 directly into the API-MS source. The
operation of the API-MS in the MS (scanning) and MS/MS
[with collision activated dissociation (CAD)] modes has been
described previously.8,10 Both positive and negative ion modes
were used in this work. In positive ion mode, protonated water
hydrates (H3O+(H2O)n), and NO+ ions generated by the
corona discharge in the chamber diluent air were responsible
for the formation of protonated molecules ([M + H]+), water
adduct ions [M + H + H2O]+, protonated homo- and
heterodimers,10 and NO+ adduct ions, while in negative ion
−
−
−
mode, O2 , NO2 , and NO3 ions were largely responsible for
formation of adduct ions.8
Reactants (including 2,5-hexanedione) and products were
collected onto Tenax TA solid adsorbent and analyzed by GC-
FID as described above. Samples were also collected, starting
immediately after the lights were turned off, for 30 min at 15 L
min−1 using an XAD-coated denuder, further coated with
PFBHA prior to sampling to derivatize carbonyls to their
oximes, and extracted as described previously.7 The sampling
entrance of the denuder extended into the chamber, thereby
eliminating any sampling line upstream of the denuder. The
extracts were analyzed by combined gas chromatography−mass
spectrometry in positive chemical ionization (PCI GC-MS)
mode and by GC-FID, with both analyses using DB-5 columns
(60 m for the GC-MS analyses and 30 m for the GC-FID
analyses). The GC-MS analyses used an Agilent 5973 Mass
Selective Detector operated in the scanning mode with
methane as the reagent gas. Each carbonyl group derivatized
to an oxime adds 195 mass units to the compound’s molecular
weight, and methane-PCI gives characteristic protonated
molecules ([M + H]+) and smaller adduct ions at [M + 29]+
and [M + 41]+.7
1,6-Hexanedial, 6-oxo-heptanal, and 1,8-octanedial were
quantified as their dioximes from replicate GC-FID analyses
of the extracts from the PFBHA-coated denuder samples. 2,5-
Hexanedione served as an internal standard and corrections for
the differing FID responses of the dioximes of the dicarbonyls
and of 2,5-hexanedione were made using the Effective Carbon
Numbers of Scanlon and Willis15 and Nishino et al.16 Five or
six separate experiments, each at a different extent of reaction,
were conducted for each cycloalkene studied.
The initial concentrations of CH3ONO, NO, and cyclo-
alkene were ∼2.4 × 1013 molecules cm−3 each, and the reactant
mixtures were irradiated for 1 min, resulting in ∼10%
consumption of the initially present cycloalkene.
Kinetic Studies of 1,6-Hexanedial and 1,8-Octanedial.
Two experiments (one in each Teflon chamber) were carried
out to investigate the importance of dark decay of 1,6-
hexanedial and 1,8-octanedial. 1,6-Hexanedial and 1,8-octane-
dial were generated in situ from irradiation of a CH3ONO−
NO−cyclohexene−cis-cyclooctene−air mixture for 6 min,
followed by monitoring 1,6-hexanedial and 1,8-octanedial in
the dark for 5.3 or 3.3 h. A 65 μm polydimethylsiloxane/
divinylbenzene solid phase microextraction (SPME) fiber was
precoated with O-(2,3,4,5,6,-pentafluorobenzyl)hydroxyl amine
(PFBHA) for on-fiber derivatization of carbonyl-containing
compounds.9 The coated fiber was then exposed to the
chamber contents for 5 min with the chamber mixing fan on,
with subsequent thermal desorption onto a 30 m DB-5 or DB-
1701 megabore column with GC-FID analysis. GC-MS analyses
(see Product Analysis by Gas Chromatography section below)
confirmed the identity of the peaks attributed to the dioximes
of 1,6-hexanedial and 1,8-octanedial.
Two series of experiments (consisting of 3 experiments in
each of the two Teflon chambers) were also carried out to
monitor the time-dependence of 1,6-hexanedial during
irradiated CH3ONO−NO−cyclohexene−air mixtures, in
order to assess the reactivity of 1,6-hexanedial toward OH
radicals. For the reaction system,
Chemicals. The chemicals used and their stated purity
levels were cyclohexene (99%), ChemSampCo; cyclohexene-
OH + cyclohexene → α 1,6‐hexanedial
(6a)
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d10 (99+% atom D), Isotec, Inc.; cis-cyclooctene (95%), cis-
cyclodecene (95%), 2,5-hexanedione (98+%), 1-methyl-1-
cyclohexene (97%), and 1,3,5-trimethylbenzene (98%), Al-
drich; O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine hydro-
chloride (99+%), Alfa Aesar; and NO (≥99.0%), Matheson
Gas Products. Methyl nitrite was prepared as described by
Taylor et al.17 and stored at 77 K under vacuum.
cyclohexene and 1-methyl-1-cyclohexene are expected to be
similar to those for the acyclic alkenes with the same degree
and position of alkyl-substitution,26,28 i.e., cis-2-butene (5.67 ×
10−11 cm3molecule−1 s−1 at 297 K4) and 2-methyl-2-butene
(8.74 × 10−11 cm3molecule−1 s−1 at 297 K4), respectively.
Consistent with this expectation, the rate constants obtained
here for OH radical addition to the CC bond in cyclohexene
and 1-methyl-1-cyclohexene are only slightly higher (by 4% and
7%, respectively) than those for OH radical addition to the C
C bonds in cis-2-butene and 2-methyl-2-butene, respectively.
Our present rate constants for cyclohexene, cis-cyclooctene,
and cis-cyclodecene can be combined with our previous rate
constants for cyclopentene and cycloheptene18 to assess the
effect of ring size on the reactivity toward OH radicals (Table
2), noting that our present and previous18 rate constants for
cyclohexene agree to within 6% (Table 2). The ring-strain
energies in cyclopentene, cyclohexene, cycloheptene, cis-
cyclooctene, and cis-cyclodecene are 5.8, 1.1, 4.9, 5.5, and
14.8 kcal mol−1, respectively.29 Previous work has shown no
significant effect of ring-strain on the rate constants for OH
radical addition to cyclic alkenes containing ≤7-membered
rings,18 and this is evident from the similarity of the rate
constants for OH radical reaction with cyclopentene and
cyclohexene (Table 2). Since the alkyl substituents on the C
C bonds in cyclopentene, cyclohexene, cycloheptene, cis-
cyclooctene, and cis-cyclodecene are all in the cis- configuration,
one could expect that the rate constants would be essentially
independent of ring size, with a slight increase from
cyclopentene through cis-cyclodecene due to H-atom abstrac-
tion of ∼1.4 × 10−12 cm3 molecule−1 s−1 per additional CH2
group.26,27 While the measured rate constants from this and our
previous18 work for cyclopentene, cyclohexene, and cyclo-
heptene are essentially identical, with an average value of 6.8 ×
10−11 cm3 molecule−1 s−1, those for cis-cyclooctene and cis-
cyclodecene are lower by factors of 1.3 and 1.6, respectively.
The reason for this decrease in rate constants for the C8- and
C10-cycloalkenes compared to those for the C5−C7 cyclo-
alkenes is not obvious, although steric factors could possibly
play a role (as has been observed in the reactions of O3 with
certain acyclic and cyclic alkenes30).
Product Analyses of the OH Radical-Initiated Reac-
tions by API-MS/MS. CH3ONO−NO−air irradiations of
cyclohexene, cyclohexene-d10, and cis-cyclooctene were carried
out with analysis by API-MS and API-MS/MS in both positive
and negative ion modes. Representative API-MS spectra are
shown in Figure 2 for the cyclohexene reaction, and the ion
peak assignments derived from API-MS/MS spectra of selected
ion peaks observed in the API-MS spectra are listed in the
caption to Figure 2. Analogous API-MS and API-MS/MS
spectra were obtained for the cyclohexene-d10 and cis-
cyclooctene reactions (see Figures S1 and S2 in Supporting
Information). In an experiment with cyclohexene with added
water vapor present, the ion peaks at 112, 144, and 258 u
(attributed to NO+ adducts; see caption to Figure 2) and at 227
u were not present, while ion peaks at 113 u ([112 + H]+) and
162 u ([161 + H]+) appeared. The ion peak at 113 u together
with its water adduct at 131 u (which was also present in the
absence of added water vapor) suggests that the ion peak at 227
u in the absence of added water vapor was [114 + 112 + H]+.
This is evidence for the presence of a molecular weight 112
product, which could be HC(O)CHCHCH2CH2CHO (or
isomers) arising after H-atom abstraction. There was some
evidence for the corresponding H-atom abstraction product
RESULTS AND DISCUSSION
■
Rate Constants for Reaction with OH Radicals. The
experimental data from CH3ONO−NO−cycloalkene−1,3,5-
trimethylbenzene−air irradiations are plotted in accordance
with eq I in Figure 1, and the rate constant ratios k6/k7 obtained
Figure 1. Plots of eq I for the reactions of OH radicals with
cyclohexene, 1-methyl-1-cyclohexene, cis-cyclooctene, and cis-cyclo-
decene, with 1,3,5-trimethylbenzene as the reference compound. The
data for cis-cyclooctene, cyclohexene, and 1-methyl-1-cyclohexene have
been displaced vertically by 0.10, 0.20, and 0.20 units, respectively, for
clarity. The data are from GC-FID analyses of samples collected onto
Tenax solid adsorbent, and 3 experiments were conducted for each
cycloalkene.
from least-squares analyses of these data are given in Table 2.
These rate constant ratios are placed on an absolute basis using
a rate constant of k7(1,3,5-trimethylbenzene) = 5.67 × 10−11
cm3molecule−1 s−1 at 297 K,4 and the resulting rate constants k6
are also given in Table 2. The available literature data for
cyclopentene,18,19 cyclohexene,18−25 cycloheptene,18 and 1-
methyl-1-cyclohexene21 are also given in Table 2; our present
measurements for cis-cyclooctene and cis-cyclodecene are the
first reported for these two cycloalkenes. As evident from Table
2, our present rate constants for cyclohexene and 1-methyl-1-
cyclohexene are in generally good agreement with the available
literature data.18−25 In particular, our present rate constant for
cyclohexene measured relative to that for 1,3,5-trimethylben-
zene is in excellent agreement with that we previously measured
relative to OH + 2-methyl-1,3-butadiene (isoprene).18
H-atom abstraction from the CH2 and CH3 groups in
cyclohexene and 1-methyl-1-cyclohexene is predicted26,27 to
account for 8% and 5% of the overall OH radical reactions with
cyclohexene and 1-methyl-1-cyclohexene, respectively. There-
fore, the rate constants for OH radical reaction with
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Table 2. Rate Constant Ratios k6/k7 and Rate Constants k6 for the Reactions of OH Radicals with Cycloalkenes at 297 2 K,
Together with Literature Data
a
cycloalkene
cyclopentene
k6/k7
1011 × k6 (cm3 molecule−1 s−1
)
reference
b
6.66 0.24
Atkinson et al.18
c
5.02 0.40
Rogers19
d
5.97 0.16
Rogers19
e
cyclohexene
6.59
Wu et al.20
Darnall et al.21
Cox et al.22
Barnes et al.23
Ohta24
f
7.56 1.52
g
6.54
g
6.75
h
6.45 0.25
b
6.70 0.17
Atkinson et al.18
Rogers19
c
5.43 0.24
i
6.18 0.81
Nielsen et al.25
this work
j
1.12 0.02
6.35 0.12
b
cycloheptene
7.37 0.23
Atkinson et al.18
this work
j
cis-cyclooctene
0.910 0.025
0.738 0.009
5.16 0.15
j
cis-cyclodecene
4.18 0.06
this work
f
1-methyl-1-cyclohexene
9.44 1.89
Darnall et al.21
this work
j
1.73 0.03
9.81 0.18
a
At 297 2 K, relative to k(OH + 1,3,5-trimethylbenzene). The indicated errors are two least-squares standard deviations. The estimated overall
b
uncertainties are the two least-squares standard deviations or 6%, whichever is larger. At 298 2 K. Relative to k(OH + 2-methyl-1,3-butadiene)
= 1.00 × 10−10 cm3 molecule−1 s−1.4 At 298 3 K. Relative to k(OH + trans-2-butene) = 6.40 × 10−11 cm3 molecule−1 s−1.4 At 298 3 K. Relative
c
d
e
to k(OH + cyclohexene) = 6.35 × 10−11 cm3 molecule−1 s−1 (this work). At 303 K. Relative to k(OH + cis-2-butene) = 5.49 × 10−11 cm3 molecule−1
s−1.4 At 305 2 K. Relative to k(OH + 2-methylpropene) = 4.94 × 10−11 cm3 molecule−1 s−1.4 At 300 K. Relative to k(OH + ethene) = 8.44 ×
f
g
10−12 cm3 molecule−1 s−1.4 At 297 2 K. Relative to k(OH + 1,5-hexadiene) = 6.20 × 10−11 cm3 molecule−1 s−1.4 At 298 2 K. Absolute rate
h
i
measurement. Placed on an absolute basis using a rate constant of k7(OH + 1,3,5-trimethylbenzene) = 5.67 × 10−11 cm3 molecule−1 s−1.4 The
j
indicated errors are two least-squares standard deviations and do not take into account the uncertainties associated with the rate constant k7.
from the cis-cyclooctene reaction, but no API-MS/MS analysis
was conducted on the small ion peak at 283 u (potentially [142
+ 140 + H]+). The molecular weights of the products observed
from the cyclohexene, cyclohexene-d10, and cis-cyclooctene
reactions were consistent with formation of two major products
from each Cn-cycloalkene: a Cn-cyclic hydroxynitrate and a Cn-
dicarbonyl. These are the products expected after initial OH
radical addition to the CC bond, with the intermediate cyclic
1,2-hydroxyalkyl peroxy radicals reacting via reactions 1
through 3a,b to form the cyclic 1,2-hydroxynitrate or the cyclic
1,2-hydroxyalkoxy radical, followed by decomposition of the
cyclic 1,2-hydroxyalkoxy radical (Scheme 1). No evidence was
observed in the API-MS analyses for product ion peaks arising
from isomerization of the intermediate cyclic 1,2-hydroxyalkoxy
radicals (see Scheme 1).
Dark Decays of 1,6-Hexanedial and 1,8-Octanedial
and Reactivity of 1,6-Hexanedial toward OH Radicals.
The relative concentrations of 1,6-hexanedial and 1,8-
Figure 2. API-MS spectra of an irradiated CH3ONO−NO−cyclo-
hexene−air mixture. The products identified were 1,6-hexanedial (MW
114) and the C6-cyclic hydroxynitrate (MW 161). Top: positive ion
spectrum. On the basis of positive ion API-MS/MS analyses, the ion
peak assignments are 112 u (present in prereaction API-MS spectra
and diminished in intensity with the extent of reaction), [cyclohexene
+ NO]+; 115 u, [114 + H]+; 131 u, [112 + H + H2O]+; 144 u, [114 +
NO]+; 180 u, [161 + H + H2O]+; 185 u, [114 + 71]+; 227 u, [114 +
112 + H]+; 229 u, [114 + 114 + H]+; 247 u, [114 + 114 + H + H2O]+;
258 u, [114 + 114 + NO]+; and 276 u, [114 + 161 + H]+. Bottom:
negative ion spectrum. On the basis of negative ion API-MS/MS
analyses, the ion peak assignments are 146 u, [114 + O2]−; 160 u, [114
+ NO2]−; 174 u, [114 + 60]−; 176 u, [114 + NO3]−; 189 u, [114 +
75]−; 193 u, [161 + O2]−; 207 u, [161 + NO2]−; 221 u, [161 + 60]−;
223 u, [161 + NO3]−; and 354 u (not shown), [161 + 161 + O2]−.
The ion peak at 237 u could not be readily assigned.
hexanedial (generated in situ from OH + cyclohexene and
cis-cyclooctene) were measured by GC-FID analyses of
PFBHA-coated SPME fibers exposed to the chamber contents
over periods of 5.3 h (chamber #1) and 3.3 h (chamber #2).
Least-squares analyses of the experimental data, in the form
ln([dicarbonyl]t /[dicarbonyl]t) against time, resulted in dark
0
decay rates for 1,6-hexanedial of (1.36 0.25) × 10−5 s−1 in
chamber #1 and (3.0 2.7) × 10−6 s−1 in chamber #2, and for
1,8-octanedial of (1.46 0.21) × 10−5 s−1 in chamber #1 and
(4.2
3.3) × 10−6 s−1 in chamber #2, where the indicated
errors are two least-squares standard deviations.
The 1,6-hexanedial concentrations (in units of GC-FID area
counts of the oximes of 1,6-hexanedial) from three irradiated
CH3ONO−NO−cyclohexene−air mixtures in chamber #1,
with three irradiation periods per experiment, are plotted
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Scheme 1. Expected Products Arising after OH Radical Addition to Cyclohexene; Products Observed by API-MS and GC-MS
Analyses Are Shown in Boxes (See Text)
extracts of PFBHA-coated denuder samples collected from the
OH radical-initiated reactions of cyclohexene, 1-methyl-1-
cyclohexene, and cis-cyclooctene showed the presence of
dioximes (with no mono-oximes) of products of molecular
weight 114, 128, and 142, respectively. This is consistent with
the API-MS analyses of the cyclohexene and cis-cyclooctene
reactions (see above), and these dicarbonyl products are
attributed to 1,6-hexanedial from cyclohexene, 6-oxo-heptanal
from 1-methyl-1-cyclohexene, and 1,8-octanedial from cis-
cyclooctene. There was no evidence in any of these reactions
for the presence of oximes of the cyclic hydroxycarbonyl
formed from reaction of the initially formed cyclic 1,2-
hydroxyalkoxy radical with O2, nor for those of the
dihydroxycarbonyls formed after isomerization of the cyclic
1,2-hydroxyalkoxy radical (shown in Scheme 1 for the
cyclohexene reaction). However, in order to obtain upper
limits to their yields, authentic standards of these potential
products would be needed to determine GC retention times of
their oximes.
1,6-Hexanedial, 6-oxo-heptanal, and 1,8-octanedial are not
commercially available, and hence, no standards could be used
to determine GC retention times or collection and analysis
efficiencies. It should be noted that no significant product peaks
were observed from GC-FID analyses of samples collected
from reacted OH + cyclohexene or OH + cis-cyclooctene
mixtures onto Tenax solid adsorbent with subsequent thermal
desorption, suggesting that 1,6-hexanedial and 1,8-octanedial
were not amenable to analysis using that procedure. For
samples collected from reacted OH + 1-methyl-1-cyclohexene
mixtures, a product peak (attributed to 6-oxo-heptanal) was
observed by this sampling and analysis procedure, but replicate
against the extent of reaction, defined as ln([cyclohexene]t /
0
[cyclohexene]t), in Figure 3. The durations of the experiments,
from the start of the first irradiation period to the midpoint of
the sampling period after the third (and last) irradiation period,
were 116−158 min, and the measured 1,6-hexanedial
concentrations were corrected for dark decay using the dark
decay rate measured in this chamber. As evident from eq II, the
dependence of the 1,6-hexanedial concentration with the extent
of reaction is determined by A (which affects the scaling of the
Y-axis) and by B = k8/k6. The value of ln([cyclohexene]t /
0
[cyclohexene]t) at which the 1,6-hexanedial concentration is a
maximum depends only on the rate constant ratio k8/k6 and is
given by ln(k8/k6)/[(k8/k6) − 1] = ln B/(B − 1).14 The data in
Figure 3 are well fitted with k8/k6 = 0.7, and considering that
small corrections had to be made for dark decay of 1,6-
hexanedial, this derived rate constant ratio is expected to be
uncertain to a factor of ∼1.3, and hence, k8/k6 = 0.7 0.2.
An independent series of analogous experiments were carried
out in chamber #2, with the GC-FID analyses of the PFBHA-
coated SPME fibers using a 30 m DB-1701 column. The 1,6-
hexanedial concentrations from three irradiated CH3ONO−
NO−cyclohexene−air mixtures in chamber #2, corrected for
the dark decay of 1,6-hexanedial in this chamber (a factor of ∼4
lower than that in chamber #1), were well fit by k8/k6 = 0.8
0.2. Thus, the two independent measurements of the rate
constant ratio k8/k6 are in good agreement, with an average of
k8/k6 = 0.75 0.2, which, when combined with our measured
rate constant k6 (Table 2), corresponds to k8 = (4.8 1.3) ×
10−11 cm3 molecule−1 s−1 at 297 2 K.
Product Analyses of the OH Radical-Initiated Reac-
tions by Gas Chromatography. PCI GC-MS analyses of
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the dicarbonyls to those of 2,5-hexanedione and taking into
account the differing FID responses of the dioximes of the
dicarbonyls and of 2,5-hexanedione,15,16 the concentrations of
the dicarbonyls in the chamber could be derived. Replicate
postreaction GC-FID analyses of the extract of each PFBHA-
coated denuder sample were conducted, with excellent
reproducibility relative to the internal standard.
The concentrations of the dicarbonyls measured in the
chamber after the reaction then need to be corrected for
reaction with OH radicals. Rate constants for the reactions of
OH radicals with 1,6-hexanedial, 6-oxo-heptanal, and 1,8-
octanedial have not been directly measured to date, and the
only room temperature rate constants available for analogous
dialdehydes and keto-aldehyes are for 1,5-pentanedial (2.4 ×
10−11 cm3 molecule−1 s−1)19 and pinonaldehyde (3.9 × 10−11
cm3 molecule−1 s−1).32 Since H-atom abstraction from −CHO
groups is generally the dominant reaction pathways for
aldehydes,4,26,32 1,5-pentanedial is expected to be more reactive
than the keto-aldehyde pinonaldehyde. Rate constants for 1,5-
pentanedial, 1,6-hexanedial, 6-oxo-heptanal, 1,8-octanedial, and
pinonaldehyde, calculated using the Kwok and Atkinson
estimation method26 are 5.75 × 10−11, 5.22 × 10−11, 3.15 ×
10−11, 5.50 × 10−11, and 2.71 × 10−11 cm3 molecule−1 s−1,
respectively, with the calculated rate constant for 1,5-
pentanedial being a factor of 2.4 higher than the measured
value19 and with the estimated rate constant for pinonaldehyde
being a factor of 1.4 lower than the currently recommended
rate constant.32 Therefore, both our estimated rate constants
and that measured for pinonaldehyde32 suggest that the
measured rate constant for 1,5-pentanedial19 may be low.
The time-dependent behavior of 1,6-hexanedial during OH +
cyclohexene reactions indicates that k8 = (4.8 1.3) × 10−11
cm3 molecule−1 s−1, consistent with the estimated26 rate
constant of 5.22 × 10−11 cm3 molecule−1 s−1 but a factor of 2
higher than that of 2.4 × 10−11 cm3 molecule−1 s−1 reported by
Rogers19 for 1,5-pentanedial (the additional CH2 group in 1,6-
hexanedial is estimated to have little effect on the rate
constant;26 see above). Therefore, corrections for reactions of
OH radicals with 1,6-hexanedial, 6-oxo-heptanal, and 1,8-
octanedial with OH radicals were made using our measured
rate constants for cyclohexene, 1-methyl-1-cyclohexene, and cis-
Figure 3. Plot of the measured relative concentrations of the dioximes
of 1,6-hexanedial, corrected for wall loss (see text), as a function of the
extent of reaction, defined as ln([cyclohexene]t /[cyclohexene]t), in
0
three OH + cyclohexene reactions conducted in chamber #1. The
differing symbols refer to different experiments. The lines are
calculated from the expression [1,6-hexanedial] = A{exp(−k6[OH]t)
− exp(−k8[OH]t)}, where k6[OH] is the rate of reaction of
cyclohexene and k8[OH] is the rate of reaction of OH + 1,6-
hexanedial. Cyclohexene was measured by GC-FID analyses of
samples collected onto Tenax solid adsorbent, and the dioximes of
1,6-hexanedial were measured by GC-FID analyses of samples
collected onto PFBHA-coated SPME fibers. The rate constant ratio
k8/k6 = 0.4 would result from combination of the rate constant for OH
+ 1,5-pentanedial reported by Rogers19 with the rate constant k6
measured here for OH + cyclohexene (Table 2).
GC-FID analyses showed that the GC peak area was quite
variable and corresponded to 6-oxo-heptanal formation yields
≤50%. This can be compared to our previous study of the OH
+ 1-methyl-1-cyclohexene reaction where samples were also
collected onto Tenax solid adsorbent with subsequent thermal
desorption and GC-FID analysis, with a measured formation
yield of 6-oxo-heptanal of 31
5%.12 Our present analyses
suggest that 6-oxo-heptanal is either not efficiently collected
onto, and/or thermally desorbed from, Tenax solid adsorbent,
or that 6-oxo-heptanal does not efficiently elute from GC
columns, and that our previously reported formation yield12
was erroneously low.
cyclooctene and calculated rate constants26 of 5.22 × 10−11
,
3.15 × 10−11, and 5.50 × 10−11 cm3 molecule−1 s−1 for 1,6-
hexanedial, 6-oxo-heptanal, and 1,8-octanedial, respectively.
The multiplicative correction factors, F, increase with the rate
constant ratio k(OH + dicarbonyl)/k(OH + cycloalkene) and
with the extent of reaction. Because the extents of reaction were
kept low (≤31%), the maximum values of F were 1.14 for the
cyclohexene reactions, 1.05 for the 1-methyl-1-cyclohexene
reactions, and 1.23 for the cis-cyclooctene reactions.
1,6-Hexanedial, 6-oxo-heptanal, and 1,8-octanedial were
therefore quantified as their dioximes from GC-FID analyses
of extracts of PFBHA-coated denuder samples. Experiments
were carried out with 2,5-hexanedione present as an internal
standard in the initial reactant mixtures (i.e., before reaction).
The rate constant for the reaction of OH radicals with 2,5-
hexanedione has been measured to be (7.13 0.34) × 10−12
cm3 molecule−1 s−1 at 298 K,31 and hence 2,5-hexanedione is
less reactive toward OH radicals than the cycloalkenes studied
here by factors of 6−14. On the basis of GC-FID analyses of
Tenax samples collected before and after reaction, the
percentages of 2,5-hexanedione which were consumed by
reaction were <6% in all cases, consistent with expectations
based on the reactivities of 2,5-hexanedione and the cyclo-
alkenes and the fractions of the cycloalkenes reacted. 2,5-
Hexanedione was observed in the PFBHA-coated denuder
samples as its dioximes (no mono-oximes were observed).
Hence, by ratioing the GC-FID peaks areas of the dioximes of
Plots of the amounts of dicarbonyl observed, corrected for
reaction with OH radicals, against the amounts of cycloalkene
reacted are shown in Figure 4. For all three cycloalkenes,
significant negative intercepts are observed, suggesting that a
loss of dicarbonyl occurs before efficient collection and
derivatization happens. Least-squares analyses (without in-
clusion of a 0,0 point) leads to the dicarbonyl formation yields
given in Table 3. Note that, even if the rate constants for the
reactions of OH radicals with 1,6-hexanedial, 6-oxo-heptanal
and 1,8-octanedial are a factor of 2 lower than those calculated
and used, the multiplicative correction factors F would change
by <12%, with the largest effect being for cis-cyclooctene for
which the 1,8-octanedial yield would decrease from 84 18%
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yields can be estimated using the data of O’Brien et al.33 for 1,2-
hydroxynitrate formation from the OH radical-initiated
reactions of a series of C2−C6 alkenes or through the
expression presented by Matsunaga and Ziemann34 to fit
their 1,2-hydroxynitrate formation yields in the aerosol phase
from the OH radical-initiated reactions of a series of C8−C17
alkenes. A linear regression of the 1,2-hydroxynitrate formation
yields of O’Brien et al.33 against carbon number results in yield
= (−1.8 + 1.2n)%, and the calculated 1,2-hydroxynitrate yields
from the cycloalkenes studied here are listed in Table 3. The
expression presented by Matsunaga and Ziemann34 was based
on the equation proposed by Arey et al.8 for secondary alkyl
nitrate formation from a series of C3−C8 alkanes, scaled by a
factor of 0.455,34 and the calculated 1,2-hydroxynitrate yields
using this approach are also listed in Table 3. The two
estimation methods predict reasonably similar hydroxyalkylni-
trate formation yields, and an average is used for the
calculations below.
The fraction of the overall reaction proceeding by H-atom
abstraction can be derived by calculating the rate constant for
H-atom abstraction using the Kwok and Atkinson estimation
method26 together with our overall OH radical reaction rate
constants, and the resulting percentage of the overall reactions
proceeding by H-atom abstraction are also given in Table 3.
Assuming that the extent of H-atom abstraction and of 1,2-
hydroxynitrate formation are each uncertain to 50%, then H-
atom abstraction, cyclic 1,2-hydroxynitrate formation (after OH
radical addition), and cyclic 1,2-hydroxyalkoxy radical decom-
position account for 90 12%, 95 13% and 108 20% of
the overall reaction pathways for the OH radical-initiated
reactions of cyclohexene, 1-methyl-1-cyclohexene, and cis-
cyclooctene, respectively. Thus, the reaction pathways are
accounted for, to within ∼20%, and under our experimental
conditions where the cyclic 1,2-hydroxyalkoxy radicals were
Figure 4. Plots of the amounts of 1,6-hexanedial, 6-oxo-heptanal, and
1,8-octanedial formed, corrected for reaction with OH radicals (see
text), against the amounts of cyclohexene, 1-methyl-1-cyclohexene,
and cis-cyclooctene, respectively, reacted with OH radicals. The data
for 6-oxo-heptanal and 1,6-hexanedial have been displaced vertically by
1.0 × 1012 and 3.0 × 1012 molecules cm−3, respectively, for clarity. The
least-squares intercepts on the Y-axis are (in units of 1012 molecules
cm−3): −0.615 0.364 for OH + cyclohexene, −0.521 0.415 for
OH + 1-methyl-1-cyclohexene, and −1.15
0.765 for OH + cis-
cycolooctene, where the indicated errors are two least-squares
standard deviations. The cycloalkene concentrations were measured
by GC-FID analyses of samples collected onto Tenax solid adsorbent,
and the dioximes of the dicarbonyls were measured by GC-FID
analyses of extracts of PFBHA-coated denuder samples (see text).
to 75 15%. The yields obtained directly from the experiment
at the greatest extent of reaction for each cycloalkene (i.e., yield
= ([dicarbonyl formed], corrected for reaction with OH
radicals)/[cycloalkene reacted]) are 69% for cyclohexene, 76%
for 1-methyl-1-cyclohexene, and 74% for cis-cyclooctene, and
these would then be lower limits.
The other products expected are the cyclic 1,2-hydroxyni-
trates formed from the cyclic 1,2-hydroxyalkyl peroxy radical +
NO reaction (Scheme 1) and products arising after H-atom
abstraction. As noted above, our API-MS analyses showed the
formation of the expected hydroxynitrates, and their formation
•
formed from the exothermic RO2 + NO reaction, decom-
position of these cyclic 1,2-hydroxyalkoxy radicals is the
dominant pathway, leading to ring-opened dicarbonyls.
ASSOCIATED CONTENT
■
S
* Supporting Information
API-MS spectra obtained from the OH + cyclohexene-d10 and
OH + cis-cyclooctene reactions. This material is available free of
charge via the Internet at http://pubs.acs.org.
Table 3. Products Observed and Quantified from the OH Radical-Initiated Reactions of Cycloalkenes, Together with Estimated
Formation Yields of Cyclic 1,2-Hydroxynitrates and H-Atom Abstraction Products
molar yield (%)
a
b
cycloalkene
cyclohexene
dicarbonyl product
decomposition
cyclic hydroxynitrate
H-atom abstraction
c
d
d
d
1,6-hexanedial
6-oxo-heptanal
1,8-octanedial
76 10
82 12
84 18
5.4 ; 6.7
8.1
c
1-methyl-1-cyclohexene
6.6 ; 9.1
5.4
c
cis-cyclooctene
7.8 ; 9.5
15.1
a
This work. Obtained from least-squares analyses of the data plotted in Figure 4. The indicated errors are two least-squares standard deviations of
the data in Figure 4, combined with estimated uncertainties in the GC-FID response factors for the cycloalkenes and 2,5-hexanedione of 5% each.
b
c
Calculated from the estimated H-atom abstraction rate constants26 and our measured overall OH radical reaction rate constants (Table 2). Based
on the hydroxyalkyl nitrate yields of O’Brien et al.33 from the reactions of OH radicals with a series of C2−C6 alkenes at 296 3 K and atmospheric
pressure of air, using a linear regression of the hydroxynitrate yields33 versus carbon number n, with yield = (−1.8 + 1.2n)%. Calculated from the
d
expression presented by Matsunaga and Ziemann,34 for hydroxyalkyl nitrate formation from the OH + alkene addition pathway, which was based on
the alkyl nitrate formation yield expression of Arey et al.8 scaled by a factor of 0.455.34 The cited yields are calculated for 297 K and 735 Torr
pressure.
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(26) Kwok, E. S. C.; Atkinson, R. Atmos. Environ. 1995, 29, 1685−
1695.
(27) Nishino, N.; Arey, J.; Atkinson, R. J. Phys. Chem. A 2009, 113,
AUTHOR INFORMATION
■
Corresponding Author
*(J.A.) Tel: (951) 827-3502. E-mail: janet.arey@ucr.edu.
(R.A.) Tel: (951) 827-4191. E-mail: ratkins@mail.ucr.edu.
852−857.
(28) Peeters, J.; Boullart, W.; Pultau, V.; Vandenberk, S.; Vereecken,
L. J. Phys. Chem. A 2007, 111, 1618−1631.
Notes
(29) NIST. Structures and Properties, version 2.0, Standard Reference
Database 25; Stein, S. E., Ed.; Chemical Kinetics and Thermodynamics
Division, National Institute of Standards and Technology: Gaithers-
burg, MD, January 1994; NIST Chemistry WebBook, NIST Standard
Reference Database Number 69, http://webbook.nist.gov/chemistry/.
(30) McGillen, M. R.; Carey, T. J.; Archibald, A. T.; Wenger, J. C.;
Shallcross, D. E.; Percival, C. J. Phys. Chem. Chem. Phys. 2008, 10,
1757−1768.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the U.S. Environmental Protection Agency (Grant
No. R833752) and the University of California Agricultural
Experiment Station for supporting this research. While this
work has been supported in part by the U.S. Environmental
Protection Agency, the results and the contents of this
publication do not necessarily reflect the views and the
opinions of the Agency.
(31) Dagaut, P.; Wallington, T. J.; Liu, R.; Kurylo, M. J. J. Phys. Chem.
1988, 92, 4375−4377.
(32) IUPAC Evaluated Kinetic Data for Atmospheric Chemistry.
IUPAC Subcommittee for Gas Kinetic Data Evaluation: Cambridge,
U.K., 2012; see http://www.iupac-kinetic.ch.cam.ac.uk.
(33) O’Brien, J. M.; Czuba, E.; Hastie, D. R.; Francisco, J. S.;
Shepson, P. B. J. Phys. Chem. A 1998, 102, 8903−8908.
(34) Matsunaga, A.; Ziemann, P. J. J. Phys. Chem. A 2009, 113, 599−
606.
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